Mechanical BE (Radiation and Propagation) by IMTS INSTITUTE

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RADIATION AND PROPAGATION CONTENTS: UNIT – I 01-22 Biological effects of radiation – Structure of the cell - Radiation effects on cells – Biological effects – Lethal dose – Radiation sickness – Stochastic and non stochastic effect. Radiation units and operational limits – Activity – Exposure – Dose – Dose Equivalent – Dose rate – Operational limits – Dose equivalent limit. UNIT – II

23-35

Interaction of changed particles with mater – Heavy charged particles – Electrons – Absorption of gamma rays by matter – Photoelectric effect – Compton scattering and pair production – Detectors of radiation – Solid state counter – G.M. counter – Nuclear emulsion plates – Scintillation counter. UNIT – III 36-62 Industrial and Analytical applications – Tracing, Gauging, Material modification, Sterilization – Food preservation and other applications, Radiation protection and safety – Area monitoring – Gun monitoring – Mini Rad Survey meter - Radiation survey meter – Personal monitoring – Film badge dosimeter – Pocket dosimeter – Control of radiation hazards – Distance and time shielding – Shielding thickness calculations. UNIT – IV 63-82 Diagnostic imaging and application to Radiation therapy – Radio isotopes used for Brach therapy – Digital Radiography – Digital X-ray detectors, digital subtraction angiography UNIT – V

83-131

Computed tomography – Nuclear medicine – Properties of radioactive pharmaceuticals – Nuclear medicine imaging – Positron emission Tomography

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UNIT-I 1.1.IONIZATION AND CELL DAMAGE As previously discussed, photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials. This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material. This creates electrons, which carry a negative charge, and atoms without electrons, which carry a positive charge. Ionization in industrial materials is usually not a big concern. In most cases, once the radiation ceases the electrons rejoin the atoms and no damage is done. However, ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other. This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed. Ionization may cause unwanted changes in some materials, such as semiconductors, so that they are no longer effective for their intended use. Ionization in Living Tissue (Cell Damage) In living tissue, similar interactions occur and ionization can be very detrimental to cells. Ionization of living tissue causes molecules in the cells to be broken apart. This interaction can kill the cell or cause them to reproduce abnormally. Damage to a cell can come from direct action or indirect action of the radiation. Cell damage due to direct action occurs when the radiation interacts directly with a cell's essential molecules (DNA). The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA). DNA is found in every cell and consists of molecules that determine the function that each cell performs. When radiation interacts with a cell wall or DNA, the cell either dies or becomes a different kind of cell, possibly even a cancerous one. Cell damage due to indirect action occurs when radiation interacts with the water molecules, which are roughly 80% of a cells composition. The energy absorbed by the water molecule can result in the formation of free radicals. Free radicals are molecules that are highly reactive due to the presence of unpaired electrons, which result when water molecules are split. Free radicals may form compounds, such as hydrogen peroxide, which may initiate harmful chemical reactions within the cells. As a result of these chemical changes, cells may undergo a variety of structural changes which lead to altered function or cell death. Various possibilities exist for the fate of cells damaged by radiation. Damaged cells can: ď&#x201A;ˇ

completely and perfectly repair themselves with the body's inherent repair mechanisms.

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die during their attempt to reproduce. Thus, tissues and organs in which there is substantial cell loss may become functionally impaired. There is a "threshold" dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome. Exceeding the threshold dose increases the level of harm. Such outcomes are called deterministic effects and occur at high doses.

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repair themselves imperfectly and replicate this imperfect structure. These cells, with the progression of time, may be transformed by external agents (e.g., chemicals, diet, radiation exposure, lifestyle habits, etc.). After a latency period of years, they may develop into leukemia or a solid tumor (cancer). Such latent effects are called stochastic (or random).

Exposure of Living Tissue to Non-ionizing Radiation A quick note of caution about non-ionizing radiation is probably also appropriate here. Non-ionizing radiation behaves exactly like ionizing radiation, but differs in that it has a much greater wavelength and, therefore, less energy. Although this non-ionizing radiation does not have the energy to create ion pairs, some of these waves can cause personal injury. Anyone who has received a sunburn knows that ultraviolet light can damage skin cells. Non-ionizing radiation sources include lasers, high-intensity sources of ultraviolet light, microwave transmitters and other devices that produce high intensity radio-frequency radiation. 1.2.CELL RADIOSENSITIVITY Radiosensitivity is the relative susceptibility of cells, tissues, organs, organisms, or other substances to the injurious action of radiation. In general, it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation. In short, this means that actively dividing cells or those not fully mature are most at risk from radiation. The most radio-sensitive cells are those which: 

have a high division rate

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have a high metabolic rate

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are of a non-specialized type

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are well nourished

Examples of various tissues and their relative radiosensitivities are listed below. High Radiosensitivity Lymphoid organs, bone marrow, blood, testes, ovaries, intestines Fairly High Radiosensitivity Skin and other organs with epithelial cell lining (cornea, oral cavity, esophagus, rectum, bladder, vagina, uterine cervix, ureters) Moderate Radiosensitivity

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Optic lens, stomach, growing cartilage, fine vasculature, growing bone Fairly Low Radiosensitivity Mature cartilage or bones, salivary glands, respiratory organs, kidneys, liver, pancreas, thyroid, adrenal and pituitary glands Low Radiosensitivity Muscle, brain, spinal cord Reference: Rubin, P. and Casarett. G. W.: Clinical Radiation Pathology (Philadelphia: W. B. Saunders. 1968). 1.3.RADIATION UNITS There are four measures of radiation that radiographers will commonly encounter. These are: Activity, Exposure, Absorbed Dose, and Dose Equivalent. A short summary of these measures and their units will be followed by more in depth information. 

Activity: The activity of a radioactive source is defined as the rate at which the isotope decays. Radioactivity may be thought of as the volume of radiation produced in a given amount of time. The International System (SI) unit for activity is the becquerel (Bq) and the curie (Ci) is also commonly used.

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Exposure: Exposure is a measure of the strength of a radiation field at some point in air. This is the measure made by a survey meter. The most commonly used unit of exposure is the roentgen (R).

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Absorbed Dose: Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter. The SI unit for absorbed dose is the gray (Gy), but the “rad” (Radiation Absorbed Dose) is commonly used. 1 rad is equivalent to 0.01 Gy. Different materials that receive the same exposure may not absorb the same amount of energy. In human tissue, one Roentgen of gamma radiation exposure results in about one rad of absorbed dose. Dose Equivalent: The dose equivalent relates the absorbed dose to the biological effect of that dose. The absorbed dose of specific types of radiation is multiplied by a "quality factor" to arrive at the dose equivalent. The SI unit is the sievert (SV), but the rem is commonly used. Rem is an acronym for "roentgen equivalent in man." One rem is equivalent to 0.01 SV. When exposed to X- or Gamma radiation, the quality factor is 1.

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Since for human tissue one Roentgen equals one rad and the quality factor for x- and gamma rays is one, radiographers can consider the Roentgen, rad, and rem to be equal in value.

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More Information on Radiation Units Activity The strength of a radioactive source is called its activity, which is defined as the rate at which the isotope decays. Radioactivity may be thought of as the volume of radiation produced in a given amount of time. It is similar to the current control on a X-ray tube. The International System (SI) unit for activity is the becquerel (Bq), which is that quantity of radioactive material in which one atom transforms per second. The becquerel is a small unit. In practical situations, radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq). The curie (Ci) is also commonly used as the unit for activity of a particular source material. The curie is a quantity 10 of radioactive material in which 3.7 x 10 atoms disintegrate per second. This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226. One curie equals approximately 37,037 MBq. New sources of cobalt will have an activity of 20 to over 100 curies, and new sources of iridium will have an activity of similar amounts. The activity of a given amount of radioactive material does not depend upon the mass of material present. For example, two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source. The concentration of radioactivity, or the relationship between the mass of radioactive material and the activity, is called the specific activity. Specific activity is expressed as the number of curies or becquerels per unit mass or volume. The higher the specific activity of a material, the smaller the physical size of the source is likely to be.

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Exposure Exposure is a measure of the strength of a radiation field at some point. It is a measure of the ionization of the molecules in a mass of air. It is usually defined as the amount of charge (i.e. the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass. The most commonly used unit of exposure is the Roentgen (R). Specifically, a Roentgen is the amount of 12 photon energy required to produce 1.610 x 10 ion pairs in one cubic centimeter of dry air at 0°C. A radiation field of one Roentgen will deposit 2.58 -4 x 10 coulombs of charge in one kilogram of dry air. The main advantage of this unit is that it is easy to directly measure with a survey meter. The main limitation is that it is only valid for deposition in air. Absorbed Dose Whereas exposure is defined for air, the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter. The most commonly used unit for absorbed dose is the “rad” (Radiation Absorbed Dose). A rad is defined as a dose of 100 ergs of energy per gram of the given material. The SI unit for absorbed dose is the gray (Gy), which is defined as a dose of one joule per kilogram. Since one joule equals 7 10 ergs, and since one kilogram equals 1000 grams, 1 Gray equals 100 rads. The size of the absorbed dose is dependent upon the strength (or activity) of the radiation source, the distance from the source to the irradiated material, and the time over which the material is irradiated. The activity of the source will determine the dose, rate which can be expressed in rad/hr, mr/hr, mGy/sec, etc. Dose Equivalent When considering radiation interacting with living tissue, it is important to also consider the type of radiation. Although the biological effects of radiation are dependent upon the absorbed dose, some types of radiation produce greater effects than others for the same amount of energy imparted. For example, for equal absorbed doses, alpha particles may be 20 times as damaging as beta particles. In order to account for these variations when describing human health risks from radiation exposure, the quantity called “dose equivalent” is used. This is the absorbed dose FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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multiplied by certain “quality” or “adjustment” factors indicative of the relative biological-damage potential of the particular type of radiation. The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness. Radiation with higher Q factors will cause greater damage to tissue. The rem is a term used to describe a special unit of dose equivalent. Rem is an abbreviation for roentgen equivalent in man. The SI unit is the sievert (SV); one rem is equivalent to 0.01 SV. Doses of radiation received by workers are recorded in rems, however, sieverts are being required as the industry transitions to the SI unit system.

The table below presents the Q factors for several types of radiation. Type of Radiation X-Ray Gamma Ray Beta Particles Thermal Neutrons Fast Neutrons Alpha Particles

Rad 1 1 1 1 1 1

Q Factor 1 1 1 5 10 20

Rem 1 1 1 5 10 20

1.4.BIOLOGICAL EFFECTS The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including: 

Type of radiation involved. All kinds of ionizing radiation can produce health effects. The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have. Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues.

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Size of dose received. The higher the dose of radiation received, the higher the likelihood of health effects.

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Rate the dose is received. Tissue can receive larger dosages over a period of time. If the dosage occurs over a number of days or weeks, the results are often not as serious if a similar dose was received in a matter of minutes.

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Part of the body exposed. Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso.

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The age of the individual. As a person ages, cell division slows and the body is less sensitive to the effects of ionizing radiation. Once cell division has slowed, the effects of radiation are somewhat less damaging than when cells were rapidly dividing.

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Biological differences. Some individuals are more sensitive to the effects of radiation than others. Studies have not been able to conclusively determine the differences.

The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic. These two terms are discussed more in the next few pages. Biological Effects of Radiation The ill effects due to various doses of radiations are numerous. Extensive studies have been carried throughout the world and reports concerning metabolic and physiological changes published. However, varied effects of radiation depend on several factors such as physical and chemical nature of radio-nuclides, their half life periods, the level of energy of the radiation, their metabolism within the body and excretion rates including the type of radiation and their penetrating power Direct radioactive contamination occurs through exposure to radiations by the radioactive particles in air, inhalation of gases and absorption of contaminants by respiratory tract. Indirect contamination occurs by consumption of radio-nuclides through food chain. Different doses of radioactive materials act differently as follows. Radiation Effects Below 10 Rem - Radiation risks are extrapolated all the way down to zero. Effects below 10 rem are not so evident because they are blurred by all other influences that affect man's health. Studies showed that residents of New Orleans receive only half the radiation dose than that of Denverites, but the cancer death rate in New Orleans is higher than in Denverites. There, other confounding factors like smoking, vehicular exhaust or even eating habits may pose serious effects. In fact, beside radiation, more than 300 agents are responsible for cancer in man (Table 2). The pertinent question that arises - Is there a threshold below which radiation causes no ill effect? Few scientists believe so any more. Dr. Arthur C. Upton, Chairman of New York University Medical Centre, Department of Environmental Medicine reported that “Any radioactive track can, in principle, deposit enough energy to cause gene mutation. Dr. Upton also stressed at extremely low levels - it would be a prodigious task to be prove what the biological effects it had caused”. The detection of radiation effects at extremely low level, that is, below 10 rem is "Like trying to listen one violin when the whole orchestra is playing." Dr. John Gofroan, one of the leading scientists to isolate the world's first workable quantity of plutonium and a co-discoverer of uranium (U-233) argues that scientific establishment is not listening hard enough to detect the low level radiation effects.

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The Radiation Effects Research Foundation, (KERF), funded jointly by the Japanese and United States Governments have been investigating radiation effects since 1947. Radiation standards are still evolving. Several experimental results hint that radiation exposed workers may be more at risk than the unexposed ones. Federal laws limit a nuclear plant worker to a maximum of 12 rem in a year while Dr.Behling, scientist of radiological health have reported that - "We do not allow a person to exceed a yearly dose of five rem." However, the persons working near towers of Three Mile Island (TMI) had received 710 millirem radiation in 1987, far below federal limits, but also equalizing about 50 X-ray exams. The average hourly exposure on the platform, not far from TMI is about 15 millirem which is like getting a chest X-ray test every hour. Recently, the International Commission on Radiological Protection (1CRP) assessed the maximum permissible weekly dose for a radiation worker as 0-3 rem. The maximum permissible total dose for whole life span has been fixed as 200 rem that corresponds to 0-1 rem per week or 5 rem per year for continuous exposure. In any case the total accumulated radiation shall never exceed 5 rem per year beyond age 18, nor shall the dose in any three months period exceed 3 rem. Experiments have shown that radiation dose from Sr-90 and Ra-226 should not exceed more than 3 and 10 pico-Curie (pCi) or micro-Curie per litre respectively. (pCi is equivalent to 3-7 -2 x 10

Effects of lower dose of radiation disintegration per second). However, somewhat increased levels are also permitted where radiation exposure is restricted to bones, skin and thyroid glands. Recently, The Federal Radiation Council has fixed a limitation to water supply that the gross beta concentration shall never exceed 1000 pCi per litre in the known absence of alpha emitters and strontium (Sr-90). Radiation Effects Between 25 to 50 Rad (Mild Dose of Radiation) - At very low doses below 25 rad (rad is the basic unit of X-ray absorbed by the tissue. One rad is equal to 100 FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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ergs/gm.), biological injuries in the body cells are observed. Between 25 to 50 rad, radiation causes several changes in blood cells, that is, in red blood corpuscles and lymphocytes etc. As the radiation dose increases, the harmful effects too increase. At 50 rad transient blood changes are detectable. Its delayed effects are also possible. Actually the radiation effects between 25 to 50 rad are determined by the amount or dose received, type and energy of radiation, mass of the organ and by where and how the body is irradiated. Effects at Moderate Dose i.e. 100 Rad - Radiation dose at 100 rad causes nausea and fatigue while possible vomiting occur above 125 rad. It also results in marked changes in blood and shortening of life expectancy in man. Effects at 200-300 Rad - Radiation hazards at 200 rad cause nausea and vomiting within 24 hours. After a week, epilation, loss of appetite, general weakness and other symptoms of sore throat and diarrhoea are observed in man. If the individual is exposed continuously to radiation, it may result in possible death in two to six weeks. A few hundred rads of whole body radiation delivered within a short time can kill most mammals. In man, a dose between 250 to 300 rads is capable of killing about 50% persons within a period of 30 days. It can cause several biological effects in other vertebrates also.

Effects at 400 Rad i.e. Sub-lethal Doses - Any radiation dose below the lethal dose is commonly referred to as 'sub-lethal dose. It can result in 50% mortality within 30 days and can be represented as LD 50/30. This sub-lethal dose results in various physiological disorders. However, the damage depends on the dose received and the time for which radiation is delivered i.e. a dose received and the time for which radiation is delivered i.e. a dose delivered in a short time interval will do more harm than the same dose given during a longer interval. The adverse health effects may appear at a later date within a few months or even after many years of exposure. Such effects are known as "late or delayed effects.” Radiations from sub-lethal doses produce a set of symptoms or syndromes whose time of onset and severity depend mostly on the size of dose. These symptoms are characterized by nausea and vomiting in a couple of hours, diarrhoea, anorexia, (loss of appetite), epilation, loss of weight, general weakness and lethargy constitute what is known as “radiation sickness”. Symptoms such as pallor, diarrhoea, nose bleeds and rapid emaciation become visible in about the fourth week, while death may occur from two to six weeks. About 50% death cases are reported to occur due to semi-lethal dose of radiation. Similarly a doses of 300 to 400 rem delivered to the whole body is very dangerous for the body tissues, Between 200 to 300 rem, radiation injury damages the body cells. Effects of Lethal Doses i.e, 600 Rad - Any radiation dose beyond 400 to 500 rad is extremely lethal within 30 days in 50% of the cases. This lethal dose results in severe nausea FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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and vomiting within a couple of hours followed by a short latent period. Other symptoms of diseases like diarrhoea, inflammation of mouth and throat appear within a week. Chronic fever, rapid emaciation and death is possible at the second week. However, eventual death of probably all the victims exposed for radiation occurs. Effects of Acute Doses - Acute doses of radiations are those doses which are received at one time against accumulated over a period in the whole body. Among the local effects of acute doses are skin burns and blisters, necrosis of the skin and deep seated tissues, reduced or abnormal reproduction of proliferative tissues such as epithelia of the gastro-intestinal tract and blood forming tissues. Such high radiation doses can impair the functions of nervous system, other differentiated systems and cause temporary or permanent sterility of gonads, that is, testes and ovaries in which germ cells are produced. These doses indicate the symptoms of reddened and ulcerated epidermis, loss of hair and anaemia. However, radiation damage is proportional to the type of radiation, the energy level, time and the frequency of exposure. Such radiations may cause recoverable and irrecoverable damages affecting the body tissues critically. At lower radiation doses, the person may suffer from radiation sickness but the damage may be recoverable. Many skin cells may have been killed but the unaffected skin may regenerate the damaged area and eventually the whole skin structure can be restored. Similarly, the blood forming cells and other growing cells of the body may lessen the radiation hazard But acute doses of radiation always result in irrecoverable damage to tissues in man. Cumulative Effects of High Radiation Doses

1. Whenever any body part accumulates radiation dosage beyond a certain limit, that is, about 6000 rem, that part of the body dies immediately, A dose of 10,000 rem will kill quickly, through damage to the central nervous system affecting brain and spinal cord causing delirium convulsion and death within few hours. 2. Sometimes a person may accumulate sufficient radiation for several years which may lead to his death without even showing any of the symptoms of radiation sickness. In such cases there is some lingering effect on the body or cumulative radiation damage from which the person can not recover.

3. High doses of radiation cause internal bleeding and blood vessels damage which become evident as red spots on the skin, FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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1. Exposure of the brain and central nervous system to high doses of ionizing radiation causes delirium, convulsions, and death within hours or days.

2. The lens of the eye is vulnerable to radiation. As its cells die, they become opaque, forming cataracts that impair sight.

Acute radiation sickness is marked by vomiting, bleeding of the gums, and, in severe cases, mouth ulcers.

Internal bleeding and bloodvessel damage may show vip as red spots on the skin.

5. Nausea and vomitting often begin a few hours after the gastrointestinal tract is exposed. Infection of the intestinal wall can kill weeks afterwards. 6. Unborn children are vulnerable to brain damage or mental retardation, especially if irradiation occurs during formation of the central nervous system in early pregnancy. 7. Acute damage to the ovaries and testes can affect the victim's fertility or offspring.

Fig. Effects of high dose of radiation

8. Damage to bone marrow, the body's blood factory, is especially harmful; it retards the body's ability to fight infections and hemorrh aging. 4. Cumulative radiation sickness is marked by vomiting, bleeding of the gum and mouth ulcers in man.

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5. Eye lens is vulnerable to high doses of radiation. It damages eye cells so that the eye lens becomes opaque forming cataract which impair sight. 6. Acute nausea and vomiting begin within few hours after the gastrointestinal tract is exposed. Infection of the intestinal wall can kill afterwards. 7. Cumulative doses acutely damage the reproductive organs like ovaries and testes that may badly affect the victim's fertility as well as their offsprings. 8.

Embryos get critically damaged. Unborn children are especially vulnerable to damage. So if irradiation occurs during formation of central nervous system in early pregnancy, it may result in brain damage and ultimate mental retardation.

9. High radiation doses cause damage to bone marrow - the body's blood factory. It is specially dangerous because it retards body's ability to fight against infection by harming the white blood corpuscles. 10. The short term damage may include anaemia, fatigue, blood, kidney and liver disorders, epilation, skin changes including erythema, pigment discolouration and premature aging. 11. The energy shed by radioactive rays while passing through the cell can lead to a cancer pathway, mutate the germ cells which pass on the effects to succeeding generations.

12. High doses of radiation cause blood haemorrhage and ultimate death of the organism. Evidences indicate that there can be shortening of life span in proportion to the amount of radiation received. In 1957, American studies on radiologists showed that their average life span was reduced to 60-6 years as compared to the general population of United States having average of 65-6 years life span. The long term effects of cumulative small doses of radiation are extremely lethal for man. Delayed Biological Effects of Radiation - Ionizing radiations can bring about more critical biological effects than other pollutants. Their chronic effects may continue in subsequent generations. Delayed biological effects are mainly of two types. (1) (2)

Somatic or Non-genetic Effects, and Genetic Effects. (1) Somatic or Non-Genetic Effects - Somatic effects include changes in the body cells which are not inherited with next generation. These effects are the direct results of radiation action on the body cells and tissues in man which show immediate or delayed symptoms. Somatic effects of radiation are given below.

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1. Dr. Jacob Thiessen, scientist of the 'Radiation Effects Research Foundation’ (RERF) reported that exposure to high radiation have resulted in acute toxicity which can kill the animal quickly. 2. The victim declines in vitality and dies from anaemia, blood cancer and haemorrhage. "Victims of Chernobyl reactor accidents have received very high doses of radiation. 3. According to the Atomic Bomb Casualty Commission (ABCC), who studied about 200,000 survivors, immediate or delayed somatic abnormalities including cancer, blood disorders, epilation, skin discolouration and haemorrhages were the main effects of radioactive exposure. 4. Studies carried on radioactive fall out victims of Hiroshima and Nagasaki revealed that among the somatic effects, they develop 50% cancers of thyroid glands, 30% cancers of blood and 20% cancers of other body organs. 5. Somatic abnormalities were the main effects of radio-active fall out. Exhaustive studies conducted in Hiroshima (1989) showed that heavily exposed ‘Hibakusha’ bomb affected people have a 29% greater chances of dying from cancer than those of normal people. 6. A recent Japanese report says that the death rate-15 per thousand among the nuclear bomb survivors is almost double than the unexposed persons due to somatic defects. 7. In India, people living in Kerala receive 5 to 10 times more radiation than elsewhere in the country. And the result ? Somatic effects, that is, occurrence of more cancer incidence or birth defects due to the deposition of radionuclides in body organs which deliver radiation doses. 8. Chronic somatic effects include thyroid changes, blood cancers, bone deformities, bone necrosis, cataract of the eye, bone sarcoma, epilation, epidermis damages including erythema, atrophy, alopecia and ulceration. The victim also suffers from lung diseases, that is, fibrosis, lung cancer, malignant tumors, cardio vascular disorders and ultimate reduction in life span. 9. Diagnostic X-ray exposure of pregnant women may increase the risk of cancer in unborn child in the womb. The delayed effects after either a single high exposure or chronic exposure include - aplastic anaemia due to destruction. or depressed functioning of the bone marrow. However, the effects of low penetrating radiation are less severe than chronic and penetrating ones. 10. Radiation sensitivity also varies with age. That is, foetuses and new born are much more sensitive to radiation hazard. 11. Radiation exposure destroys the body immune response in which the body becomes less resistant towards a variety of diseases. Severely affected tissues include spleen, bonemarrow, lymph nodes and intestine. FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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12. Other somatic effects include carcinoma where there is uncontrolled growth of cancerous cells in the leukocytes i.e., white blood corpuscles in the blood. 13. Experiments conducted on laboratory animals such as mice, rats and rabbits have concluded that radiation can induce cancer in a variety of organs such as lungs, mammary glands, lymph glands, ovaries and skin etc. (2) Genetic Effects of Radiation - Genetic effects of radiation include gene mutations and chromosome abnormalities which are transmitted to the next generation. These disorders are produced both by natural background and man made radiations. Many gene mutations, which are recessive, show their symptoms in subsequent generations. Some effects remain latent and appear at a later stage. Genetic effects generally imply inheritable changes in the reproductive cells. Radiation causes damage to the germ cells in two ways. (i). Lethal Mutations called Genetic Death, and (ii). Non-lethal Mutations. 1. Acute doses of radiation affect the reproductive organs so the gametes produced contain deleterious gene mutations which are carried in the unborn children. 2. Radiation causes breakage or disintegration within the gametes so that genetic mechanism of chromosomes become damaged. 3. Ionizing radiations may bring about abnormalities within growing cells which deprive them of the ability to divide and grow. The stimulus to multiply the cells is inhibited or destroyed. The cells may also continue to grow until they become giant cells and eventually die because they become inefficient due to irregular large size. Abnormality may be of any organ of the body which may result in death of the individual. 4. It has been reported that many descendents of the Hiroshima and Nagasaki (1945) victims, even after a lapse of 48 years developed abnormalities inherited from their grandparents. 5. Disorderly genetic effects may lead to the death of embryo, neonatal death or may cause birth of defective off springs. 6. Recent reports have indicated defective births of an uncommonly large number of babies in some villages of Rajasthan in the neighbourhood of Kota nuclear power-station. The atomic energy establishment of India reacted quickly and pointed out that release of radio active chemicals from the power plant is enough to cause the observed birth defects. 7. More serious are the genetic effects observed in second generation. The Atomic Bomb Casualty Commission (ABC) have reported mental retardation, slower physical growth and higher rate of leukaemia among infants exposed to radiation in the womb during

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bomb explosion. Even where the children of survivors do not show obvious genetic defects, the fear lingers that the effects may manifest in the next generation. 8. In fact,-over the years, the name Hibakusha that connotes defect, disease and disgrace in Japanese, has been tagged on to the bomb survivors. 9. Higher animals are more susceptible to genetic damages than lower animals such as flies and insects. Studies on Drosophila have shown that mutation rates go very high due to radiation exposures. The radionuclides enter the metabolic cycle and thereby incorporate into DNA molecules in animal cells causing genetic damage in them. Mutagenic Effects of Radiation - Radiation can induce ionizing and photochemical reactions in DNA molecules causing them to mutate. Mutation is a sudden and stable inheritable change in chromosomal or non-chromosomal (as chloroplast and mitochondria in plants) deoxyribo nucleic acid (DNA), H.J. Muller of Indiana University, USA, showed the development of mutants in Drosophila with the help of X-rays. His studies also established the mutagenic property of low-energy radiations. The mutagenic producing ability, that is, mutagenecity depends on the energy of electromagnetic radiation which in turn is inversely proportional to wave length and directly proportional to the frequency. Greater the frequency, more is the penetrating power and hence greater will be the mutagenecity. Mutagenic changes are caused by both ionizing and nonionizing radiations. Mutagen and Radiation Dose - Experiments have shown that induced mutation rates depend much on radiation dosage. Larger the dosage greater is the mutation rate. When the ionizing radiations strike a target they interact and produce certain changes at atomic, molecular or nuclear levels resulting in several primary and secondary events. Radiation changes the electron number of stable atoms or molecules transforming them into reactive ionic states. Radiologists have reported that water present in the cell is a major source of ions which indirectly produces mutagenic and genetic damage in presence of oxygen. For example, spermatids are generally higher in oxygen content than spermatozoa. So they show high mutational susceptibility to X-rays. Dr. Tikvah Alper of Hammer Smith Hospital, London, UK has reported that biological membrane is the critical site of primary radiation damage. Autoradiography had established that the carrier of genetic information 'DNA' is attached to the membrane during interphase and the point of attachment in the origin of DNA replication. Radiation damages DNA threads and reproductive organs which result in mutagenic changes in the succeeding generations. These changes may cause reduced fertility and increased likelihood of still-births. It also results in malformation and diseases like leukaemia in children. FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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Radiation Effects on Developing Embryos - Growing embryos of man and animals are more sensitive to radiation damage as they contain rapidly growing and dividing cells. The degree of sensitivity and damage varies with the embryonic stage (Table). Younger embryos are more susceptible to undergo radiation damage. A great variation is observed in the radiosensitivity of different species of living organisms. Experiments reveal that a dose of 200 R will kill some insect embryos in the cleavage stage, 5000 R sterilizes various species of insects, while 100,000 R may kill adult individuals of the higher resistant species. It has been observed that the less complex the organism the greater is the resistance towards radiation damage. Thus human beings, the most sensitive species get easily attacked by radiation, while microbes are more resistant to damage.

Table - RADIATION DAMAGE TO RAT EMBRYOS Radiation Dose Causing 100% Age of Embryos Mortality 100 R 200 R 400 R 600 R 750 R 900 R

6 days 8 days 10 days 11 days 6 weeks Mature adult

A radiation does of 600 R, when administered within a period of day, would be sufficient to kill a human being, a dose of 900 R can kill adult rats and even the plants like onion can be killed by a dose of 2500 R. It seems that with more DNA content and greater chromosome area, there would be greater chances of destruction to DNA. 1.5.STOCHASTIC EFFECTS Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects. Stochastic effects often show up years after exposure. As the dose to an individual increases, the probability that cancer or a genetic effect will occur also increases. However, at no time, even for high doses, is it certain that cancer or genetic damage will result. Similarly, for stochastic effects, there is no threshold dose below which it is relatively certain that an adverse effect cannot occur. In addition, because stochastic effects can occur in individuals that have not been exposed to radiation above background levels, it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure. While it cannot be determined conclusively, it often possible to estimate the probability that radiation exposure will cause a stochastic effect. As mentioned previously, it is estimated that FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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the probability of having a cancer in the US rises from 20% for non radiation workers to 21% for persons who work regularly with radiation. The probability for genetic defects is even less likely to increase for workers exposed to radiation. Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur. Radiation-induced hereditary effects have not been observed in human populations, yet they have been demonstrated in animals. If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation, hereditary effects could occur in the progeny of the individual. Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and, during certain periods in early pregnancy, may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high. More on Specific Stochastic Effects Cancer Leukemia Genetic Effects Cataracts

1.6.NONSTOCHASTIC (ACUTE) EFFECTS Unlike stochastic effects, nonstochastic effects are characterized by a threshold dose below which they do not occur. In other words, nonstochastic effects have a clear relationship between the exposure and the effect. In addition, the magnitude of the effect is directly proportional to the size of the dose. Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time. These effects will often be evident within hours or days. Examples of nonstochastic effects include erythema (skin reddening), skin and tissue burns, cataract formation, sterility, radiation sickness and death. Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (i.e. acute vs. chronic exposure). There are a number of cases of radiation burns occurring to the hands or fingers. These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter. Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1,768 R/s. Contact with the source for two seconds would expose the hand of an individual to 3,536 rems, and this does not consider any additional whole body dosage received when approaching the source.

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More on Specific Nonstochastic Effects Hemopoietic Syndrome The hemopoietic syndrome encompasses the medical conditions that affect the blood. Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy). This disease is characterized by depression or ablation of the bone marrow, and the physiological consequences of this damage. The onset of the disease is rather sudden, and is heralded by nausea and vomiting within several hours after the overexposure occurred. Malaise and fatigue are felt by the victim, but the degree of malaise does not seem to be correlated with the size of the dose. Loss of hair (epilation), which is almost always seen, appears between the second and third week after the exposure. Death may occur within one to two months after exposure. The chief effects to be noted, of course, are in the bone marrow and in the blood. Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs. In this case, however, spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow. An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow. Gastrointestinal Syndrome The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines. This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater, and is a consequence of the desquamation of the intestinal epithelium. All the signs and symptoms of hemopoietic syndrome are seen, with the addition of severe nausea, vomiting, and diarrhea which begin very soon after exposure. Death within one to two weeks after exposure is the most likely outcome. Central Nervous System A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system, as well as all the other organ systems in the body. Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days. The rapidity of the onset of unconsciousness is directly related to the dose received. In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy), the victim was ataxic and disoriented within 30 seconds. In 10 minutes, he was unconscious and in shock. Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident. Other Acute Effects Several other immediate effects of acute overexposure should be noted. Because of its physical location, the skin is subject to more radiation exposure, especially in the case of low energy x-rays and beta rays, than most other tissues. An exposure of about 300 R (77 mC/kg) of low energy (in the diagnostic range) x-rays results in erythema. Higher doses may cause changes in pigmentation, loss of hair, blistering, cell death, and ulceration. Radiation dermatitis of the FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century. The reproductive organs are particularly radiosensitive. A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men. For women, a 300 rad (3 Gy) dose to the ovaries produces temporary sterility. Higher doses increase the period of temporary sterility. In women, temporary sterility is evidenced by a cessation of menstruation for a period of one month or more, depending on the dose. Irregularities in the menstrual cycle, which suggest functional changes in the reproductive organs, may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization. The eyes too, are relatively radiosensitive. A local dose of several hundred rads can result in acute conjunctivitis. 1.7.EXPOSURE SYMPTOMS Listed below are some of the probable prompt and delayed effects of certain doses of radiation when the doses are received by an individual within a twenty-four hour period. Dosages are in Roentgen Equivalent Man (Rem) • 0-25 No injury evident. First detectable blood change at 5 rem. • 25-50 Definite blood change at 25 rem. No serious injury. • 50-100 Some injury possible. • 100-200 Injury and possible disability. • 200-400 Injury and disability likely, death possible. • 400-500 Median Lethal Dose (MLD) 50% of exposures are fatal. • 500-1,000 Up to 100% of exposures are fatal. • 1,000-over 100% likely fatal. The delayed effects of radiation may be due either to a single large overexposure or continuing low-level overexposure. Example dosages and resulting symptoms when an individual receives an exposure to the whole body within a twenty-four hour period. 100 - 200 Rem First Day First Week Second Week Third Week Fourth Week 400 - 500 Rem First Day First Week Second Week

No definite symptoms No definite symptoms No definite symptoms Loss of appetite, malaise, sore throat and diarrhea Recovery is likely in a few months unless complications develop because of poor health Nausea, vomiting and diarrhea, usually in the first few hours Symptoms may continue Epilation, loss off appetite

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Third Week Fourth Week

Hemorrhage, nosebleeds, inflammation of mouth and throat, diarrhea, emaciation Rapid emaciation and mortality rate around 50%

Exposure Limits

As discussed in the introduction, concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895. Over the years, numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups..

In general, the guidelines established for radiation exposure have had two principle objectives: 1) to prevent acute exposure; and 2) to limit chronic exposure to "acceptable" levels Current guidelines are based on the conservative assumption that there is no safe level of exposure. In other words, even the smallest exposure has some probability of causing a stochastic effect, such as cancer. This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure "as low as reasonable achievable" (ALARA). ALARA is a basic requirement of current radiation safety practices. It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible. Regulatory Limits for Occupational Exposure Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world. In the United States, annual radiation exposure limits are found in Title 10, part 20 of the Code of Federal Regulations, and in equivalent state regulations. For industrial radiographers who generally are not concerned with an intake of radioactive material, the Code sets the annual limit of exposure at the following: 1) the more limiting of: ď&#x201A;ˇ ď&#x201A;ˇ

A total effective dose equivalent of 5 rems (0.05 Sv) or The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (0.5 Sv).

2) The annual limits to the lens of the eye, to the skin, and to the extremities, which are:

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A lens dose equivalent of 15 rems (0.15 Sv) A shallow-dose equivalent of 50 rems (0.50 Sv) to the skin or to any extremity.

The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.007 cm averaged over and area of 10 cm2. The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.3 cm The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation. This value is the dose equivalentPregnant at a tissueWorkers depth of and 1 cm.Minors Declared The total effective dose equivalent is the dose equivalent to the whole-body.

pregnant women can receive no more than 0.5 rem during the entire gestation period. This is 10% of the dose limit that normally applies to radiation workers. Persons under the age of 18 years are also limited to 0.5rem/year. Non-radiation Workers and the Public The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit. Therefore, a non-radiation worker can receive a whole body dose of no more that 0.1 rem/year from industrial ionizing radiation. This exposure would be in addition to the 0.3 rem/year from natural background radiation and the 0.05 rem/year from man-made sources such as medical x-rays.

1.8.RADIATION POISONING

Radiation poisoning, also called "radiation sickness" or a "creeping dose", is a form of damage to organ tissue due to excessive exposure to ionizing radiation. The term is generally used to refer to acute problems caused by a large dosage of radiation in a short period, though this also has occurred with long term exposure. The clinical name for "radiation sickness" is [1][2][3] acute radiation syndrome (ARS) as described by the CDC. A chronic radiation syndrome does exist but is very uncommon; this has been observed among workers in early radium source production sites and in the early days of the Soviet nuclear program. A short exposure can result FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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in acute radiation syndrome; chronic radiation syndrome requires a prolonged high level of exposure. Radiation exposure can also increase the probability of contracting some other diseases, mainly cancer, tumors, and genetic damage. These are referred to as the stochastic effects of radiation, and are not included in the term radiation sickness. The use of radionuclides in science and industry is strictly regulated in most countries (in the U.S. by the Nuclear Regulatory Commission). In the event of an accidental or deliberate release of radioactive material, either evacuation or sheltering in place will be the recommended measures.

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UNIT – II DETECTORS OF NUCLEAR RADIATIONS 2.1. INTRODUCTION

Most of the nuclear reactions are accompanied by the emission of charged particles like

-

particles, protons, electrons and radiations like



- rays.

In order to understand these

particles and their interaction with atomic nuclei, precise information about their mass, momentum, energy, etc., are necessary. We shall describe in this chapter some of the common techniques employed for the detection of nuclear radiations and for analysing their energies.

Several nuclear radiation detectors depend for their operation on the ionization that is produced in them by the passage of charged particles.

This group of detectors includes

ionization chambers, proportional counters, G-M counters, semiconductor radiation detectors, cloud chambers and spark chambers. In other detectors, excitation and sometimes molecular dissociation also play important roles. These phenomena, in combination with ionization, bring about the luminescence in scintillation detectors and the latent images in photographic emulsions.

2.2. INTERACTION BETWEEN ENERGETIC PARTICLES AND MATTER

(a) Heavy Charged Particles : A heavy charged particle (like a proton,

 -particle

fission fragment) has a fairly definite range in a gas liquid, or solid. The particle loses energy primarily by the excitation and ionization of atoms in its path. The energy loss occurs in a large number of small increments. The primary particle has such a large momentum that its direction is usually not seriously changed during the slowing process. Enentually it loses all its energy and comes to rest. The distance traversed is called the range of the particle. The energy loss per unit length (- dE / dx) is called the stopping power. The rate – dE/dx at which a heavy particle of charge ze and speed v loses energy in an absorber of atomic number Z which contains N atoms per unit volume whose average ionization energy is I is given by



dE z 2 e4 NZ  dx 4 0 2 mo v 2

  2mo v 2   v2  v2   In 1   In      c 2  c 2     I 

...(1)

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mo is the electron rest mass. The range can be calculated by integrating Eq. (1) over the energies of the particle.

R



 dE    dx 

1

... (2)

(b) Electrons : Electrons interact through coulomb scattering from atomic electrons, just like heavy charged particles. There are however, a number of important differences :

1) Electrons travel at relativistic speeds.

2) Electrons will suffer large deflections in collisions with other electrons, and therefore will follow erratic paths. The range will therefore be very different from the length of the path that the electron follows.

3) Very energetic electrons (E >1 MeV) lose an appreciable fraction of their energies by producing continuous X-rays (also called Bremsstrahlung). The cross section for this process increases with increasing E.



– Rays : The interaction of

different from that of charged particles such as

 or





-rays with mater is markedly

particles.



-rays are extremely

penetrating so that they are able to pass through considerable thicknesses of matter.



-rays

show the exponential absorption in matter. If radiation of intensity I is incident upon an absorbing layer of thickness d x , the amount of radiation absorbed dI is proportional both to d x and I. Hence,

dI   I dx or I  I 0 e  x Here,



is a constant of proportionality which is a characteristic property of the medium,

known as linear absorption coefficient. The mass absorption coefficient dividing



by the density of the medium.

m   /  .

m may be obtained by

The above relation gives the intensity

(number of quanta per unit area per second) of the beam of initial intensity I o, after traversing a

x of the homogeneous material. At low energies (0.1 MeV to 25 MeV) there are three important processes through which  photons are absorbed by matter. (Fig. 2.1). thickness

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Fig. 2.1

(i). Pair Production

In this process, the photon disappears and is converted to an electron-position pair. This 2

process can take place only when the photon energy exceeds 2m 0c .

The pair production

process cannot occur in free space and usually takes place in the presence of a nuclear field. The nucleus recoils in this process conserving momentum. But the K.E. carried away by the nucleus is negligibly small due to its large mass compared with that of the electron. Photon 2

energy, if any, in excess of 2m 0c is shared as K.E. by the product particles. (ii). Photoelectric Effect

The photoelectric effect was first noted by Heinrich Hertz in 1887 during an experiment to confirm one of Maxwell’s predictions. Photoelectric effect is the emission of electrons from a clean metallic surface when electromagnetic radiation falls into that surface. There are at least four different phenomena which can cause electrons to be released from a metallic surface. Besides photoelectric effect there is also thermionic emission, field emission and secondary emission.

Thermionic emission is the process that creates the electron current in vacuum tubes. The metal object is heated to incandescence and this imparts thermal energy to the free electrons. Some of the more energetic surface electrons actually ‘boil off’ from the surface into space.

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Field emission is the attraction of electrons from the surface by a strong electric field.

Secondary emission is a problem in ordinary vacuum tubes and the mode of operation of X-ray generator vacuum tubes. This occurs when a rapidly moving electron strikes the metallic plate, imparting some of its kinetic energy to electrons in the plate.

If enough energy is

transferred to these electrons, they may jump off the surface, creating a secondary emission of electrons.

Figure 2.2 shows an experiment used to study the photoelectric effect. When light strikes the positive anode, electrons will be emitted. These electrons will be moving with a kinetic energy of :

E = Â˝ mev where, me v

-31

mass of an electron (9.11 x 10

kg)

velocity of the electrons in meters per second.

Fig. 2.2 Apparatus used to demonstrate the photoelectric effect.

The external power supply creates an electric field that opposes the electrons and creates a retarding influence on their motion. At some potential V 0 even the most energetic electrons will be retarded so that no current flows in the external circuit. The energy situation under these circumstances is :

eV0 = Â˝ meVM

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where, e

electron charge

external voltage

velocity of the most energetic electrons

One interesting aspect of the photoelectric effect is that V0 is independent of light intensity but is dependent upon the frequency (colour) of the impinging light wave. Light photons striking the metallic surface have energy E = hF presented earlier. Each photon can only give up all of its energy to a single electron (fractions are not permitted), so we can write : hF = ½ me  M + hF0 2

The extra term, hF0, is called the work function and is a property of the particular material comprising the metallic cathode surface. Frequency F0 is the critical frequency that must be exceeded for the photoemission of electrons to occur. Photoemission can only occur if hF is greater that hF0. (iii). Compton Effect

The Compton effect is a phenomenon by which a photon can impart only a part of its energy to a charged particle such as an electron. This situation is illustrated in Fig. 2.3. In part (a) of the figure, an electron is at rest and lying in the path of an incident photon with energy level hF. This photon ‘collides’ with the electron and some of its energy is imparted to the electron. This, incidentally, implies that photons carry momentum. In fact, momentum of the photon is equal to :

P  E / C  hF / c  h /  where,

energy

F c

= =

frequency of the photon speed of light

Planck’s constant



wavelength

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Fig. 2.3. The Compton effect is a means by which an incident photon can give up only part of its energy to a nearby electron. The photon will still exist after the collision because it only imparts a portion of its energy to the electron. It will, however, exist at a lower frequency (longer wavelength) because of the lost energy. The energy lost to the electron will become kinetic energy, setting the electron in motion.

A particle at rest has a potential energy of U = mc , so by conservation of energy : hF + U = hF ' + U '

where U ' is the potential energy of the re-coiling electron and the other terms have been previously defined.

The loss of energy means that photon hF ' has a longer wavelength,

 '.

The difference

in wavelength,  , is given by :

  1/ F  1/ F '  (h / Mc) (1  cos ( ))

where



is the angle of deflection shown in Fig. 2.3.

The kinetic energy of the moving electron is :

   E  hF       

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2.3. SOLID â&#x20AC;&#x201C; STATE DETECTORS

A p-n junction, used as a particle detector, is shown in Fig. 2.4. p-n

junction

n-type

between

silicon

layer

p-type

n-type

silicon.

Contact

It consists of a made

with

the

thin

evaporated film of gold. In order to minimise the current flowing in the detector, when no radiation is striking it, a reverse biased diode is always used.

The positive (reverse) bias

applied to the gold film

will

push

all the positive

Fig. 2.4.

charge carriers away from the junction and produce a depletion layer, indicated in the figure. The depletion layer contains almost no carriers of either sign. When an energetic charged particle travels through the depletion layer, its interaction with the electrons in the crystal, produces electron-hole pairs. There is an electron-hole pair for every 3.5 eV (in Si) of energy lost by the charged particle. The electrons and holes are swept away by the applied electric field and registered as a voltage pulse over the resistor R. The number of charge carrier pairs produced in a semiconductor material is approximately 10 times as large as the number of ion pairs produced in a gas ion chamber i.e., the energy extended per pair is about 3.5 eV in silicon, compared to about 30 eV for gases. The voltage pulse will therefore be about 10 times larger. Hence this detector has much better energy resolution than other radiation detectors. In solid state detectors for charged particles, silicon has been used most because of its low intrinsic conductivity. This means that the detector can be operated at room temperature without excessive leakage current. For gamma ray work, germanium detectors are much better than silicon because of the larger density of germanium.

2.4. GEIGER â&#x20AC;&#x201C; MULLER COUNTER It consists of a metal chamber C containing air or some other gas at a pressure of about 10cm of Hg. A fine tungsten wire (W) is stretched along the axis of the tube and is insulated from it by ebonite plugs EE (Fig. 2.5). The wire is connected to the positive terminal of a high tension FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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battery (about 1000 to 3000 volts) through a high resistance R (about 100 megohms) and the negative terminal is connected to the chamber C. The D.C. voltage is kept slightly less than that which will cause a discharge between the electrodes.

When an ionizing particle (say an

ď Ą particle)

enters the counter, ionisation takes place

and a few ions are produced. If the applied P.D. is strong enough, these ions are

Fig. 2.5.

multiplied by further collisions. An avalanche of electrons moves towards the central wire and this is equivalent to a small current impulse which flow through the resistance R. The critical potential is lowered momentarily, causing a sudden discharge through the resistance R. The p.d. thus developed across R is amplified by vacuum tube circuits and is made to operate a mechanical counter.

In this way single particles can be registered.

The sudden pulse of

discharge sweeps away the ions from the chamber and the counter is ready to register the arrival of the next particle.

The voltage characteristics of a Geiger â&#x20AC;&#x201C; Muller counter are shown in Fig. 2.6. This is a plot of the counting rate against the counter potential with a

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radioactive source placed near the counter. It is seen that there is a threshold below which the tube does not work. This can be several hundred volts.

As the applied

potential is increased, the counting begins and rises rapidly to a flat portion of the curve called the plateau. This is the region of the counter operation where the counter Fig. 2.6.

operation where the counting rate is, more or less, independent of small changes in p.d. across the tube. Beyond the plateau the applied electric field is so high that a continuous discharge takes place in the tube as shown in Fig. 2.6 and the count rate increases very rapidly. It does not require any ionizing event for this to happen so that the tube must not be used in this region.

The efficiency of the counter is defined as the ratio of the observed counts/sec. to the number of ionizing particles entering the counter per second. Counting efficiency is defined as the ability of its counting, if at least one ion pair is produced in it.

Counting efficiency =

  1  e slp

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where s = specific ionization at one atmosphere;

atmospheres and

pressure

l = path length of

the ionization particle in the counter. The efficiency

 of a GM counter, as

a function of pressure for air and hydrogen, is illustrated in Fig. 2.7.

The counter set-up is portable (with the transistorised electronics) and

serves

best

for

mineral

prospecting, apart from its several

Fig. 2.7

other applications in cosmic ray work. A virtue of the Geiger counter is that the pulse height is constant over a range of applied voltages, as in Fig. 2.6. So the power supply does not have to be precisely regulated as it does for a proportional counter. Also, the pulses are several volts in height, which makes amplifiers unnecessary.

Disadvantages of the Geiger counter are : (i) it is insensitive for a period of 200 to 400

s

following each pulse, which prevents its use at very high counting rates. (ii) it cannot provide

information about the particle or photon causing a pulse.

2.5. NUCLEAR EMULSIONS

It has been known for a long time that charged particles affect a photographic plate. A heavy charged particle traversing a photographic emulsion produces a latent image of its track. The track is revealed when the plate is developed. Ordinary optical photographic emulsions are not suitable for quantitative studies with nuclear radiations. The sensitivity of such emulsions is low. Further, the tracks due to charged particles have non-clear range because the developed crystal grains are large and widely spaced. The composition of the emulsion was changed so as to make it more suitable for the study of various ionising particles, such as

 - particles, protons,

mesons and even electrons. The nuclear emulsions differ from the optical emulsions in that they have considerably higher silver halide content and smaller grain size. In nuclear emulsion, the thickness is greater than that of optical emulsion (Table 2.1). The smaller the grain size, the

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RADIATION AND PROPAGATION more sensitive is the emulsion to ionising particles.

33 This different commercially available

emulsions, differing cheifly in grain size, can be used to discriminate between different particles.

Table 2.1. Nuclear and Optical emulsions

Property

Optical Emulsion

Nuclear Emulsion

AgBr : Gelatin (mass)

47 : 53

80.20

AgBr : Gelatin (volume)

15 : 85

45 : 55

Grain size (micron)

1 – 3.5

0.1 – 0.6

Thickness (,,)

2–3

25 – 2000

Sensitivity to light

Very high

Poor

Dense blackening

Individual tracks

Response to

 particle



Moderate

Faint fog



Faint

Almost none

Advantages.

(1)

The emulsion is relatively light and cheap. Because of their lightness, they can be sent in balloons, spaceships etc., for high altitude cosmic ray experiments. The cosmic ray events once recorded can be studied by developing the exposed plates conveniently in the laboratory. Emulsions were widely employed in cosmic ray studies and led to the discovery of the

(2)



and K mesons.

The high density of the emulsion gives it a stopping power about a thousand times that of standard air. Unstable high energy particles are brought to rest in the emulsions and their decay schemes can thus be studied.

(3)

The emulsion is continuously sensitive and is consequently always available to record an event.

But the cloud chamber is sensitive only for a fraction of a second after an

expansion and remains ineffective for several seconds between successive expansions. Limitations.

(1)

The main drawback of nuclear emulsions is that their sensitivity and thickness are affected by temperature, humidity, age of the emulsions before development and the

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conditions under which they are developed. The scanning of the plates and the analysis of the tracks obtained are also laborious, when done manually.

(2)

As compared with cloud chamber, photographic emulsions have the following drawback. It is very difficult to determine the sign of the charge and the momentum of a particle from observations of the curvature of the path in a strong magnetic field. The actual pathlength in the emulsion is small compared to that in a cloud chamber.

2.6. THE SCINTILLATION COUNTERS

One of the earliest methods of radiation detection was the spinthariscope (Fig. 2.8). It consists of a small wire, the tip of which is dipped in Radium bromide (R) or any other radioactive salt. It is placed in front of a zinc sulphide screen S and viewed

through

 or  -particle

microscope.

falls on the

screen, they produce light

When

zinc sulpide flashes

which

can be seen by a

Fig. 2.8

microscope (M) in a dark room. The visible

luminescence excited in zinc sulphide by

particles was used by Rutherford for counting the particles.

-

The process of counting these

scintillations through a low power microscope is a tedious one and the limitations of observation with the eye restrict the counting rate to about 100 per minute. This process, whereby the energy of the particle is converted to light, is the basis of scintillation counter.

The

main

parts

scintillation

counter

are

shown

Fig. 2.9.

atoms of the phosphor are excited or ionised by the energy loss of an impinging

Fig. 2.9

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RADIATION AND PROPAGATION

 ,  or 

ray. When the atoms return to their ground states, photons are emitted, in the blue

and ultraviolet regions of the optical spectrum. The phosphor is optically coupled to the envelope of a photomultiplier tube. The photons strike the photocathode, causing the ejection of photoelectrons (Fig. 2.10). As these photo-electrons leave the photocathode, they are directed by a focusing electrode to the first multiplier electrode or dynode. This property of emitting three, four

electrode has

the

or five electrons for

Fig. 2.10

every single electron which strikes its surface. There may be from 10 to 14 such multiplier stages in a given tube. Hence, from the emission of one single electron from the cathode, a burst of one million electrons may impinge on the final stage in the tube (the anode). The output pulse from the photomultiplier is fed to a pulse amplifier followed by a scaler circuit.

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UNIT-III 3.1.CONTROLLING RADIATION EXPOSURE When working with radiation, there is a concern for two types of exposure: acute and chronic. An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time. An acute exposure has the potential for producing both nonstochastic and stochastic effects. Chronic exposure, which is also sometimes called "continuous exposure," is long-term, low level overexposure. Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures. The three basic ways of controlling exposure to harmful radiation are: 1) limiting the time spent near a source of radiation, 2) increasing the distance away from the source, 3) and using shielding to stop or reduce the level of radiation.

3.1.1.Time

The radiation dose is directly proportional to the time spent in the radiation. Therefore, a person should not stay near a source of radiation any longer than necessary. If a survey meter reads 4 mR/h at a particular location, a total dose of 4mr will be received if a person remains at that location for one hour. In a two hour span of time, a dose of 8 mR would be received. The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area. Dose

Dose

Rate

Time

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When using a gamma camera, it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source. Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators. This is illustrated in the images at the bottom of this page.

Dose = Dose Rate x Time Example Calculations 1 A technician is in an area for 10 minutes and the reading on the survey meter is 5mR/h. What dose of radiation does the technician receive? 5mR/h / 60 min./h = 0.0833 mR/min. 0.0833 mR/min. x 10 minutes = 0.833 mR total dose. Example Calculations 2 A technician wants to receive no more than a 1.0 mR dose knowing the above conditions. What is the maximum time the technician can stay in the area? 1.0 mR / 0.0833 mR/min. = 12 minutes The calculated dosages would be approximations. The actual dosages may vary due to scattering and other considerations. The TLD or Film Badge should be used to determine dosage received by an individual. 3.1.2.Distance

Increasing distance from the source of radiation will reduce the amount of radiation received. As radiation travels from the source, it spreads out becoming less intense. This is analogous to standing near a fire. The closer a person stands to the fire, the more intense the heat feels from the fire. This phenomenon can be expressed by an equation known as the inverse square law, which states that as the radiation travels out from the source, the dosage decreases inversely with the square of the distance. Inverse

Square

Law:

I1/

D2 /

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Calculating Intensity with the Inverse Square Law 2

I1/ I2 = D2 / D1

Where: I1 =

Intensity 1 at D1

I2 =

Intensity 2 at D2

D1 = D2 =

Distance source Distance source

from

Example Calculation 1 The intensity of radiation is 530 R/h at 5 feet away from a source. What is the intensity of the radiation at 10 feet? Rework the equation 2 2 I2 = I1 x D1 / D2 Plug

solve

in 2 2 I2 = 530R/h x (5ft) / (10ft)

Solve

the

for

the

intensity

known

distance

values

I2 = 132.5 R/h In this instance the distance has been doubled and the intensity at that point has decreased by a factor of four. Example Calculation 2 A source is producing an intensity of 456 R/h at one foot from the source. What would be the distance in feet to the 100, 5, and 2 mR/h boundaries. Convert R/hour to mR/hour 456R/h x 1000 = 456,000 mR/h Rework the equation to solve for D2

Plug in the known values and solve

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D2= 67.5 feet Using this equation the 100mR/h boundary would be at 68 feet, the 5mR/h boundary would be at 301.99 feet, and the 2mR/h boundary would be at 477.5 feet. Sources are seldom operated for an entire hour, and collimators are often used which reduce these distances considerably. 3.1.3.Shielding The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation. In general, the denser the material the more shielding it will provide. The most effective shielding is provided by depleted uranium metal. It is used primarily in gamma ray cameras like the one shown below. The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera. Depleted uranium and other heavy metals, like tungsten, are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms. Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials. Concrete is commonly used in the construction of radiation vaults. Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside.

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3.2.SHIELDING DESIGN Shielding reduces the intensity of radiation exponentially depending on the thickness. A standard measure of the effectiveness of a shielding material is the halving thickness or half value layer,

, the thickness that reduces gamma or x-ray radiation by half. If

of radiation on the source side and gets through,

is the intensity

is the thickness of the shield, the intensity of radiation that

, is:

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This means when added thicknesses are used, the shielding multiplies. For example, a practical shield in a fallout shelter is ten halving-thicknesses of packed dirt, which is 90 cm (3 ft) of dirt. This reduces gamma rays by a factor of 1/1,024, which is 1/2 multiplied by itself ten times. Halving thicknesses of some materials, that reduce gamma ray intensity by 50% (1/2) include (see Kearney, ref): 

9 cm (3.6 inches) of packed soil



6 cm (2.4 inches) of concrete



1 cm (0.4 inches) of lead



0.2 cm (0.08 inches) of depleted uranium



150 m (500 ft) of air

The effectiveness of a shielding material in general increases with its density. 3.2.1.Half-Value Layer (Shielding) As was discussed in the radiation theory section, the depth of penetration for a given photon energy is dependent upon the material density (atomic structure). The more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur and the radiation will lose its energy. Therefore, the denser a material is the smaller the depth of radiation penetration will be. Materials such as depleted uranium, tungsten and lead have high Z numbers, and are therefore very effective in shielding radiation. Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults. Since different materials attenuate radiation to different degrees, a convenient method of comparing the shielding performance of materials was needed. The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level. At some point in the material, there is a level at which the radiation intensity becomes one half that at the surface of the material. This depth is known as the half-value layer for that material. Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to onehalf its unshielded value. Sometimes shielding is specified as some number of HVL. For example, if a Gamma source is producing 369 R/h at one foot and a four HVL shield is placed around it, the intensity would be reduced to 23.0 R/h. Each material has its own specific HVL thickness. Not only is the HVL material dependent, but it is also radiation energy dependent. This means that for a given material, if the radiation energy changes, the point at which the intensity decreases to half its original value will also change. Below are some HVL values for various materials commonly used in industrial radiography. As can be seen from reviewing the values, as the energy of the radiation increases the HVL value also increases.

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Approximate HVL for Various Materials when Radiation is from a Gamma Source Half-Value Layer, mm (inch) Source

Concrete

Steel

Lead

Tungsten

Uranium

Iridium-192

44.5 (1.75)

12.7 (0.5)

4.8 (0.19)

3.3 (0.13)

2.8 (0.11)

Cobalt-60

60.5 (2.38)

21.6 (0.85)

12.5 (0.49)

7.9 (0.31)

6.9 (0.27)

Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source Half-Value Layer, mm (inch) Peak Voltage (kVp)

Lead

Concrete

0.06 (0.002)

4.32 (0.170)

100

0.27 (0.010)

15.10 (0.595)

150

0.30 (0.012)

22.32 (0.879)

200

0.52 (0.021)

25.0 (0.984)

250

0.88 (0.035)

28.0 (1.102)

300

1.47 (0.055)

31.21 (1.229)

400

2.5 (0.098)

33.0 (1.299)

1000

7.9 (0.311)

44.45 (1.75)

Note: The values presented on this page are intended for educational purposes. Other sources of information should be consulted when designing shielding for radiation sources. 3.3.SAFETY CONTROLS Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent, a variety of safety controls are used to limit exposure. The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls. Engineered controls include shielding, interlocks, alarms, warning signals, and material containment. Administrative controls include postings, procedures, dosimetry, and training.

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3.3.1.Engineered control

Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a "radiation vault." Fixed shielding materials are commonly high density concrete and/or lead. Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced. Warning lights are used to alert workers and the public that radiation is being used. Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present. Safety controls should never be tampered with or bypassed.

When portable radiography is performed, it is most often not practical to place alarms or warning lights in the exposure area. Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation. Occasionally, radiographers will use battery operated flashing lights to alert the public to the presence of radiation. Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection. Sheets of steel, steel beams, or other equipment may be used for temporary shielding. It is the responsibility of the radiographer to know and understand the absorption value of various materials. More information on absorption values and material properties can be found in the radiography section of this site.

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3.3.2.AdministrativeControls As mentioned above, administrative controls supplement the engineered controls. These controls include postings, procedures, dosimetry, and training. It is commonly required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation. Normal operating procedures and emergency procedures must also be prepared and followed. In the US, federal law requires that any individual who is likely to receive more than 10% of any annual occupational dose limit be monitored for radiation exposure. This monitoring is accomplished with the use of dosimeters, which are discussed in the radiation safety equipment section of this material. Proper training with accompanying documentation is also a very important administrative control. 3.4.RESPONSIBILITIES Working safely with radiation is the responsibility of everyone involved in the use and management of radiation producing equipment and materials. Depending on the size of the organization, specific responsibilities may be assigned to various individuals and/or committees.

Radiation safty officer All organizations that are licensed to use ionizing radiation must have a Radiation Safety Officer. The RSO is the individual authorized by the company to serve as point of contact for all activities conducted under the scope of the authorization. The RSO ensures that radiation safety activities are being performed in accordance with approved procedures and regulatory requirements. Some of the common responsible for the RSO include: 

Ensuring that all individuals using radiation equipment are appropriately trained and supervised.



Ensuring that all individuals using the equipment have been formally authorized to use the equipment.



Ensuring that all rules, regulations, and procedures for the safe use of radioactive sources and X-ray systems are observed.

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

Ensuring that proper operating, emergency, and ALARA procedures have been developed and are available to all system users.



Ensuring that accurate records of the use of the sources and equipment are maintained.



Ensuring that required radiation surveys and leak tests are performed and documented.



Ensuring that systems and equipment are protected from unauthorized access or removal.

The minimum qualifications, training, and experience for RSOs for industrial radiography are as follows: (1) Completion of the training and testing requirements of Sec. 34.43(a) of Part 10 of the Federal Code of Regulations, (2) 2000 hours of hands-on experience as a qualified radiographer in industrial radiographic operations, and (3) Formal training in the establishment and maintenance of a radiation protection program. Radiation Safety Committee Some organizations may have a Radiation Safety Committee (RSC) to assist the RSO. The RSC often provides oversight of the policies, procedures and responsibilities of an organizations radiation safety program. System Users The individuals authorized to use the X-ray producing system or gamma sources are responsible for ensuring that: 

All rules, regulations, and procedures for the safe use of the X-ray system are followed.



An accurate record of the use of the system is maintained.



All safety problems with the system are reported to the RSO and corrected before further use.



The system is protected from unauthorized access or removal.

3.5.PROCEDURES Written operating procedures must be developed and made available to anyone that will be working with radiation sources or X-ray producing equipment. These procedures must be specific to the equipment and its use in a particular application. Simply making the equipment manufacturers operating instructions available to workers does not satisfy this requirement. The operating procedure must be followed at all times unless written permission to deviate is received from the Radiation Safety Officer.

Standard operation procedures As a minimum, operating procedures must include instructions for the following:

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

Appropriate handling and use of licensed sealed sources and radiographic exposure devices so that no person is likely to be exposed to radiation doses in excess of the established exposure limits.



Methods and occasions for conducting radiation surveys.



Methods for controlling access to radiographic areas.



Methods and occasions for locking and securing radiographic exposure devices, transport and storage containers and sealed sources.



Personnel monitoring and the use of personnel monitoring equipment.



Transporting sealed sources to field locations, including packing of radiographic exposure devices and storage containers in the vehicles, placarding of vehicles when needed, and control of the sealed sources during transportation.



The inspection, maintenance, and operability checks of radiographic exposure devices, survey instruments, transport containers, and storage containers.



The procedure(s) for identifying and reporting defects and noncompliance.



Maintenance of records.

Emergency procedures Procedures must also be developed that guide workers in the event of an emergency. A few of the items that could be covered include: 

Steps that must be taken immediately by radiography personnel in the event a pocket dosimeter is found to be off-scale or an alarm ratemeter alarms unexpectedly.



Steps for minimizing exposure of persons in the event of an accident.



The procedure for notifying proper persons in the event of an accident.



Radioactive source recovery procedure if licensee will perform the recovery.

3.6.RADIATION SAFETY EQUIPMENT

Instruments used for radiation measurement fall into two broad categories: - rate measuring instruments and - personal dose measuring instruments.Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity). Survey meters, audible alarms and area monitors fall into this category. These instruments present a radiation intensity reading relative to time, such as R/hr or mR/hr.

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Dose measuring instruments are those that measure the total amount of exposure received during a measuring period. The dose measuring instruments, or dosimeters, that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual. An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units. The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages. 3.6.1.Area Monitoring Instruments Radiation monitoring instruments are used both for area monitoring and for individual monitoring. The instruments used for measuring radiation levels are referred to as area survey meters (or area monitors) and the instruments used for recording the equivalent doses received by individuals working with radiation are referred to as personal dosimeters (or individual dosimeters). All instruments must be calibrated in terms of the appropriate quantities used in radiation protection. The assessment of radiation levels at different locations in the vicinity of radiation installations is generally known as area monitoring. The essential purpose of this assessment is to minimize personnel exposure. Area monitoring instruments,generallyConsists of a probe and associated electrode circuitry. The ionization Chamber and G.M. counters are used as area monitoring instruments which are already discussed in radiation detectors. Ionisation chamber being operatable at relatively low voltages and also being robust, the instrument would be versatile as a portable monitor. As the ionization current depends on the size of the chamber a wide range of radiation levels can be measured by suitable choice of the volume of the chamber. The d.c. amplifier used with ionization chamber is sensitive to dust and humidity which may result in the failure of the instrument, if proper care is not taken in the fabrication and maintenance of the instrument .This mainly used to monitor higher radiation levels. G.M. counter compared to ionization chamber is more sensitive due to ion avalanche formation. Hence, the use of G.M. counter technique is attractive at low radiation levels and the size of the counter would be relatively smaller. These instruments, due to their small size and handling conveniences are extensively used in radiation monitoring. In addition they are rugged and are insensitive to environmental conditions. In addition to normal monitoring facilities, it is necessary to install gamma radiation monitors with alarm facility, at suitable places, especially where there is a likehood of changes in the radiation level, due to the nature of work being carried out in that area.

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3.6.1.1.Survey Meters

The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation. A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter. There are many different models of survey meters available to measure radiation in the field. They all basically consist of a detector and a readout display. Analog and digital displays are available. Most of the survey meters used for industrial radiography use a gas filled detector. Gas filled detectors consists of consists of a gas filled cylinder with two electrodes. Sometimes, the cylinder itself acts as one electrode, and a needle or thin taut wire along the axis of the cylinder acts as the other electrode. A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge). The gas becomes ionized whenever the counter is brought near radioactive substances. The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode. This results in an electrical signal that is amplified, correlated to exposure and displayed as a value.

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Depending on the voltage applied between the anode and the cathode, the detector may be considered an ion chamber, a proportional counter, or a Geiger-M端ller (GM) detector. Each of these types of detectors have their advantages and disadvantages 3.6.2.Personal Monitoring 3.6.2.1.Pocket Dosimeter Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays. As the name implies, they are commonly worn in the pocket. The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter. Direct Read Pocket Dosimeter A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen. The dosimeter contains a small ionization chamber with a volume of approximately two milliliters. Inside the ionization chamber is a central wire anode, and attached to this wire anode is a metal coated quartz fiber. When the anode is charged to a positive potential, the charge is distributed between the wire anode and quartz fiber. Electrostatic repulsion deflects the quartz fiber, and the greater the charge, the greater the deflection of the quartz fiber. Radiation incident on the chamber produces ionization inside the active volume of the chamber. The electrons produced by ionization are attracted to, and collected by, the positively charged central anode. This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position. The amount of movement is directly proportional to the amount of ionization which occurs.

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By pointing the instrument at a light source, the position of the fiber may be observed through a system of built-in lenses. The fiber is viewed on a translucent scale which is graduated in units of exposure. Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts. During the shift, the dosimeter reading should be checked frequently. The measured exposure should be recorded at the end of each shift. The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure. It also has the advantage of being reusable. The limited range, inability to provide a permanent record, and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter. The dosimeters must be recharged and recorded at the start of each working shift. Charge leakage, or drift, can also affect the reading of a dosimeter. Leakage should be no greater than 2 percent of full scale in a 24 hour period.

Digital Electronic Dosimeter Another type of pocket dosimeter is the Digital Electronic Dosimeter. These dosimeters record dose information and dose rate. These dosimeters most often use Geiger-M端ller counters. The output of the radiation detector is collected and, when a predetermined exposure has been reached, the collected charge is discharged to trigger an electronic counter. The counter then displays the accumulated exposure and dose rate in digital form. Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of

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exposure. Some models can also be set to provide a continuous audible signal when a preset exposure has been reached. This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary.

3.6.2.2.Film Badges

Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. The detector is, as the name implies, a piece of radiation sensitive film. The film is packaged in a light proof, vapor proof envelope preventing light, moisture or chemical vapors from affecting the film. A special film is used which is coated with two different emulsions. One side is coated with a large grain, fast emulsion that is sensitive to low levels of exposure. The other side of the film is coated with a fine grain, slow emulsion that is less sensitive to exposure. If the radiation exposure

causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted, the fast emulsion is The film is contained inside a film holder or removed and the dose is computed badge. The badge incorporates a series of filters to using the slow emulsi

determine the quality of the radiation. Radiation of a given energy is attenuated to a different extent by various types of absorbers. Therefore, the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter. By comparing these results, the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy. The badge holder also contains an open window to determine radiation exposure due to beta particles. Beta particles are effectively shielded by a thin amount of material.

The major advantages of a film badge

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that it provides a permanent record, it is able to distinguish between different energies of photons, and it can measure doses due to different types of radiation. It is quite accurate for exposures greater than 100 millirem. The major disadvantages are that it must be developed and read by a processor (which is time consuming), prolonged heat exposure can affect the film, and exposures of less than 20 millirem of gamma radiation cannot be accurately measured. Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives. Whole body badges are worn on the body between the neck and the waist, often on the belt or a shirt pocket. The clip-on badge is worn most often when performing X-ray or gamma radiography. The film badge may also be worn when working around a low curie source. Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation. A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body.

3.7.RADIATION STERILIZATION Methods of sterilization exist using radiation such as electron beams, X-rays, gamma rays, or [26] subatomic particles. 

Gamma rays are very penetrating and are commonly used for sterilization of disposable medical equipment, such as syringes, needles, cannulas and IV sets. Gamma radiation requires bulky shielding for the safety of the operators; they also require storage of a radioisotope (usually Cobalt-60), which continuously emits gamma rays (it cannot be turned off, and therefore always presents a hazard in the area of the facility).



Electron beam processing is also commonly used for medical device sterilization. Electron beams use an on-off technology and provide a much higher dosing rate than gamma or x-rays. Due to the higher dose rate, less exposure time is needed and thereby any potential degradation to polymers is reduced. A limitation is that electron beams are less penetrating than either gamma or x-rays.



X-rays, if low energy, are less penetrating than gamma rays and tend to require longer exposure times, but require less shielding. They are generated by an X-ray machine that can be turned off for servicing and when not in use.



Ultraviolet light irradiation (UV, from a germicidal lamp) is useful only for sterilization of surfaces and some transparent objects. Many objects that are transparent to visible light absorb UV. UV irradiation is routinely used to sterilize the interiors of biological safety cabinets between uses, but is ineffective in shaded areas, including areas under dirt (which may become polymerized after prolonged irradiation, so that it is very difficult to remove). It also damages many plastics, such as polystyrene foam. Further information: Ultraviolet Germicidal Irradiation



Subatomic particles may be more or less penetrating, and may be generated by a radioisotope or a device, depending upon the type of particle.

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Irradiation with X-rays or gamma rays does not make materials radioactive. Irradiation with particles may make materials radioactive, depending upon the type of particles and their energy, and the type of target material: neutrons and very high-energy particles can make materials radioactive, but have good penetration, whereas lower energy particles (other than neutrons) cannot make materials radioactive, but have poorer penetration. Irradiation is used by the United States Postal Service to sterilize mail in the Washington, DC area. Some foods (e.g. spices, ground meats) are irradiated for sterilization (see food irradiation). Radioactive tracer A radioactive tracer, also called a radioactive label, is a substance containing a radioisotope(which is an isotope that has an unstable nucleus and that stabalizes itself by spontaneously emitting energy and particles). Tracers can be used to measure the speed of chemical processes and to track the movement of a substance through a natural system such as [1] a cell or a tissue. Radioactive tracing was developed by George de Hevesy. In medicine tracers are applied in autoradiography and nuclear medicine, including single photon emission computed tomography (SPECT), positron emission tomography (PET) and scintigraphy.

Material Evaluation Prior to using Gamma or Electron Beam Radiation for the sterilization of healthcare products, it is essential to determine the effect radiation will have on the materials used in the product, its components and packaging. Because each polymer reacts differently to ionizing radiation, it is important to verify that the maximum dose likely to be administered during the sterilization process will not adversely affect the quality,safety or performance of the product throughout its shelf life.The device manufacturer is ultimately responsible for ensuring that the final product meets its labeling claims Experimental samples of the product should be irradiated to at least the highest dose to be encountered during routine processing.For example, a product that is to receive a sterilizing dosage of 25 to 40 kiloGray (kGy) should be tested by irradiating samples to at least 40 kGy. A preferred, more conservative approach is to irradiate samples at twice the anticipated maximum dose. Since various device applications call for certain performance properties or functional characteristics, it is important to test each device in an appropriate manner. Physical and functional methodsfor evaluating plastic materials can be found in the ANSI/AAMI/ISO 111371994 document. It is important to note that standards call for the development of a test program that will address â&#x20AC;&#x153;variations in the manufacturing processes, tolerances, radiation doses, radiation 2 source, raw materials and storage conditions.â&#x20AC;? Tests that more closely approximate the actual mechanical application of the product may also be performed by the product engineering or research staff.Evaluation and test results FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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are to be maintained in the product’s device history file, serving as physical confirmation that all productclaims and specifications have been met. If product testing indicates a potentially adverse effect from high levels of radiation, a maximum permissible dose should be established by the manufacturer and emphasized in the processing instructions provided to the contract sterilizer. Sterilization Dose Selection The process of selecting a sterilization dose is intended to establish the minimum permissible dose necessary to provide the required or desired sterility assurance level (SAL),meaning the “probability of a viable microorganism being present on a product unit after sterilization.”2 This requirement is dependent upon the intended use of the product. For example, a product, which is to be used in the body’s fluid path, is considered a Class III device. Under this classification, the product must receive a sterilization dose high enough to ensure that the probability of an organism surviving the dosage is no greater than one in one million units tested (10-6).

The chances of one organism surviving after irradiation decreases logarithmically with increasing dosages. However, it is important to consider microbial population characteristics that define a product’s pre-sterilization bioburden (“population of viable microorganisms on a product”2). Relevant characteristics include the magnitude of the population and the resistance of the population to radiation. Product Dose Mapping A dose mapping study is to be performed in order to identify minimum and maximum dose zones within the product load using a predetermined loading pattern. This verifies the minimum sterilization dose is achieved while material integrity is maintained by staying within the maximum allowable dosage. In addition, the dose mapping study establishes the reproducibility of the sterilization process and is used in the selection of the dose monitoring locations for routine processing . Certification Information that is gathered or produced during the validation process is to be documented and reviewed for acceptability by a designated individual or group and maintained in the product’s device history file. Sterilization Dose Audit In accordance with ANSI/AAMI/ISO 11137-1994, an audit must be performed to determine the continued validity of the sterilization dose any time there is a change in the manufacturing process that could significantly affect level or nature of the bioburden. In the absence of any change, a sterilization dose audit is to be performed every three months. FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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Summary In order to conduct and maintain a successful validation process, it is important that themanufacturer, contract sterilizer and testing laboratory work cooperatively. Detailed results of each phase of the validation process are to be kept in the manufacturer’s device history file.This standardization procedure will optimize the sterilization process, maintain material integrity and allow similar results to be produced in the future. References 1. AAMI Recommended Practice–“Process Control Guidelines for Gamma Radiation Sterilization of Medical Devices,” ISBN No. 0-910275-38-6, pp. 7-21, 1984. 2American National Standard, ANSI/AAMI/ISO 11137-1994, Sterilization of health care products– Requirements for validation and routine control–Radiation sterilization, 1994 3American National Standard, ANSI/AAMI ST32-1991, Guideline for Gamma Radiation Sterilization, 1991. 4American National Standard, ANSI/AAMI ST31-1990, Guideline for Electron Beam Radiation Sterilization of Medical Devices, 1990. 5 Genova, Hollis, Crowell and Schady, “A Procedure for Validating the Sterility of an Individual Gamma Radiation Sterilized Production Batch,” Journal of Parenteral Science and Technology, Volume 41, No.1, pp. 33-36, Jan 1987. 6 Gaughran and Morrissey, “Sterilization of Medical Products,” Volume 2, ISBN-0-919868-14-2, pp. 35-39, 1980. 2015 Spring Road, Suite 650 _ Oak Brook, IL 60523 _ 800.472.4508 3.8.RADIATION FOR FOOD PRESERVATION FOOD irradiation, one of the beneficial applications of atomic energy, is an important innovation in food preservation since the development of canning in the 19th century. It provides an effective alternative to fumigants, which are being phased out owing to their adverse effects on the environment and human health. Moreover, exposure of food material to radiation has strong advantages over conventional methods of preservation such as cold storage, fumigation, salting and drying because it does not lead to loss of flavour, odour, texture or quality.

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Swapnesh Kumar Malhotra, Head of the Public Awareness Division, Department of Atomic Energy (DAE), says: "Radiation technology can complement existing technologies to ensure food security and safety." He said radiation processing could be used for anti-infestation of foodgrains and pulses; inhibition of sprouting in onions, potatoes, garlic, yam and ginger; preventing microbial contamination of spices; extending shelf-life under recommended conditions of storage; and overcoming quarantine barriers in international trade. The technology can be used for sterilising cut-flowers, pet food, cattlefeed, aquafeed, ayurvedic herbs and medicines, and packaging material. India is one of the few countries that have the expertise in the deployment of radiation technology. The process involves the controlled application of energy of ionising radiation such as gamma rays, X-rays and accelerated electrons. Irradiation is a direct, simple and efficient onetime process. Application of low doses of radiation (0.15 kilo Gray) can arrest the sprouting of potatoes and onions. (Gray is the unit of absorbed radiation energy.) As a result, storage losses of tubers and bulbs due to sprouting, and their dehydration can be reduced substantially. Low-dose applications (less than one kGy) also lead to the disinfestation of insects in stored grain, pulses and food products, and the destruction of parasites in meat and meat products. A medium dose (one to ten kGy) eliminates microbes in fresh fruits, meat and poultry products, destroys food pathogens in meat, and helps in the hygienisation of spices and herbs. A high dose

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(above 10 kGy) produces shelf-stable foods without resort to refrigeration, and the sterilisation of food for special requirements. The irradiation process has been approved by the Food and Agriculture Organisation (FAO), the World Health Organisation (WHO), the International Atomic Energy Agency (IAEA) and the Codex Alimentarius Commission. About 100 countries have approved the process for application in more than 100 items of food. India first approved them in 1994. Today, the Directorate of General Health Service, under the Prevention of Food Adulteration Act, has approved more than 20 commodities to be processed using this method. Food irradiation is done in plants approved by the Atomic Energy Regulatory Board (AERB), and local food and drug administrations. The DAE has set up two technology demonstration facilities for irradiating food, agricultural and other products. One is at Vashi, Navi Mumbai, for the application of high doses for spices and dry vegetables. Another is at Lasalgaon, near Nashik, for onions, cereals, pulses and their products and cut-flowers, which require low doses. Malhotra said entrepreneurs in private and cooperative sectors have shown interest in setting up radiation processing plants. A women's non-governmental organisation, Annapurna Mahila Mandal, is selling radiation-processed spices in and around Mumbai. Malhotra said: "There is a need to deploy and integrate this technology with the national system of procurement, storage, distribution and marketing of agro-produce. Radiation processing plants designed to process several products requiring a specified range of radiation doses need to be set up in private, cooperative and public sectors." The Food Irradiation and Processing Laboratory of the Bhabha Atomic Research Centre (BARC) is one of the foremost laboratories of its kind in the world. Irradiation techniques developed at this centre have been effective in arresting sprouting in onions and potatoes; delaying the ripening of fruits such as bananas, mangoes and papayas; disinfesting grains; extending the shelf-life of fish and meat; eliminating pathogens from frozen seafood; and preventing microbial and insect contamination of spices. India is the largest producer of onions in the world. Onion is planted during winter (rabi crop) in most parts of the country and harvested during April-May. More than 1.5 lakh tonnes of onion is stored in Nashik district from June to October in conventional chawls. In Nashik, low ambient temperatures and high humidity conditions during the rainy season, beginning from July, lead to the sprouting of onions. Similarly, the sprouting of stored onions occurs following the rainy season in Mahuwa, a major onion production and storage centre in Bhavnagar district of Gujarat. Sprouted onions shrivel faster owing to increased water loss by transpiration. But irradiation at very low dose levels inhibits sprouting in onions. The DAE says that the process consists of exposing onions to gamma rays in a shielded room for a specified duration. The onions are brought into and taken out of the room by conveyors or carriers. According to the DAE, gamma irradiation is the answer to problems of producers and exporters of spices. Since gamma rays have high penetrating power, spices can be irradiated

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after packaging, irrespective of the size of the carton. This ensures that there is no contamination at the time of the opening of the package. 3.9.APPLICATIONS On the basis of the dose of radiation the application is generally divided into three main categories as detailed under: Low Dose Applications (up to 1 kGy) 

Sprout inhibition in bulbs and tubers 0.03-0.15 kGy



Delay in fruit ripening 0.25-0.75 kGy



Insect disinfestation including quarantine treatment and elimination of food borne parasites 0.07-1.00 kGy

Medium Dose Applications (1 kGy to 10 kGy) 

Reduction of spoilage microbes to prolong shelf-life of meat, poultry and seafoods under refrigeration 1.50–3.00 kGy



Reduction of pathogenic microbes in fresh and frozen meat, poultry and seafoods 3.00– 7.00 kGy



Reducing the number of microorganisms in spices to improve hygienic quality 10.00 kGy

High Dose Applications (above 10 kGy) 

Sterilisation of packaged meat, poultry and their products which are shelf stable without refrigeration. 25.00-70.00 kGy



Sterilisation of Hospital diets 25.00-70.00 kGy



Product improvement as increased juice yield or improved re-hydration

It is important to note that these doses are above those currently permitted for these food items by the FDA and other regulators around the world. The Codex Alimentarius Standard on [18][19] Irradiated Food does not specify any upper dose limit. NASA is authorized to sterilize frozen [20] meat for astronauts at doses of 44 kGy as a notable exception. Irradiation treatments are also sometimes classified as radappertization, radicidation and [21] radurization. 3.10.USES Ionizing radiation has many uses, such as to kill cancerous cells. However, although ionizing radiation has many applications, overuse can be hazardous to human health. For example, at one time, assistants in shoe shops used X-rays to check a child's shoe size, but this practice was [2] halted when it was discovered that ionizing radiation was dangerous.

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Technical uses of ionizing radiation Since ionizing radiations can penetrate matter, they are used for a variety of measuring methods. Radiography by means of gamma or X rays This is a method used in industrial production. The piece to be radiographed is placed between the source and a photographic film in a cassette. After a certain exposure time, the film is developed and it shows internal defects of the material if there are any. Gauges Gauges use the exponential absorption law of gamma rays 

Level indicators: Source and detector are placed at opposite sides of a container, indicating the presence or absence of material in the horizontal radiation path. Beta or gamma sources are used, depending on the thickness and the density of the material to be measured. The method is used for containers of liquids or of grainy substances



Thickness gauges: if the material is of constant density, the signal measured by the radiation detector depends on the thickness of the material. This is useful for continuous production, like of paper, rubber, etc.

Applications using ionization of gases by radiation 

To avoid the build-up of static electricity in production of paper, plastics, synthetic 241 textiles, etc., a ribbon-shaped source of the alpha emitter Am can be placed close to the material at the end of the production line. The source ionizes the air to remove electric charges on the material.



Smoke detector: Two ionisation chambers are placed next to each other. Both 241 contain a small source of Am that gives rise to a small constant current. One is closed and serves for comparison, the other is open to ambient air; it has a gridded electrode. When smoke enters the open chamber, the current is disrupted as the smoke particles attach to the charged ions and restore them to a neutral electrical state. This reduces the current in the open chamber. When the current drops below a certain threshold, the alarm is triggered.



Radioactive tracers for industry: Since radioactive isotopes behave, chemically, mostly like the inactive element, the behavior of a certain chemical substance can be followed by tracing the radioactivity. Examples: o Adding a gamma tracer to a gas or liquid in a closed system makes it possible to find a hole in a tube. o Adding a tracer to the surface of the component of a motor makes it possible to measure wear by measuring the activity of the lubricating oil.

Potential electricity generation through nanomaterials Using layers of carbon nanotubes interlaced with gold and lithium hydride, has been shown to produce a current when the gold particles are hit by radiation, releasing

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electrons which can travel through the carbon nanotubes to the lithium hydride, and then [3] to electrodes in order to generate electricity. Biological and medical applications of ionizing radiation In biology, radiation is mainly used for sterilization, and enhancing mutations. For example, mutations may be induced by radiation to produce new or improved species. A very promising field is the sterile insect technique, where male insects are sterilized and liberated in the chosen field, so that they have no descendants, and the population is reduced. Radiation is also useful in sterilizing medical hardware or food. The advantage for medical hardware is that the object may be sealed in plastic before sterilization. For food, there are strict regulations to prevent the occurrence of induced radioactivity. The growth of a seedling may be enhanced by radiation, but excessive radiation will hinder growth. Electrons, x rays, gamma rays or atomic ions may be used in radiation therapy to treat malignant tumors (cancer). Furthermore, just like in industrial application, x rays can also be used in radiography to create images of hard-to-image objects, such as inside one's body. Tracer methods are used in nuclear medicine in a way analogous to the technical uses mentioned above.

3.11. BENEFICIAL ASPECTS OF RADIATION Food Preservation by Irradiation The question of food preservation by ionizing radiations have been the subject of extensive research in a number of countries. Technologically a large number of applications of radiations are advantageous in industry, medicine and agriculture. Beneficial Aspect of Food Irradiation – Every year India loses about 10% of its food production through ill storage which accounts Rs.10,000 crores loss. Previously salting, canning, drying, heat treatment, freezing and dehydration were used to prevent food putrefaction. Nuclear technology has added another process called ‘food irradiation’, that is, bombarding food substances with relatively low energy gamma rays emanating from a radio-isotope such as cobalt-60 or caesium – 137. Following are the benefits of food irradiation. 1. 2.

It can totally eliminate micro-organisms thus making the food indefinitely safe from spoilage. Irradiated cereals, fruits, onions, garlic, spices like pepper, cardamom, clove prevent their sprouting, prolongs their storage and lessen the dangers of microbial contamination and subsequent food poisoning. Irradiation of soyabeans during germination result in the reduction of oligosaccharide content of the beans leading to a product which does not cause flatulence.

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10. 11. 12.

13. 14. 15.

With citrus fruits the fungicidal control can be attained as is obtained by wrapping the fruit with diphenyl paper. Irradiation reduces bacterial growth sufficiently to retard spoilage. Radiation disinfestation is practical proposition to destroy insects in cereals, grain, flour, peanuts, dried fruits and walnuts etc. It inactivates pathogens and parasites that endanger health. Irradiation effectively control physiological processes such as ripening in fruits, sprouting in tubers and bulbs to increase their shelf life. Irradiation sterilization can be used successfully to store ham, pork, bacon, sausage, beef, chicken, canned beef, cakes, cod fish and shrimp for long durations under non-refrigerated conditions. Recently DRDO scientists showed that jot just wheat, onion or spices, but even processed food like chapati and pulao etc. can also be safely irradiated. Irradiation can replace or drastically reduce the use of food additives and fumigants which pose geno-toxic and occupational health hazards. Through its ability to inhibit sprouting of root crops to prevent reproduction of insects, to kill food spoilers and to delay the ripening of fruits, irradiation is used as a potent method for reducing post harvest losses. Co-60 water insoluble so there is no risk of environmental contamination. Co-60 after its use gets converted into non-radioactive nickel. So there is no problem of radioactive waste disposal. Cost of irradiation and the low energy requirement for the process compares favourably with non-conventional food preservation techniques.

Presently 36 countries have approved one or more food items processed by ionizing radiations for human consumption. Japan was the first country to market successfully the irradiated potatoes. The technical feasibility of using ionizing radiations to preserve high perishable protein foods has been proved under US Army Radiation Preservation of Food Programmes. High does radiation sterilization of meat, meat products and poultry is a practical possibility if it is combined with a mild treatment to inactivate radiation resistant proteolytic enzymes of the tissues. Dose Application to Food For complete elimination of micro-organisms, a high does of gamma rays (a few million rads or mega rads) would be needed while for low doses (few thousand rads) would suffice shelf life extension. The higher doses called rad - appretisation, which is higher than 3.5 M rad, can inactivate spores of clostridium botulinum, a toxin producer. The latter process, known as rad urization is similar to heat pasteurisation and causes selective destruction of spoilage micro â&#x20AC;&#x201C; organisms and helps in controlling ripening and sprouting of fruits and vegetables.

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Gamma rays emitted from Co-60 kill pathogens such as salmonella, bugs and fungus on produce and inhibit maturation to extend shelf life. Food Monitoring – Government of India cleared irradiated processing for treatment of onions, frozen sea foods and spices for export in 1986. An apex body, namely National Monitoring Agency under Ministry of Health was constituted to define guidelines and to oversee all aspects of using radiation technology for food preservation. Effects of irradiated food – how safe they are? In India, Bhabha Atomic Research Centre (BARC) at Trombay, a unit of the Department of Atomic Energy has worked on food irradiation and its safety from hazardous radiation effects. It is, however, realized that early international protocols of whole someness testing of irradiated food could not detect subtle population hazards. Scientists have observed following adverse effects of irradiated food. 1.

Food irradiated by high doses of gamma rays cuse cytotoxic, teratogenic, carcinogenic and mutagenic actions in bacteria, fruitfly Drosophila, plants and mammalian tissue culture system. Genetic damages were observed in mice fed with irradiated wheat or potatoes. The cyto-toxic and clastogenic (chromosome breaking) effects of irradiated media were traced in sugar compounds and in plant cells when grown in irradiated sucrose solutions. Cytotoxic compounds, produced in irradiated sugars and lipids were identified as

, 

unsaturated carbony1 compounds, which become nearly double in presence

of oxygen.

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UNIT – IV DIGNOSTIC IMAGING AND APPLICATION TO RADIATION THERAPY X-RAY MACHINES AND DIGITAL RADIOGRAPHY

X-rays were discovered by the German physicist Wilhelm Konrad Rontgen in November 1895. He called the 'new kind of ray' or X-rays, X for the unknown. With these new rays, he could make a photograph of his wife's hand - showing the bones and her wedding ring. Soon afterwards, their usefulness to visualize the internal anatomy of humans was established. Today, imaging with X-rays is perhaps the most commonly used diagnostic tool with the medical profession, and the techniques from a simple chest radiography to a digital subtraction angiography or computer tomography depend on the use of X-rays.

4.1

BASIS OF DIAGNOSTIC RADIOLOGY

A radiological examination is one of the most important diagnostic aids available in the medical practice. It is based on the fact that various anatomical structures of the body have different densities for the X-rays. When X-rays from a point source penetrate a section of the body, the Internal body structures absorb varying amount of the radiation. The radiation that leaves the body has a spatial intensity variation, i.e. an image of the internal structure of the body. The commonly used arrangement for diagnostic radiology is shown in Fig. 4.1. The X-ray intensity distribution is visualized by a suitable device like a photographic film. A shadow image is generated that corresponds to the X-ray density of the organs in the body section. The examination technique varies according to the clinical problem. The main properties of X-rays, which make them suitable for the purposes of medical diagnosis, are their: 

Capability to penetrate matter coupled with differential absorption observed in various materials; and



Ability to produce luminescence and its effect on photographic emulsions.

The X-ray picture is called a radiograph, which is a shadow picture produced by X-rays emanating from a point source. The X-ray picture is usually obtained on photographic film placed

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in the image plane. The skeletal structures are easy to visualize and even the untrained eye can sometimes observe fractures and other bone abnormalities.

Fig. 4.1

Basic set-up for a diagnostic radiology image formation process

Chest radiographs are mainly taken for examination of the lungs and the heart. Because of the air enclosed in the respiratory tract, the larger bronchi are seen as a negative contrast, and the pulmonary vessels are seen as a positive contrast against the air-filled lung tissue. Different types of lung infections are accompanied by characteristic changes, which often enable a diagnosis to be made from the location, size and extent of the shadow.

Heart examinations are performed by taking frontal and lateral films. The evaluation is performed partly by calculating the total heart volume and partly on the basis of any changes in shape. For visualization of the rest of the circulatory system and for the special examinations of the heart, use is made of injectible, water-soluble organic compounds of iodine. A contrast medium is injected into an artery or vein, usually through a catheter placed in the vessel. Therefore, all the larger organs of the body can be examined by visualizing the associated vessels and this technique is called angiography. The examination is designated according to the organ examined -, e.g. for coronary angiography - the coronary vessels of the heart -, angiocardiography - the heart, and cerebral angiography -, the brain.

The entire gastro-intestinal tract can be imaged by using an emulsion of barium sulphate as a contrast medium. It is swallowed or administered to diagnose common pathological conditions such as ulcers, tumours or inflammatory conditions. Negative and positive contrast media are used for visualizing the spinal canal, the examination being known as myelography. The central nervous system is usually examined by pneumography, i.e., filling the body cavities FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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with air. It may, however, be mentioned that computerized tomography has greatly reduced the need for some of the invasive neuro-radiological methods, which involve discomfort and a certain risk for the patient.

4.2

NATURE OF X-RAYS

X-rays are electromagnetic radiation located at the low wavelength end of the electromagnetic spectrum. The X-rays in the medical diagnostic region have wavelength of the -10

order of 10 m. They propagate with a speed of 3 x 10

cm/s and are unaffected by electric and

magnetic fields. According to the quantum theory, electromagnetic radiation consists of photons, which are conceived as 'packets' of energy. Their interaction with matter involves an energy exchange and the relation between the wavelength and the photon is given by

E  hv  h

 -34

where h

Planck's constant = 6.32 x 10

velocity of propagation of photons = 3 x 10 cm/s

frequency of radiation



wavelength

A vibration can be characterized either by its frequency or by its wavelength. In the case of X-rays, the wavelength is directly dependent on the voltage with which the radiation is produced. It is, therefore, common to characterize X-rays by the voltage, which is a measure of the energy of the radiation. 4.2.1

Properties of X-rays

Because of short wavelength and extremely high energy, X-rays are able to penetrate through materials which readily absorb and reflect visible light. This forms the basis for the use of X-rays for radiography and even for their potential danger. X-rays are absorbed when passing through matter. The extent of absorption depends upon the density of the matter. X-rays produce secondary radiation in all matter through which they pass. This secondary radiation is composed of scattered radiation, characteristic radiation and electrons. In diagnostic radiology, it is scattered radiation which is of practical importance.

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X-rays produce ionization in gases and influence the electric properties of liquids and solids. The ionizing property is made use of in the construction of radiation-measuring instruments.

X-rays also produce fluorescence in certain materials to help them emit light. Fluoroscopic screens and intensifying screens have been constructed on the basis of this property. X-rays affect photographic film in the same way as ordinary visible light.

4.2.2

Units of X-radiation

The International Commission on Radiological Units and Measurements has adopted Rontgen as a measure of the quantity of x-radiation. This unit is based on the ability of radiation to produce ionization and is abbreviated 'R'. One R is the amount of x-radiation which will 9

produce 2.08 x 10 ion pairs per cubic centimetre of air at standard temperature (0°C) and pressure (760 mmHg at sea level and latitude 45°). Other units derived from the Rontgen are the millirontgen (mR = 1 /1000 R) and the microrontgen (  R = 10 R). The unit of x-radiation has -6

been based on the ionization produced by the rays and not on other effects like the blackening of a photographic film due to the ease and accuracy with which ionization in the air can be measured.

The biological effects of X-rays are due to energy imparted to matter: Therefore, these effects are more closely correlated with the absorbed dose than with exposure. The unit of absorbed dose is rad. One rad is the radiation dose which will result in an energy absorption of -2

l.0 x 10 J/kg in the irradiated material. It is approximately equal to the dose absorbed by soft tissue exposed to one Rontgen of X-rays.

The Rontgen and the absorbed dose D are related as D = f R where f is a proportionality constant and depends upon both the composition of the irradiated material and quality of the radiation beam. The value of f for air is 0.87 rad / R. For soft tissues, f = 1 rad / R and hence the absorbed dose is numerically equal to the exposure. However, for bone, f is larger but significantly decreases with an increase in kV. Therefore, if the contrast requirements permit, the patient's absorbed dose can be decreased by using suitably high kV.

The ionization produced by different types of radiation is not a sufficiently good criterion of biological effect. Another concept is that of the so-called dose equivalent (DE) H. DE is defined as the product of the absorbed dose D and a modifying quality factor (QF), i.e.

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DE = (QF) D

The film badge readings and radiation guides in the form of a maximum permissible dosage are expressed in rems or millirems.

In short, Rontgens express incident energy, rads give an indication of how much of this incident energy is absorbed and rems are a measure of the relative biological damage caused.

4.2.3. X-ray Image Intensifier Television System

An X-ray image intensifier consists of a large evacuated glass tube with an input screen diameter ranging from 15-32 cm. The input screen converts the X-ray image into a light image. The light image thus produced is transmitted through the glass of the tube to a photo-cathode which converts the light image to an equivalent electron image. The image intensification takes place because of the very small output screen size and electron magnification in the tube.

Figure 4.2 shows the general structure of an X-ray image intensifier tube. It consists of an input screen, the surface of which is coated with a suitable material to convert X-rays into a light image. First generation X-ray image intensifiers used zinc cadmium sulphide as the input fluorescent screen. However, this material has a poor X-ray absorption efficiency. Modern X-ray image intensifiers make use of a thin layer of cesium iodide (CsI) which has the advantage of high X-ray absorption and packing density. The X-ray quanta, after getting converted to light quanta, falls on the photo-cathode in which the light quanta produce electrons. Under the influence of an electrical field, the electrons are emitted from the photo-cathode and accelerated towards the output phosphor, while being focused by the electrostatic lens system. The electrons impinging with high kinetic energy on this screen produce light quanta resulting in a much brighter and minified output image. The brightness gain is due to the acceleration of the electrons in the lens system and the fact that the output image is smaller than the primary fluorescent image. The gain is several hundred times. It not only allows the X-ray intensity to be decreased tremendously but makes it possible to observe the image in a normally illuminated room.

The output window which permits us to examine the light image presented by the output viewing screen (15-30 mm diameter) located inside the bulb near the window, is flat and allows for image transfer through large numerical aperture objective (F/0.75). Certain tubes (Thomson CSF THX 475) are made with a fibreoptic

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Fig. 4.2. Constructional details of an X-ray image intensifier tube (a) inputsur)'ace consists of scintillation layer and photocathode (b) three focusing electrodes with typical bias voltages (c) electrode and output phosphor output window. This permits picking up the image by mechanical coupling with a camera tube that has a fibreoptic input window (Ebrecht, 1977).

Figure 4.3 shows a system incorporating the X-ray image intensifier system which can be coupled to a closed circuit television, cine camera, photo-spot camera and video recording facilities. Although the image intensifier is a fundamental element of the chain, the final image observed on the TV monitor depends, to a large extent, on the characteristics of the TV chain, which greatly affects the signal constancy and the spatial resolution. The most commonly used TV pick-up tube is the 2.5 cm vidicon, whose target will accept a 15 mm diameter image. The sensitivity of the system depends on the conversion efficiency of the vidicon target. Available optical systems have a luminous yield of the order of 20% of green light onto the vidicon target. Also, the optical system suffers from 'vignetting' (brightness fall-off at the edges of the image). The use of a system with fibreoptic coupling solves this problem.

The combination of X-ray image intensifiers and the TV system must control the X-ray generator to produce constant density changes, despite variations in patient thickness. This is done by automatic dose rate control, also known as automatic brightness control. In the FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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arrangement shown in Fig. 4.4 the dose control of the X-ray tube is linked to the exposure control of the camera tube and photo-camera. The intensity of the light leaving the intensifier is measured by a light sensor (LS). The exposure control circuits drive the voltage and current of the generator (XG). If the image intensifier is switched to a higher magnification, the current in the X-ray tube i

A. Two channel attachment - can take a TV camera and a 70 mm or cine camera. B. Holder and lens. This produces a parallel light beam for the cameras. C. Photo pick-up. D. Electrical signal from photo pick-up. It provides the control for 70 mm and cinefluorography.

Fig. 4.3

X-ray image intensifier system

Fig. 4.4 Automatic dose control in an X-ray image intensifier system

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increased in inverse proportion to the diameter on the X-ray screen. At the same time, the lead diaphragm output size between the patient and the X-ray tube is reduced to cut down on the area of patient exposure. Diaphragms in front of the TV camera and the photo-camera can be opened up if the light output is still not sufficient, in spite of maximum dose input. The input screen technology of image intensifiers has improved so greatly in recent years that spatial resolution comes close to that of conventional X-ray films. But after the first step of conversion, image quality worsens because of the image intensifier electron optics, the optical lens system, and to a large extent, the TV camera. The Siemens AG 1249 line X-ray video system optimizes these steps and provides 50% better resolution as compared with the common 525 / 625 systems (Riemann and Marholff, 1981). With 1249 television lines and the bandwidth of 25 MHz, it is possible to achieve a vertical resolution of 2.8 line pairs/mm and a horizontal resolution of 3.0 line pairs/mm.

Cine-film or 35 mm format recording of the image intensifier output image is usually performed with a 16 or 22 cm field and is mainly employed in cardiovascular examination for quantitative measurements of certain phenomena, such as heart volume during cardiac muscle contraction. The measurement of apparent heart cross-section compared to its volume permits detection of insufficiencies that may lead to cardiac failure. The sensitivity of the system is such that a radiation dose of 20-30

ď R

is sufficient to produce a good image. The common optical

lens system of 50 mm to 82 mm focal length can give images that are well-suited to the film format. The overall resolution is of the order of 30 Ip/cm.

The use of 105-mm spot film is becoming increasingly popular, even tending to support traditional radiographic film systems for certain cardiovascular examinations. Reduction in radiation dose, high speed cine-film capacity (12 images), easy filing and the low cost of film are some of the advantages of using spot film.

In video flouroscopic X-ray systems, the detector or rather the detector chain, embodies the key technology. The CCD (Charge Coupled Device) camera, introduced in some systems, is replacing the vidicon tube camera and offers significant improvements in image quality. It offers higher resolution wherein, for certain applications, 2048 x 2048 pixel matrices are preferable. In addition, continuous improvements in the detector chain have led to radically novel approaches which dispense with the need for an image intensifier and TV camera. The introduction of

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selenium- a photo-conductor with optimal properties for use in X-ray detectors â&#x20AC;&#x201C;facilitates the image to be obtained directly in digital format. The next generation of X-ray systems will contain a new type of flat solid state detectors. In these detectors, the optical image is provided by the cesium iodide input screen, which is directly detected by a high-resolution amorphous silicon photo-diode matrix and a thin film transistor array. This is described in detail in the next section.

4.3.

DENTAL X-RAY MACHINES

X-rays are the only media available to detect location of the teeth, their internal condition and the degree of decay at an early stage. Since the object-film distance is rather low, and the tissue and the bone thickness are limited, an X-ray machine of low power is adequate to obtain the radiograph with sufficient contrast. In practice, most dental units have a fixed tube voltage, in the region of 50 kV, and a fixed tube current of about 7 mA. The system combines the high voltage transformer and X-ray tube into a singly small case, thus greatly simplifying handling and positioning as no high voltage cables are required.

The primary winding of the transformer is fed with mains voltage via an exposure timer and the high voltage developed in the secondary windings is fed to the self rectifying X-ray tube. The complete assembly is contained in a metal case filled with special insulating oil. The X-ray tube is of special design and employs a third electrode, called a 'grid', between the anode and the cathode electrodes. The grid restricts electrons from leaving the cathode until the high voltage reaches its peak value, whereupon all electrons are released and impinge on the anode at a very high velocity. Consequently, the x-radiation generated contains fewer useless soft X-rays and more hard rays. The total radiation is, therefore, more effective and can be compared mathematically to a much higher output resulting in shorter exposure times.

4.4

PORTABLE AND MOBILE X-RAY UNITS In many situations, portable and mobile X-ray units are necessary for X-ray patients who,

for some reason, may not be able to go to X-ray departments. Such situations emerge when the patient is too ill to be moved from the hospital bed, is seriously ill at home or undergoing surgery in the theatre. Thus, the need arises to have X-ray equipments, which can be moved to the patients rather than the patient moving to the X-ray machine.

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Portable Units

A portable unit is so designed that it can be dismantled, packed into a small case and conveniently carried to the site. The tube head is so constructed that the X-ray tube and the high voltage generator are enclosed in one earthed metal tank filled with oil. The X-ray is usually a small stationary anode type, operating in the self-rectifying mode and connected directly across the secondary winding of the transformer. The only connection required from the control desk is for the low voltage supply. The controls provided are fairly limited and include a mains voltage compensator, combined kV and current switch and time selector. As the unit is designed to be used on the domestic supply, the current must be limited to 15 A. Thus the maximum radiographic output commonly found on portable units is in the range 15-20 mA at 90-95 kV.

The tube head is mounted on a cross arm which is carried on a vertical column. The cross arm may be moved up and down this column by means of a rack and pinion drive.

4.4.2

Mobile Units

A mobile unit carries the control table and the column supporting the X-ray tube permanently mounted on the mobile base. Mobile units could be much heavier than the portable units and are capable of providing higher outputs. Mobile units provide a greater selection of mA and kV values. The high voltage generator has a full wave rectification circuit feeding a double focus rotating anode X-ray tube. Most mobile units have a radiographic output of up to 300 m A and a maximum of 125 kV. This type of output requires a main supply current of 30 A. The units are, therefore, usually fitted with a 30 A plug and special sockets need to be provided throughout the hospital for this purpose.

Because of the high current requirements of the mobile units, the mains resistance becomes a problem, specially if the mobile is to be used in different parts of the hospital. In order to ensure consistent results from one power supply socket to another, a means for mains resistance calibration is provided and must be adjusted to suit each location before making an exposure.

Where there are limitations on the electrical supply, mobile units make use of stored energy. This may be from the capacitor discharge or battery-powered inverter circuits. The former releases stored energy from the capacitor during exposure, while the latter converts energy stored in the battery.

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Mammographic X-ray Equipment

Mammography is an X-ray imaging procedure used for examination of the female breast. It is primarily used for diagnosis of breast cancer and in the guidance of needle biopsies. The female breast is highly radiation-sensitive. Therefore, the radiation dosage during mammography should be kept as low as possible. Also, it is required to achieve better spatial resolution than other types of film/screen radiographs. In order to achieve these goals, an X-ray tube with a small focal spot size is used to minimize the possibility of geometric blur. The film/screen cassette has a single emulsion film and a single screen, and is designed to provide excellent film/screen contact.

Mammographic X-ray equipment can either be used with special film/screen cassette or as xero-radiographic units. The units intended for film/screen use have a molybdenum target Xray tubes with a beryllium window and a 0.03 mm molybdenum filter. Radiographs are usually taken at 28-35 kV. Xero-radiographic systems use X-ray tubes with tungsten targets and about 1 mm aluminum filter. Radiographs with this technique are taken at 40-50 kV. Hence, both types of mammographic units operate at low peak voltages.

Film-based mammography has several disadvantages such as limitations in detection of micro-calcifications and other fine structures within the breast, and inefficiency of grids in removing the effects of scattered radiation. Many of these limitations can be effectively removed by using a digital mammography system in which image acquisition, display and storage are performed independently, allowing for optimization of each process. However, the availability of a suitable X-ray detector for this purpose is still a challenge that precludes the widespread use of digital mammography. Various detector technologies which are under evaluation in digital mammography are large area CCDs (Charge Coupled Devices), photo-stimulable phosphors, amorphous silicon coupled to scintillators, amorphous selenium and other solid-state devices.

4.5.

PHYSICAL PARAMETERS FOR X-RAY DETECTORS

The physical parameters used to characterize X-ray detectors are as follows:

Detector Quantum Efficiency (DQE): The DQE describes the efficiency of a detector, i.e. the percentage of quanta for a given dose that actually contributes to the image. It is a function of dose and spatial frequency and is, by definition, effected by the various components of the system.

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Dynamic Range: The dynamic range of a detector is the range from minimum to maximum radiation intensity that can be displayed in terms of either differences in signal intensity or density differences in conventional film.

Modulation Transfer Function (MTF): The MTF describes how the contrast of the image component is transmitted as a function of its size or its spatial frequency. It is expressed in line pairs per millimetre (1p/mm).

Contrast Resolution : It is the smallest detectable contrast for a given detail size that can be shown by the imaging system with different intensity (density) or the whole dynamic range. The threshold contrast is a measure for imaging of low contrast structures and is largely determined by the DQE of the detector.

4.6

DIGITAL RADIOGRAPHY Ever since the original discovery of X-rays, film has been the preferred medium for

producing medical X-ray images. This means that the same medium is employed for image acquisition, presentation and storage. Consequently, images which are produced with less than optimal quality cannot be easily manipulated to improve information retrieval.

The conventional screen film system has a moderately good detector quantum efficiency (20-30% at 60 keV) and a similarly good MTF for frequencies above 3 1p/mm. The strength of screen-film combinations lies in their high nominal spatial resolution (>31p/mm) and the high contrast resolution at optimum exposure.

In both radiography and fluoroscopy, there are definite advantages of having a digital image stored in a computer. This allows image processing for better displayed images, the use of lower doses, avoiding repeat radiography and opening up of the possibility of digital storage with a PACS (Picture Archieving and Communication System) or remote image viewing via teleradiology. Digitally formatted images would permit digital storage, retrieval, transfer and display of X-ray images with vast possibilities of image-related processing and manipulations, as each function can be individually optimized.

Digital X-ray imaging systems consist of the following two parts: (i) X-ray imaging transducer or data collection; and (ii) Data display, storage and processing.

The digitally compatible X-ray imaging transducers can be divided into the following two categories: FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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(i)

Image intensifier TV system; and

(ii)

(ii) Radiographic (film replacement) systems.

The application of X-ray image intensifier TV systems in digital X-ray imaging evolved from their use in angiography. Angiography is a diagnostic and rapidly developing therapeutic modality concerned with diseases of the-circulatory system. The procedure is carried out by using a contrast material to opacity vascular structures because the radiographic contrast of blood is essentially the same as that of soft tissue. Contrast material is an iodine-containing compound which is injected through a catheter (diameter ranging from 1 to 3 mm). Radiographic images of the contrast-filled vessels can be viewed on a TV screen or are recorded by using either film or video. The most important application of digital technology is the development of digital subtraction angiogrphy (DSA). In this technique, a pre-injection image (mask) is acquired, the injection of contrast agent is then performed, then images of the opacified vessels are acquired and subtracted from the mask. This greatly helps in contrast enhancement there by providing increased contrast sensitivity. To illustrate this, Fig. 4.5(a) represents the transmitted X-ray intensity through the cross-section of a patient. It is obvious that small contrast changes due to vessels are masked by a large anatomical background contrast change. Any attempt to amplify these small signals would merely produce saturation of the display system by the large background signals. Subtraction of the constant background signal away from the contrastenhanced signal (Fig. 4.5(b) produces a more meaningful and uniform signal. This enables the vessel signals to be amplified greatly prior to display, which improves their visibility.

Fig. 4.5 (a)

X-ray transmission cross-section of a patient with contrast enhanced vessel

images superimposed (b) subtracted profile with uniform background to vessel image

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Figure 4.6 shows a digital subtraction angiography system based on the use of image intensifier. The output of the video camera, which is in the analog form, is first digitized in an analog to digital converter and fed into two semiconductor memories. Theoretically, noise is added to the image due to quantization errors associated with digitization process. This additional noise can be kept to an insignificantly small degree by using a 10 or 12-bit analog-to-digital

Fig. 4.6 . Block diagram of a digital video subtraction unit

converter in order to have a sufficiently high number of digital levels.

Storage of digital images is required for both online and archival purposes.

Online

storage is provided by real-time digital disks. Archival needs are mostly met by the storage of hard copy films generated from the digital images. Hard copy devices include multiformat cameras (laser or video) and video thermal printers. Alternatively, digital streamer tape cassettes

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and digital optical disks have been used to record angiographic procedures and associated images.

Film replacement digital X-ray imaging transducers make use of a number of technologies for scanning the area of interest for radiography. Storage phosphor screens are the most widely used detectors for digital radiography. Their QE is less than that of a screen-film combination while the MTF is moderate and depends on the type of phosphor screen. The use of selenium as a detector material has long been known from xero-radiography. Here, the digital electrostatic X-ray imaging system involves the direct electrical read-out of the latent X-ray image formed by xero-radiography. This process, a thin layer of selenium on which a surface voltage has been induced is exposed to X-rays. The charges liberated by absorption of the X-ray energy migrate to the surface where they neutralize the deposited charge layer, thus forming an electrostatic image. The latent charge images thus produced are scanned by guarded electrometers. A time-dependent signal is induced, which a high input impedance pre-amplifier detects. The detected signal from the electrometers is then multiplexed, amplified and digitized. As in all digital systems, the resolution is determined by the pixel size (0.2 mm; 2166 x 2448 matrix) and is nominally below that of film. The dynamic range is extremely wide (1:10000) with a linear relationship between dose and signal over a wide exposure range. Xero-radiography tends to suffer from a lack of sensitivity when thicker body sections are being imaged. Therefore, its potential use may be limited to X-ray examinations of extremities of the body.

The image is displayed on the monitor immediately after acquisitions, so that the image quality can be checked while the patient is still in the examination room. Any changes in the patient position or the equipment settings can be made immediately, without having to wait until a film has been developed. The digital images can be automatically combined with the patient and the exposure data in the system and transferred online to the laser camera or the diagnostic workstation. 4.6.1. Flat Panel Detectors for Digital Radiography

The next step towards the digital integration of the classic X-ray acquisition technique is the use of electronic image detectors. The system under development is based on new X-ray detectors employing large area amorphous silicon semiconductor sensors. Amorphous (noncrystalline) silicon (a-silicon) is used in place of the classic microchip with mono-crystalline silicon, as this is necessary for achieving the large detector area. The a-Si layer is brought onto a glass carrier as a thin layer and structured into an array of sensors (photo-diodes) using conventional FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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photolithographic methods. A switching element (a diode or a transistor) is allocated to each individual sensor so that the sensor can be connected to a read-out line in the column direction. The switching element are controlled via corresponding address lines in the row direction (Fig. 4.7). The signal from the individual sensors is led to pre-amplifiers, amplified and given to analogto-digital converters.

Fig. 4.7 Flat X-ray solid state detector with an amorphous silicon active readout matrix

The signals from all the sensors are read out until the whole X-ray image has been completed. As the process is electronic, it can achieve very high transfer rates. The X-ray image is displayed on the monitor of a workstation with a gray scale resolution of 12 bits. Silicon by itself is not sufficiently sensitive for detecting X-rays in the energy range used in diagnostic radiology. For this reason, an image converter layer is applied over the layer of amorphous silicon. Generally, cesium iodide (CsI) is used as the image converter layer. This is a flourescent material, which is also used as the input screen of the X-ray image intensifier.

The pixel size in the X-ray image is determined by the size of the sensors. In the a-Si detector, it is 143 ( ď m x 143

ď m.

This allows a resolution of more than 3.5 Ip/mm to be

achieved, sufficient for most of the radiographic applications except for mammography. With a detector size of 43 x 43 cm, a matrix of 3000 x 3000 pixels is created on the flat a-Si detector. The flat sandwich structure of the image detector allows for a compact construction so that the flat detector can be easily integrated in the bucky table.

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A flat panel detector has the potential to close the gap between digital systems already in use (CT, MR etc.) and radiography, and thus represents one more important step towards the fully digital hospital.

4.7. PHYSICAL ASPECTS OF BRACHYTHERAPY TECHNIQUES

Introduction

The treatment of malignant disease by irradiation with small sealed sources, applied in close proximity to the tumour is an important branch of radiotherapy. The sources may be placed in or near the body surface, within natural body cavities or implanted directly into the tumour tissues. These types of therapy-surface, intracavitary and interstitial are collectively called 'plesiotherapy' or brachytherapy. plesio means near and brachy means short.

The main advantage of this technique is that it delivers localized high' dose to the tumour volume. The dose outside falls off very rapidly, thereby giving less integral dose. It is a continuous irradiation which is biologically better because of less oxygen dependence. Cosmetic and functional results are excellent. By a combination of external and brachy irradiation much higher doses can be delivered because surrounding normal tissues get less dose. If the tumour recurs or complications occur, surgical salvage is still possible because of less dose to the surrounding tissues and hence less problems with healing.

The limitations of brachytherapy are the accessibility of tumour volume and its size. If lesion is too big too many radioactive foci are required. The most important drawback is the high risk of exposure during preparation and application of sources. Source reconstruction and dosimetry are laborious, in the absence of treatment planning computer facility.

Although, radium and radon have been used for many years with good success, the hazards associated with the use of radium forced many radiotherapy centers to discontinue this technique. The main disadvantage of radium is that its daughter product radon is a highly toxic gas which may be released if any rupture occurs due to pressure build up within the encapsulation. Re-encapsulation cost is very high and not undertaken by any supplier. Radium is a bone seeker and highly toxic. It emits a wide spectrum of gamma energies, the maximum is about 2.45 MeV which demands heavy shielding. The maximum beta energy is about 3.2 MeV which necessitates heavy filtration and results in bulky sources. An ideal brachytherapy source should have the following properties.

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1. Gamma Energy

Preferably monoenergetic and sufficiently high to avoid differential absorption in bone. If energy is too high, if leads to radiation protection problems. Ideal energy is about 0.2-0.4 MeV.

2. Half Life

A long half life is a distinct advantage for temporary implants because it allows permanent or semipermanent stock of sources. Short half life sources necessitates frequent replacement and dosimetric correction for decay. Very short half life sources are useful for permanent implantation.

3. Beta energy

Should be low as otherwise larger sheathing will be required.

4. Physical characteristics

Radionuclide should preferably be nontoxic and insoluble form and should not powder or' disperse, if damaged.

5. Specific Activity

Higher specific activity allows fabrication of miniature sources which gives greater flexibility.

6. Cost, availability etc. are the other important parameters.

Taking into account all these parameters, quite a few radionuclides have been considered, although none of them meets with all the requirements. However, suitable alternatives can be found for specific applications. e.g. Caesium-137, Cobalt-60 and Iridium-192 are reasonably good substitutes for intracavitary and interstitial applications. Gold-198 and Iodine - 125 are successfully employed for permanent implants in place of radon.

Cobalt - 60 is available locally and cheaper. It is supplied in the form of pellets, tubes and needles. The main disadvantage is the higher gamma energy which necessitates thicker

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shielding. Further, when Co-60, sources are procured at regular intervals of say, 1 or 2 years, inventory of sources become somewhat laborious because of comparatively smaller half- life. Because of high specific activity, Co-60 sources are generally preferred for high intensity remote controlled afterloading units.

Caesium-137 has many advantages including relatively long half life, medium energy gamma etc. It is supplied in the form of insoluble powders or ceramic microspheres. Presently many manual afterloading systems and, some remote afterloading units use

137

Cs sources.

Iridium-192 (alloy of 25% Ir and 75% Pt) sources in the form of seeds and wires are increasingly used for interstitial after loading techniques. It has a complicated gamma ray spectrum with an average energy of 0.38 MeV. Iridium is preferred because of lesser gamma energy and high specific activity which allows fabrication of thin sources. Gold-198 seeds are used for permanent implant. A gold seed is typically 2.5 mm long and 0.8 mm diameter.

Iodine - 125 seeds are now increasingly used in place of radon-222 and gold-198 for permanent implants. Its advantage is the low photon energy which requires less shielding. However, dosimetry is more complex than other conventional sources. Iodine-125 seed consists of a titanium tube containing two ion exchange resin spheres separated by a radio opaque gold marker. The ion exchange resin beads are impregnated with 0.8 mm

ď Śx

4.5 mm

125

I in the form cylindrical capsule of

I decays by electron capture and emits 35.5 keV - photons. Characteristic

X-ray in the range of 27-35 keV are also produced. Another isotope which needs to be considered for brachytherapy is calif ornium-252. It is a source of spontaneous fission neutrons. The average energy of neutron is 2.35 MeV and that of gamma 140 keV. The half life is 2.65 yrs . The main advantages of Cf-252 sources are low OER and high RBE. Sources are available as needles, tubes and seeds. The main problem is the shielding of neutrons which restraints its use for manual loading. Other promising isotopes are Caesium-131, Chromium-51, Barium-133 etc. Cs-131 has a half life of about 10 days and gamma energy of 30 keV. It is ideal for permanent implants but is not generally available. Cr-51 has a gamma ray energy 0.32 MeV. But its half life (28 days) is neither long enough for permanent stock nor short enough for permanent implantation. Barium-133 with half of 7.2 days and gamma energy of 0.36 MeV has low specific activity. Its production in millicurie level is rather difficult at present.

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The characteristics of various isotopes are given in Table 1. Most of the gamma ray sources are



- emitters. These rays produce necrosis of the layers of tissue surrounding the

source. To cut off the

 -rays an absorber is used. The choice of absorber (sheathing) should be

such that it should be non-toxic to body fluids, non corrosive, preferably of high absorption coefficient and density. The sheathing materials generally used are platinum, gold, silver, stainless steel etc. The physical properties of some of the commonly used radionuclides are given in the Table -1. TABLE – 1 : COMMONLY USED RADIONUCLIDES IN BRACHYTHERAPY

Radio

Energy of

Half

(MeV)

Radiation nuclide Cs – 137

HVL in Water

HVL in Lead

(cm)

(mm)*

Life

Gamma (Ave)

Beta

30 y

0.662

0.51 – 1.17

8.2

5.5

0.67

6.3

2.5

0.31

10.8

11.0

Ir – 192

0.38 73.8d (0.14-1.06)

Co-60

1.25 5.26y (1.17, 1.333)

Au – 198

2.7 d

0.412

0.96

7.0

2.5

I – 125

60 d

0.028

2.0

0.025

Pd – 103

17 d

0.021

1.0

0.008

0.02 – 3.26

10.6

12.0

Ra – 226*

0.83 1600y (0.05 – 2.45)

* filtered with 0.5 mm platinum.

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UNIT â&#x20AC;&#x201C; V CHAPTER - 20 X-RAY COMPUTED TOMOGRAPHY 5.1. COMPUTED TOMOGRAPHY

There are two main limitations of using conventional X-rays to examine internal structures of the body. Firstly, the super-imposition of the three-dimensional information onto a single plane makes diagnosis confusing and often difficult. Secondly, the photographic film usually used for making radiographs has a limited dynamic range and, therefore, only objects that have large variations in X-ray absorption relative to their surroundings will cause sufficient contrast differences on the film to be distinguished by the eye. Thus, whilst details of bony structures can be clearly seen, it is difficult to discern the shape and composition of soft tissue organs accurately. In such situations, growths and abnormalities within tissue only show a very small contrast difference on the film and consequently, it is extremely difficult to detect them, even after using various injected contrast media. The problem becomes even more serious while carrying out studies of the brain due to its overall shielding of the soft tissue by the dense bone of the skull.

Various techniques have been applied in an effort to overcome these limitations, but the most powerful technique which has shown dramatic results is computed tomography, which was invented and developed by G.N. Hounsfield at the Central Research Laboratories of EMI Ltd, UK, and introduced on a commercial scale in 1972. Since then, its impact on the medical world has been as great as the discovery of X-rays itself. Despite the inherently high cost of the equipment, several thousands of these are now installed in hospitals around the world.

Tomography is a term derived from the Greek word 'tomos', meaning 'to write a slice or section' and is well-understood in radiographic circles. Conventional tomography was developed to reduce the super-imposition effect of simple radiographs. In this arrangement, the X-ray tube and photographic film are moved in synchronisation so that one plane of the patient under examination remains in focus, while all other planes are blurred. In computed tomography (CT), the picture is made by viewing the patient via X-ray imaging from numerous angles, by mathematically reconstructing the detailed structures and displaying the reconstructed image on a video monitor. FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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The early CT scanners were specifically designed for neuro-radiological investigations. Computed tomography enabled radiologists to distinguish, for the first time, between different types of brain tissue, and even between normal and coagulated blood. With CT images, radiologists could easily visualize the ventricles of the brain and repositories of the cerebro-spinal fluid. This capability made obsolete a rather unpleasant procedure known as 'pneumoencephalography', in which air is pumped into the ventricles to displace the fluid and provide radiographic contrast.

The desirability of having body scanners was soon realized. The examination of the body sections, however, represents widely differing problems. Some of these problems include the movement of organs, the patient's respiratory action, the broader range of tissue densities encountered and the wide range of body sizes that have to be accommodated. In spite of these difficulties, whole body CT scanners (Fig. 5.1) with a very wide range of clinical capabilities have been made commercially available. Since respiration does not normally involve gross movements of the head, its effect on the quality of brain pictures is negligible. Consequently, a brain scanner does not need to operate at the speed of a whole body machine.

5.1.1. Basic Principle

Computed tomography differs from conventional X-ray techniques in that the pictures displayed are not photographs but are reconstructed from a large number of absorption profiles taken at regular angular intervals around a slice, with each profile being made up from a parallel set of / absorption values through the object.

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Fig. 5.1 Whole body CT scanner (Courtesy : M/s General Electric Company, USA)

In computed tomography, X-rays from a finely collimated source are made to pass through a slice of the object or patient from a variety of directions. For directions along which the path length through-tissue is longer, fewer X-rays are transmitted as compared to directions where there is less tissue attenuating the X-ray beam. In addition to the length of the tissue traversed, structures in the patient such as bone, may attenuate X-rays more than a similar volume of less dense soft tissue. In principle, computed tomography involves the determination of attenuation characteristics for each small volume of tissue in the patient slice, which constitute the transmitted radiation intensity recorded from various irradiation directions. It is these calculated tissue attenuation characteristics that actually compose the CT image.

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For a monochromatic X-ray beam, the tissue attenuation characteristics can be described by x

Ioe

Incident radiation intensity

Transmitted intensity

Thickness of tissue



Characteristic attenuation coefficient of tissue

If a slice of heterogeneous tissue is irradiated (Fig. 5.2), and we divide the slice into volume elements or voxels with each voxel having its own attenuation coefficient, it is obvious that the sum of the voxel attenuation coefficients for each X-ray beam direction can be determined from the experimentally measured beam intensities for a given voxel width. However, each individual voxel attenuation coefficient remains unknown. Computed tomography uses the knowledge of the attenuation coefficient sums derived from X-ray intensity measurements made at all the various irradiation directions to calculate the attenuation coefficients of each individual voxel to form the CT image.

Fig. 5.2

X-rays incident on patient from different directions. They are attenuated by different amounts, as indicated by the different transmitted X-ray intensities

Figure 5.3 shows a block diagram of the system. The X-ray source and detectors are mounted opposite each other in a rigid gantry with the patient lying in between, and by moving one or both of these around and across the relevant sections,

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Fig. 5.3. The technique of producing CT images. The x-ray tube and the detector are rigidly coupled to each other. The system executes translational and rotational movement and transradiates the patient from various angular projections. With the aid of collimators, pencil thin beam of X-ray is produced. A detector converts the x-radiation into an electrical signal. Measuring electronics then amplify the electrical signals and convert them into digital values. A computer then processes these values and computes them into a matrix-line density distribution pattern which is reproduced on a video monitor as a pattern of gray shade (Courtesy: Siemens, W. Germany).

which is how the measurements are made. The patient lies on a motorized couch and is moved into the aperture of the gantry, with the location to be accurately determined by means of a narrow strip of light that falls on the body from the gantry and illuminates the section to be examined. From the keyboard mounted on the operating console, details such as, the patient's code, the name of the hospital, etc., are fed into the system and settings for X-ray parameters for the scan are made.

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In one system which employs 18 traverses in the 20s scanning cycle, 324,000 (18 x 30 x 600) X-ray transmission readings are taken and stored by the computer. These are obtained by integrating the outputs of the 30 detectors with approximately 600 position pulses. The position pulses are derived from a glass graticule that lies between a light emitting diode and photo-diode assembly that moves with the detectors. The detectors are usually sodium-iodide crystals, which are thallium-doped to prevent an after-glow. The detectors absorb the X-ray photons and emit the energy as visible light. This is converted to electrons by a photo-multiplier tube and then amplified. Analog outputs from these tubes go through signal conditioning circuitry that amplifies, clips and shapes the signals. A relatively simple analog-to-digital converter then prepares the signals for the computer. Simultaneously, a separate reference detector continuously measures the intensity of the primary X-ray beam. The set of readings thus produced enables the computer to compensate for fluctuations of X-ray intensity. Also, the reference readings taken at the end of each traverse are used to continually calibrate the detection system and the necessary correction is carried out.

After the initial pre-processing, the final image is put onto the system disc. This allows for direct viewing on the operator's console. The picture is reconstructed in either a 320 x 320 matrix of 0.73 mm squares giving higher spatial resolution or in a 160x 160 matrix of 1.5 mm squares which results in higher precision, lower noise image and better discrimination between tissues of similar density. Each picture element that makes up the image matrix has a CT number, say between -1000 and +1000, and therefore, takes up one computer word. A complete picture occupies approximately 100 K words, and upto eight such pictures can be stored on the system disc. There is a precise linear relationship between the CT numbers and the actual X-ray absorption values, and the scale is defined by air at -1000 and by water at 0.

Obviously, the quality of the reconstructed image is a matter of the differentiation between

ď (X-ray attenuation coefficient) at different points and of the size of each pixel (square

dots of light whose intensity varies to reflect the attenuation). The differences in

of the various

body tissues are slight and that typical tissue contains mostly elements of low atomic weight. At the photon energy employed in CT scanners, the interaction of the photons with the tissue result in the 'Compton Effect', in which the impact of an X-ray photon with an electron is accompanied by a transfer of energy and a fall in the X-ray frequency. The loss in energy is proportional to the density of electrons which results in linear relationship between tissue density and attenuations. The differential attenuation coefficient is thus well-correlated with the specific gravity. Hence, the image reconstructed as a result of computerized tomography can be considered as the mapping of densities (Table 5.1) with respect to that of water.

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5.1.2. Contrast Scale

The linear attenuation coefficient of tissue is represented by the scanner computer as integers that usually range in values from-1000 to +1000. These integers have been given the name 'Hounsfield

Table 5.1

Specific Gravity and Attenuation Coefficient for Various Materials

Materials

Specific gravity



Water

1.00

0.205

Whole blood

1.034

0.214

4.3

-1

(cm )

 above water %

0.322 Heart muscle

1.04

0.212

3.4

Fat

0.93

0.190

- 7.8

Breast

0.97

0.189

- 8.4

Brain white matter

0.215

4.8

Grey matter

0.218

3.9

Menignioma

1.05

0.214

4.3

units, and are abbreviated as H. They are also denoted by CT numbers. The relationship between the linear attenuation coefficient and the corresponding Hounsfield unit is:

H 

where

   water  water

 1000

 water = attenuation coefficient of water. The CT number scale (Fig. 5.4) is defined in such a way that 0 is assigned to water and -

1000 to air. The value +1000 represents highly dense materials. Since neither the human eye nor the television display system is able to differentiate all the 2000 steps in this scale, only a section of the scale is represented on the video monitor.

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Fig. 5.4

CT number scale as used in computed tomography

Therefore, viewing systems have window level and window width controls (Fig. 5.5). These controls determine where and over what range of CT numbers will the video gray scale possibly lie. A decrease in window width enables us to see very small changes in tissue density more clearly, since the gray scale becomes spread over a smaller range and any given change in tissue density shows an increased contrast. However, it is necessary to move the window up and down the CT scale by means of the level control so that the absorption values under examination will be displayed between black and white on the monitor.

The equipment is provided with a facility for the selection of the window widths either in steps of 0, 32, 64, 128, 256, 512 and 1024, or freely. The middle position of the window can be set between -1024 and +1023 or between -512 and +1535.

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Fig. 5.5

Window level and window width control in CT

In the relevant range of the effective radiation energy between 60 and 80 keV, the differences in the attenuation values of plexiglass and water are largely constant. The measured CT values and the known

 -values

of these substances are employed in a CT number scale,

and the ratio of the difference of the CT values to the corresponding difference of the values is given by the slope of the straight line:

CTplex  CT H 2O I  K  plex   H 2O where

 plex   H O  0.024 cm1 2

 H O  0.197 cm1 at 66 keV 2

K  1.9  104 cm1 / CT value FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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The

value for various tissues can thus be computed by simply multiplying the

measured CT value by K.

It may be noted that this is, in fact, correct for all materials equivalent to water or plexiglass, e.g., soft tissue. But in a combination of bone and soft tissue, the actual

values

cannot be easily extracted from the CT numbers because the X-ray radiation is polychromatic, and subject to so-called beam hardening (Dichiro et al. 1978).

Most commercial CT machines have a spatial resolution of around 2 mm and almost without exception, the manufacturers claim a noise level corresponding to a 0.5% change in the X-ray absorption coefficient. Achieving this level of discrimination in a normal radiograph would require X-ray intensity differences of about 0.02% to be rendered visible on the film. (The linear 1

absorption coefficient of soft tissue is about 0.2 cm and the change of intensity would be 0.2 cm 1

x 0.2 cm X 0.5% = 0.02%). With conventional projection radiography, an intensity change of

0.02% is two orders of magnitude smaller than the maximum detectable change possible with that modality.

5.2. SYSTEM COMPONENTS

All computer tomography systems consist of the following four major sub-systems:

Scanning system - This takes suitable readings for a picture to be reconstructed, and includes X-ray source and detectors.

ii.

Processing unit - This converts these readings into intelligible picture information.

iii.

Viewing part - It presents this information in visual form and includes other manipulative aids to assist diagnosis.

iv.

5.2.1.

Storage unit - This enables the information to be stored for subsequent analysis.

Scanning System

The purpose of the scanning system is to acquire enough information to reconstruct a picture for an accurate diagnosis. A sufficient number of independent readings must be taken to allow picture reconstruction with the required spatial resolution and density discrimination for diagnostic purposes. The readings are taken in the form of 'profiles'. When a plane parallel X-ray beam is passing through a required section, a profile is defined as the intensity of the emergent beam plotted along a line perpendicular to the X-ray beam. This profile represents a plot of the FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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total absorption along each of the parallel X-ray beams. It thus follows that the higher the number of profiles obtained, the better is the resulting picture. In practice, 180 such profiles at 1째 intervals are normally needed to construct a diagnostically useful picture. There are several designs of scanning gantry commercially available from various manufacturers. They use different mechanical configurations. An excellent review of the physical aspects of X-ray transmission computed tomography is given by Webb (1987).

First Generation - Parallel Beam Geometry : In the basic scanning process, a collimated X-ray beam passes through the body and its attenuation is detected by a sensor that moves on a gantry along with the X-ray tube (Fig. 5.6(a)). The tube and detector move in a straight line, sampling the data 180 times. At the end of the travel, a 1째 tilt is made and a new o

linear scan begins. This assembly travels 180 around the patient's position. This arrangement is known as 'Traverse and Index' and was used in the earliest commercial system, the EMI MKI Brain Scanner. This procedure results in 32,400 independent measurements of attenuation, which are sufficient for the systems computer to produce an image. Obviously, this is a fairly slow procedure and requires a typical scan time of 5 minutes. It is essential for the patient to keep still during the entire scan period and for this reason, the early scanners were limited in their use to only brain studies.

Although this type of system is slow, its picture quality and hence its diagnostic utility, is exceptionally good. Scanning the brain with the early scanners provided the maximum immediate clinical benefit since traditional X-ray pictures of the brain are notoriously difficult to interpret.

Fig. 5.6 (a) Scanning arrangement of the early CT machines. They made a linear traverse before taking a 1째 rotation. The system employed single-source and singledetector system. It took long measuring times.

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However, in order to speed up the information gathering and to achieve a reasonable patient throughput, a pair of detectors was used so that two contiguous slices could be examined simultaneously.

The slowness of the earlier brain scanners precluded the possibility of scanning areas of the body other than the head. At best, the patient could be sedated to eliminate head movement, but it is obviously not possible to eliminate respiratory movements. Therefore, there could be chances of blurring the reconstructed image caused by movement of the patient or of internal organs, which necessitates reduction of the examination period to within breath-holding times. The inherent mechanical constraints of a traverse/index system mean that each traverse must take at least 1 s. So, it was unlikely that the machines based on this principle could ever be made so fast as to scan in less than 180 s.

Second Generation - Fan Beam, Multiple Detectors: An improved version of the traverse-index arrangement consists in using a bank of detectors and a fan beam of X-rays (Fig. 5.6(b)). This system effectively takes several profiles with each traverse and thus permits greater index angles. For example, by using a 10째 fan beam, it is possible to take 10 profiles, at 1째 intervals, with each traverse and then index through 10째 before taking the next set of profiles. Therefore, a full set of 180 profiles can be obtained with 18 traverses. This method has permitted a reduction in the scan time, and at the rate of approximately 1 s for each traverse, it has led to the systems operating in the 18-20 s range.

Third Generation - Fan Beam, Rotating Detectors: The main obstacle for a further increase in speed with the conventional computer tomographs arises from the mechanically unfavourable multiple alterations between the translational and rotational movement of the measuring system. Since the scanning of radiation absorption profiles of the object slice to be reproduced from several different projection directions is essential for the construction of a computer tomogram, the rotational movement of the radiation source cannot be dispensed with. On the other

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Fig. 5.6(b) Using a fan-shaped beam and an array of detectors, larger steps can be taken and the scanning process speeded up.

hand, the linear scanning movement can be avoided by using a sufficiently wide fan-shaped Xray beam which encompasses the whole object cross-section, and a multiple detector system mechanically tied to the tube which permits a simultaneous measurement of the whole absorption profile in one projection direction (Fig. 5.6(c)). Also, on account of the largeness of the measuring system consisting of X-ray tube and detectors, the rotational movement must not be stepwise but continuous.

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If the fan-beam is large, no traverse motion is needed. Only rotational movement of the scanning frame is required, thus offering considerable improvement in measuring time

Pure rotational machines have been developed on the basis of this principle. The simplest of these has the X-ray source and detectors mounted on a common frame and rotate around the patient, usually through 360째. The system gives a wide fan beam, typically between 30째 and 50째. The frame can be made to travel quite fast, so that a complete rotation takes only a few seconds.

This configuration has two major disadvantages. Firstly, it has a fixed geometry. With a fan beam set for the largest patient, the arrangement proves to be inefficient for smaller objects, particularly heads. Secondly, calibration of the detectors during scanning is not possible since the patient is always within the beam. Therefore, any drifts or faults in the detection system tend to produce a significant degradation in the picture quality.

Fourth Generation - Fan Beam, Fixed Detectors : In order to overcome the difficulties encountered in the rotating detectors configuration, rotational machines have been designed in which only the X-ray source rotates within a full circle of stationary detectors arranged around the patient. The system employs as many as 2000 detectors to maintain a good spatial resolution. The individual detectors are lined up practically without gaps, so that the radiation which has penetrated the patient is optimally used (Fig. 5.6(d)). The system permits calibration during scanning, which eliminates the problem of detector drift. However, the cost of such machines would obviously be high.

Fig. 5.6(d) The x-ray tube rotates while detectors remain stationary. This arrangement overcomes many problems of pure rotational systems

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Fifth Generation - Scanning Electron Beam : The 0.7 to 1 second time resolution limit of mechanical CT scanners makes phase-resolution imaging of the beating heart possible only through manipulations involving ECG triggering. The acquisition of all the cardiac phases within a single cardiac cycle can only be realized using a data acquisition system which does not contain any moving mechanical parts. One such system is the electron beam tomography (EBT) scanner (Schwierz and Kirchgeorg, 1995). Basically, the electron beam computed tomography differs from conventional CT in terms of speed and the method of generating the X-ray. In conventional CT scanning, an X-ray tube and an X-ray detector are mounted across each other on a circular frame and rotate around the patient. In electron beam tomography, the electron beam sweeps back and forth through a magnetic field. The impact of the electron beam on a semi-circular tungsten array underneath the patient generates the X-rays and the X-ray detectors are mounted on a semi-circular array above the patient (Fig. 5.7). Because an X-ray tube and X-ray detector are heavy moving parts, weighing as much as 250 kg, it takes one second or more to take all the snapshots which are later reconstructed to form an image of one slice of the body with a conventional CT scanner. Since an electron beam can be moved back and forth

Fig. 5.7

Schematic of ultrafast electron beam CT scanner

through a magnetic field very quickly, the time for scanning a slice can be of the order of 50 ms with electron beam tomography.

When combined with ECG triggering, EBCT can permit a comprehensive cardiac imaging and examination including the quantitation of flow rate over multiple heart beats. Cardiac images obtained with a conventional CT may be blurred due to motion artifact. In contrast, images of the

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heart obtained with electron beam tomography are precise and reproducible (Guerci and Kornhausee, 1994). The detector array consists of two continuous ranges of 216° with 432 channels each. Luminascent crystals coupled to silicon photo-diodes are used. The scanning electron beam emitted by an electron gun is accelerated by 130 - 440 kV, electromagnetically focused and deflected over a target in a typical time of 50-100ms. It was originally designed for cardiac examinations. The unit was equipped for this purpose with four anode rings and two detector rings which enabled eight contiguous slices, an area of approximately 8 x 8 mm, to be scanned without movement of the patient. The basic difference between an electron beam scanner and conventional units is that the patient is encircled by stationary anode rings which can thus be cooled directly. By serially scanning all four rings, a multiple-slice examination can be performed (Webb, 1987). Spiral /Helical Scanning : This is a scanning technique in which the X-ray tube rotates continuously around the patient while the patient is continuously translated through the fan beam. The focal spot therefore, traces a helix around the patient. The projection data thus obtained allow for the instruction of multiple contiguous images. This operation is often referred to as helix, spiral, volume or three-dimensional CT scanning. This technique has been developed for acquiring ^images with faster scan times and to obtain fast multiple scans for three-dimensional imaging to obtain and evaluate the ‘Volume’ at different locations. Figure 5.8 illustrates the spiral scanning technique, which causes the focal spot to follow a spiral path around the patient. Multiple images are acquired while the patient is moved through the gantry in a smooth continuous motion rather than stopping for each image. The projection data for multiple images covering a volume of the patient can be acquired in a single breath hold at rates of approximately one slice per second. The reconstruction algorithms are more complex because they need to account for the spiral or helical path traversed by the X-ray source around the patient (Kalender,1993).

Fig. 5.8 The spiral CT scan principle in multislice scanning (after Theobald et al, 2000)

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Spiral CT has a special advantage in that it allows images to be reconstructed at arbitrary positions and arbitrary spacing, also resulting in overlapping. This offers a great advantage if slices at small spacings are required for the clear proof of a small lesion. The continuous acquisition of whole sections of the body, largely independent of respiration or movement, also permit the reliable localization of small lesions. Continuous data acquisition in the trunk of the body with the possibility of the reconstruction of overlapping slices could not previously be achieved (Becker et al, 1999).

A fundamental difference between and potential disadvantage of spiral CT as compared with conventional CT is the fact that slice sensitivity profiles are blessed by the movement of the patient in the Z direction. The degree of blurring depends upon the speed at which the patient is moved and has a corresponding influence on the spatial resolution perpendicular to the scan slice. However, this can be largely minimized by using suitable de-blurring software. In the normal case, this blurring is almost negligible if the selected table speed per 360째 revolution is the same as the slice thickness (Theobald, et al 2000).

The SOMATOM Plus from Siemens and Toshiba 900S were the first units which offered spiral CT in 1987 and for the first time, made possible scan times of only 1 second per 360 degree scan.

Use of Slip Rings : In a conventional CT scanner, the input power is applied to the transformer, which is located separately from the gantry. The transformer steps up the voltage to the level of 80-150 kV. The high voltage is supplied by special cables, which are attached to the X-ray tube in the gantry. A sophisticated cable management system allows the tube free access for about 400 degrees of rotation in either direction. Therefore, the tube must rotate first in one and then in the opposite direction during scanning.

In third and fourth generation CT systems, it was realized that the power and signal cables would have to be eliminated as they would otherwise have to be re-wound between scans. A completely new concept to achieve this, was developed, by using self-lubrication slip ring technology, to make the electrical connections with rotating components. In the high voltage slip ring CT scanner, the input power is applied to the transformer which is located separately from the gantry. The high voltage is then connected to a ring inside the gantry. The X-ray tube has cables which are attached to metal brushes that make physical contact with the ring and transfer the high voltage to the X-ray tube. This allows for unlimited freedom of rotation in either direction for the X-ray tube. The high voltage slip rings have proven to be virtually maintenance-free and extremely reliable over several years of testing. The special arrangement of the slip rings has FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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100

rendered the use of oil or gas for insulating the high voltage unnecessary and thus precluded the possible danger of a leak. In practical operation, electric power of upto 40 kW and voltage of up to 140 kV can be transmitted (Alexander and Krumme, 1988).

In an alternative arrangement, low voltage slip rings can be used to connect the input power directly to the ring inside the gantry. A small high frequency transformer is located inside the gantry at a distance of about one metre from the X-ray tube. The transformer has cables which make physical contact with the ring through metal brushes to transfer the low voltage to the transformer. The high voltage generated by the transformer is then supplied to the X-ray tube by short high voltage cables. This allows for unlimited freedom of rotation, in either direction for the X-ray tube.

X-ray Source: In CT scanners, the highest image quality, free from disturbing blurring effects, is obtained with the aid of pulsed X-ray radiation. During rotation, high voltage (120 kV) is applied at all times, A grid inside the tube prevents the electron current from striking the anode except when desired, allowing the X-rays to be emitted in bursts. As the gantry rotates, an electric signal is generated at certain positions of the rotating system, e.g., in the 4.8 second scan, 288 electrical pulses are generated at intervals of 1/60 s around the circle. Each pulse turns on the Xrays for a short period of time. The number of pulses, the pulse duration and tube current determine the dose to the patient. These factors can be selected by the operator in the same way that they are selected in conventional X-ray systems.

Since the beam is on for only a short period of time, the motion of the patient during the measurement has to be minimized to ensure that the resolution does not get degraded. For producing a fan beam, a collimator is incorporated between the X-ray tube and the patient. A filter inside the collimator housing shapes the beam intensity. Actually, in body scanners, there are two filters, one for bodies and the other for heads which are automatically selected by the computer. These filters produce an intensity variation which, when coupled with the roughly-round shape of the patient, significantly reduces the dynamic range requirements on the electronics.

The fan of X-rays extends beyond the patient diameter so that X-rays which are not attenuated can enter the detector. The intensity of these non-attenuated X-rays is measured in order to correct the data for variations in the X-ray tube output.

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Two main types of X-ray tubes have been utilized for computed tomography. The first is an oil-cooled fixed anode line - focus continuous tube, which was principally, used in first and second generation CT scanners. They utilized a tungsten target with a target angle of about 20 degrees. The line focus is provided by a 2 x 16 mm spot. The second type of tube used in the later generations of the scanners is the rotating anode air-cooled pulsed X-ray source. These tubes have a higher power capability for exposure times in the 2-20 second range. The power requirements of these tubes are generally variable within 100-160 kV. Typical power requirements of these tubes are 120 kV at 200-500 mA, producing X-rays with an energy spectrum ranging from approximately 30-120 keV. Most systems have two possible focal spot sizes, approximately 0.5 x 1.5 mm and 1.0 x 2.5 mm. A collimeter assembly is used to control the width of the fan beam between 1.0 and 10 mm, which, in turn, controls the width of the imaged slice.

All modern systems use high frequency generators, typically operating between 5 and 50 kHz. With the production of X-rays in the X-ray tube being an inefficient process, most of the power delivered to the tube results in heating up of the anode. A heat exchanger on the rotating gantry is used to cool the tube. Spiral scanning especially places heavy demands on the heat storage capacity and cooling rate of the X-ray tube. A new X-ray tube based on liquid-metal-filled, spiral-groove bearings which allow very high continuous power, has been developed to meet this requirement. New applications such as CT angiography have become possible with these developments.

The major sources of drifting in CT scanners are variations in output of the X-ray tube and detector electronics. The reference channels included in the system correct the X-ray tube drifts. The electronics have two built-in stability circuits. The first is a switch at the input of each channel which can connect the electronic amplifiers to a battery and resistor to measure and correct for any type of electronic drift. This is done automatically by the computer. The second electronic calibration occurs on every detector channel between each X-ray pulse. Since X-rays are not present, the circuits provide zero electronic output between pulses. It may be appreciated that it is not only the number of projections and the measured data per projection which are of importance for the detail resolution that can be obtained, but also the size of the detector, the dimensions of the X-ray beam impinging upon the detectors and the path of the focal spot over the duration of the X-ray pulse, which are important factors the limit the resolving power. In principle, the factors giving rise to 'unsharpness' that have to be taken into account, are very similar to those encountered in any other X-ray imaging system.

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Detectors : For a good image quality, it is important to have a stable system response and in that, detectors play a significant role. The detectors used in CT systems must have a high overall efficiency in order to minimize the patient radiation dose, have large dynamic range, be very stable with time and insensitive to temperature variations within the gantry. Figure 5.9 shows the three types of detectors commonly used in CT scanners. Fan-beam rotational scanners mostly employ xenon gas ionization detectors. The schematic diagram of the detector shows that X-rays enter the detector through a thin aluminium window. The aluminium window is a part of a chamber that holds the xenon gas, which fills the entire chamber. Only one gas volume is present so that all detector elements are under identical conditions of pressure and gas purity.

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The detector volume is separated into several hundred elements or cells. In a typical scanner, these cells subtend the 42 cm maximum patient diameter. There are 511 data cells and 12 reference cells for simultaneous data collection per view. The detector cells are defined by thin tungsten plates. Every other plate is connected to a common 500 V power supply. The alternate

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plates are collector plates and are individually connected to electronic amplifiers. X-rays which enter the gas volume between the plates interact with xenon, thus producing positive ions and negative electrons. The positive voltage accelerates the ions to the collector plate and produces an electric current in the amplifier. The resulting current through the electrode is a measure of the incident X-ray intensity.

The xenon detector is inherently a stable detector. Since the detector operates in an ionization mode rather than a proportional mode, small changes in voltage and temperature produce no measurable change in detector output. This is vastly different from photo-multiplier tubes which require almost continual calibration. The main advantages of xenon gas detectors are that they can be packed closely and that they are inexpensive. The entrance width can be as small as 1 mm. In the fixed detectors-rotating source scanners, the detectors do not have to be packed closely. Therefore, scintillation detectors are employed as opposed to ionization gas chambers. Most scintillation detectors are made of sodium iodide, bismuth germanate and cesium iodide crystals. The crystals transform the kinetic energy of the secondary electrons into flashes of light which can be detected by a photo-multiplier.

The scintillator-photo-multiplier detectors suffer from the disadvantage that the smallest commercially available photo-multiplier tube has a diameter of 12 mm. Consequently, they are employed only in translation-rotation and stationary detector arrays.

Siemens employs the SCINTILLARC detector system comprising scintillation crystals and photo-diodes in their SOMATOM machines. In this system, 520 CsI crystals, assembled with photo-diodes, are arranged on a 42째 arc. In the radiation entrance plane, the detectors have very small dimensions of only 1.2 mm x 13.5 mm, thus permitting a good resolution. Owing to the finegrid like separation of the scattered radiation collimator, high percentages (75%) of the X-ray quanta actually reach the detectors. Also, about 97% of the incident quanta can be converted into an electrical signal.

Many modern scanners use solid state detectors such as-single crystal CdWO4 or ceremic Gd2O2S, with photo-diodes which have some inherent advantages such as a higher efficiency in detecting X-ray photons. One of the next developments to be expected is the use of multi-array detectors, i.e. a number of parallel rings of solid state detectors. This will allow for faster volume scanning.

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105

Data Acquisition System: Although good detector properties are a pre-requisite for obtaining optimal image quality, the measuring electronics must have a large dynamic range to back up the detector. The dynamic range defines the ratio of the smallest, just detectable signal to the largest signal without causing saturation. The dynamic range in a typical situation is 1:4,00,000. This implies that with such systems, an optimal image will always be obtained irrespective of whether the patient is obese or thin, or whether we are concerned with bones or soft tissues.

A typical data acquisition system is shown in Fig. 5.10. It consists of precision preamplifiers, current to voltage converter, analog integrators, multiplexers and analog-to-digital converters. Data transfer rates of the order of 10 Mbytes/s are required in some scanners. This can be accomplished with a direct connection for systems having a fixed detector array. The third generation slip ring systems make use of optical transmitters on the rotating gantry to send data to fixed optical receivers.

Fig. 5.10

Data acquisition system in a CTscanner

Processing Unit : Although for the CT image, the patient slice is divided up into numerous three dimensional voxels, the image of the slice is a two-dimensional picture in which each picture element (pixel) value corresponds to the attenuation coefficient of a voxel in the object slice. Figure 5.11 illustrates how the iterative or successive approximation method may be used to obtain an image of attenuation coefficients from the measured intensity data. Suppose the attenuation coefficients of the objects (not known before hand) in the first row is 4 and 6, and in the second row it is 1 and 8, representing the characteristics of tissue within the patients. When the object is scanned with X-rays, the sum of the values along various rays / directions are obtained. For example, for scan I, the vertical sums 5 and 14 are obtained; for scan II, the diagonal sums are 1,12 and 6, and for scan III, the horizontal sums of 10 and 9 are obtained. This scan data will be now used to calculate the image matrix.

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As the first step, the data from scan I is back-projected or distributed along the appropriate vertical column with equal weighting, by making the first estimate by placing 5/2 (2.5) in each pixel of that column. Similarly, the second column data value 14, is back-projected, giving 14/2 (7) for pixel in the second column. The matrix of the resulting image of the first iteration is next summed up diagonally and its ray sums of 2.5,9.5 and 7 are compared with the experimental data of 1,12 and 6 obtained from scan II. The differences of -1.5,2.5 and -1 are back-projected with equal weighting diagonally so as to match the experimental data of the scanned object. Similarly, the resulting image matrix of the second iteration is now summed up horizontally to obtain the third iteration result. It is obvious that with more and more iterations, the image matrix matches more and more closely with the object matrix, thereby generating the image of an unknown object with the help of a computer

Fig. 5.11

Principle of iterative reconstruction method

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The information received by the computer from the scanning gantry needs to be processed for reconstructing the pictures. The data from the gantry contains information on the following parameters: 

Positional information, such as which traverse is being performed and how far the scanning frame is along its traverse;



Absorption information including the values of attenuation coefficient from the detectors;



Reference information that is obtained from the reference detector that monitors the Xray output; and



Calibration information that is obtained at the end of each traverse. The first stage of computation is to analyse and convert all the collected data into a set of

profiles, normally 180 or more. However, the main part consists of processing the profiles to convert the information which can be displayed as a picture and then be used for diagnosis. In general, the reconstruction methods can be classified into the following three major techniques:  Back projection, which is analogous to a graphic reconstruction;  Iterative methods, which implement some form of algebraic solution; and  Analytical methods, where an exact formula is used. Two of these are filtered-back projection, which incorporates the convolution of the data and Fourier filtering of the image, and the two-dimensional Fourier reconstruction technique. The method of back projection without any further processing is simple and direct. In this method each of the measured profiles is projected back over the image area at the same angle from which it was taken. At the same time, each projection contributes not only to the points that originally formed the profile, but also to all the other points in its path. This technique in fact produces 'starred' images (Fig. 5.12(a)) and blurring, which makes it totally unsuitable for providing pictures of adequate clarity for medical diagnosis.

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Fig. 5.12(a) By adding the back projections produced by the shadow functions, the backprojected rays are added to the reconstructed image as artefacts or unwanted points. The original circular structure is transformed into a star shaped display. The earlier brain scanners used the iterative technique which took a succession of back projections correcting at each stage until an accurate reconstruction was achieved. The method requires several steps to modify the original profiles into a set of profiles which can be projected to give an unblurred picture. The technique, however, tends to require long computation.

Current commercial scanners use a mathematical technique known as convolution (Fig. 5.12(b)) or filtering. This technique employs a spatial filter to remove the blurring artifacts. This is achieved by convolving the shadow function with a filter so that each point in the projection has a negative value instead of 0, at every point other than its proper place in the projection. The resulting profiles are then

Fig. 5.12(b) Filtered back projection technique of eliminating the unwanted cusp like tails of the projection. The projection data are convolved (filtered) with a suitable processing function before back projection. The filter function has FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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negative side lobes surrounding a positive core, so that in summing the filtered back projections, positive and negative contribution cancel outside the central core, and the reconstructed image resembles the original object back-projected and added. Thus, the negative portion of each shadow function cancels out image artifacts that would otherwise be caused by other functions. Mathematically, the method of fast Fourier transform offers a powerful tool in making the required computations and special purpose high speed computers are now available to meet this requirement. The use of this method enables pictures to be reconstructed within a few seconds. Figure 5.13 shows a block diagram image reconstruction computer, used in CT scanners.

In principle, the blurring effect is counteracted in the convolution process by means of a weighing of the scan profiles. The nature and degree of the weighing is determined by the 'convolution kernel', wherein the convolution has an effect on the image structures. Thus, for example, it can be edge-enhancing, so that the bone / soft part interfaces within the skull are particularly clearly emphasized or it can have a 'smoothing effect' with the aim of producing a more uniform image structure. The 'smoothing' convolution kernel reduces image noise and such errors which, for example, can occur with motion artifacts. However, the details are more poorly resolved. The convolution kernels for the head take account of the bones forming the outer housing of the head, in such a way that the so called 'cupping' effect is suppressed. Gilbert et al. (1981)

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Fig. 5,13

110

Block diagram of the image computer. The synchronous reconstruction of the image permits the representation of the tomogram on the video monitor immediately upon completion of the scan (Courtesy: Siemen, Germany).

review several computer software and special purpose digital hardware implementations of different forms of algorithms, either proposed or actually implemented in commercial or research CT scanners.

Computer Systems : The computer system plays a central role in CT scanning because without it, there would be no image computation and formation. The computer controls X-ray generation, gantry and table motion, data acquisition, image formation, display and storage. Usually, the CT computer system includes a microcomputer for control functions, an array processor and video memory to enable viewing of the reconstructed images. The image can be viewed on a console and a hard copy can be made on a multi-format camera. Figure 5.14 illustrates a typical computer system employed in a CT scanner. It uses twelve independent processors connected by a 40 Mbyte /s multibus configuration. A multiple array processor is used to achieve the computational speed of 200 Mflops (million floating-point operations per second). The reconstruction time from such a configuration is approximately five seconds to produce an image on a 1024 x 1024 pixel display. A multiuser and multi-tasking environment is provided by a simplified UNIX operating system.

5.2.3. Viewing System

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In most of the CT systems, the final picture is available on a television type picture tube. The picture is constructed by a number of elements in a square matrix wherein each element has a value representative of the absorption value of the point in the body which it represents. This technique facilitates a much larger dynamic range than the eye can possibly have. The absorption values are displayed on a linear scale corresponding to air through tissue to dense bone, etc. Several values have been assigned to the two terminal points of the scale. For example, in some cases like the original EMI scale, air is assigned the value of -500, water the value of 0 and bones, that of + 500. In the CT scanners that are presently being manufactured, the picture points are divided into 2000 steps. The resolution of the scale thus obtained is 1 promille difference in absorption related to the attenuation value for water.

Fig. 5.14

Typical computer system organization for a CT. The system makes use of

motorola family 68000, 16 bit microprocessors (Courtesy: M/s Pickers, USA).

In order to facilitate image display, it would be better if the scale could be expanded within this range. For this purpose, recourse is made to the selection of an image window. The information content of this window is spread over the representable range of colour or gray scale.

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As long as the original image data of the CT scan is present in the image reconstruction store of the image computer or in the image display memory of the monitor, the image window of interest can be varied in two parameters, namely window level and window width. These two parameters can be varied at will within the range of the absorption values.

Windowing is a powerful aspect of CT and shows the underlying mathematical nature of the displayed picture. This helps in defining the region of interest from where various calculations can be performed on the enclosed elements. The commonly used calculations are of the area, mean value and standard deviation which may well show an identifiable difference between healthy and diseased tissue. It is also possible to subtract one picture from another to demonstrate differences that have occurred during treatment.

5.2.4. Storing and Documentation

For subsequent processing or evaluation of a CT picture, various methods of storage are used. The picture is stored in the digital form so that the evaluation is convenient on a computerassisted programme. For this purpose, the data carriers generally employed are magnetic disc, magnetic tape and floppy disc. Most manufacturers of CT units use the magnetic tape or floppy disc. The floppy disc provides a medium-range storage. The capacity of a bilaterally coated disc is around 20 pictures, having a matrix of 256 x 256 and depth of information of 10 bits. Floppy discs offer advantages such as ease of handling, low maintenance costs of the drive mechanism, the considerably short access time and the possibility of patient-related storage of the data carrier. For long-term storage, magnetic tapes are preferred. They are inexpensive and extremely reliable, but retrieval of the image is time-consuming and they are sensitive to environmental influences.

There are other possibilities for storing pictures which would normally only store one specific setting of window width and window level. The most common of these is a photograph which is taken from a slave monitor. It is a replica of the picture displayed on the screen at any instant. For photography, one can choose between the multiple-format camera and the 100 mm cut film camera which is available for the magazine technique. The multiple-format camera has a capacity of nine 70 mm pictures or four 100 mm pictures per sheet. A100 mm cut film camera with the magazine technique and automatic exposure control permits the recording of up to 100 pictures without changing the magazine.

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Hard copy print-outs can be made in the gray scale or in colour shades. In the gray scale picture, the shades of gray are restricted to 8 or 10. Besides this, the picture obtained is rather fuzzy and poor in contrast. The CT picture in colour can be made by using 32 colour shades using the ink-writing system, the image resolution being 256 x 256 points. A further possibility of image documentation is the print out of isodensity lines. The hard copy print-out permits the possibility of obtaining an image scaled 1:1. This scale is a prerequisite to be met particularly in therapy planning. Figure 5.15 shows a set of typical CT scans.

Fig. 5.15 (a)Typical CT scan of the brain

(b) CT scan of the abdominal region

NUCLEAR MEDICAL IMAGING SYSTEMS 5.3. RADIO-ISOTOPES IN MEDICAL DIAGNOSIS

Radio-isotopes are used in medicine both for therapeutic as well as diagnostic applications. In diagnostic practice, small amounts of radioactive chemicals, called 'tracers' (or radio-pharma-ceuticals), are injected into an arm vein or administered through ingestion or inhalation. The amount of radioactivity at different points within the patient's body, or in body fluids, is then examined by radiation detectors. Using these detectors, the amount of radioactivity can be measured within parts of organs as well as within the whole organ. The images show where biochemical processes are occurring normally and where they are occurring too slowly or too quickly.

Among the earliest procedures involving the use of radioactive tracers in medicine was measurement of the uptake of radioactive iodine by the thyroid gland. In the early 1940s, it was found that the rate of uptake of iodine by the thyroid greatly increased in patients with disease

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characterized by increased production of thyroid hormone (hyperthyroidism), a disease that led to nervousness, tremor, weight loss and, in extreme cases, even death. Other patients exhibited decreased iodine uptake (hypothyroidism) by the thyroid and had symptoms and signs of diminished thyroid function. Thus, the radioactive tracers have been in use in medical diagnostics for a long time and even today, the thyroid gland continues to be the organ most frequently examined by nuclear medicine.

In nuclear medical diagnostics, the imaging of organ functions is carried out noninvasively.

In contrast to other imaging diagnostic modalities (ultrasound, X-ray, MRI), the

nuclear medical examination approach is primarily function-oriented. In this case, vital processes such as blood circulation, metabolism and vitality of organs and tumours can be displayed as functional images.

The clinical use of radio-nuclide imaging depends on obtaining a suitable distribution of the radio-nuclide in the patient. The radio-nuclide is labelled to a compound which will be taken up or metabolized in some way by the human tissue to be studied. The patient receives the material, usually by intravenous injection, and after a suitable delay, which may be minutes or hours, to allow uptake in the target tissues and clearance from the blood, imaging can commence. In this way useful static images, each taking 2-10 minutes, can be produced of the bone, brain, thyroid, lung etc. Dynamic studies can be performed with the gamma camera, starting at the moment of injection and capturing frames of the data either photographically or digitally, at times ranging ' from minutes down to fractions of a second. Numerical analysis of such data can produce useful information on organ function, blood flow, clearance rates, etc. (Keyes, 1987).

Tecnetium-99m (Tc-99m) has proven to be the most important imaging radio-nuclide used to examine the brain, liver, lungs, bones, thyroid, kidney and heart. It combines the advantages of optimum radiation properties (emission of exclusively gamma radiation with suitable energy, short half-life of six hours) and general availability as a generator nuclide. However, TC-99m cannot be coupled with all required biologically active substances, so that with this radio-nuclide the spectrum of radio-pharmaceuticals for examinations of the organ metabolism is limited. For example, Iodine-123 labelled substances, which are used in many clinical examinations, therefore represent an important supplement to technetium studies.

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5.4. PHYSICS OF RADIOACTIVITY

From the theory of atomic structure, it is known that some elements are naturally unstable and exhibit natural radioactivity. On the other hand, elements can be made radioactive by bombarding them with high-energy charged particles or neutrons, which are produced by either a cyclotron or a nuclear reactor. This process will alter the ratio of photons to neutrons in the atoms, thus creating a new unstable nucleus which could undergo radioactive decay. The extra neutron disintegrates and in the process, releases energy in the form of gamma radiation. Radioactive emissions take place in the following three different forms:

Alpha Emissions : Alpha particles are composed of the two protons and two neutrons. They are least penetrating and can be stopped or absorbed by air. They are most harmful to the human tissue.

Beta Emissions : Beta particles are positively or negatively charged, high speed particles originating in the nucleus. They are not as harmful to tissue as alpha particles, because they are less ionizing, but are much more harmful than gamma rays.

Gamma Emissions: Like X-rays, Gamma particles constitute electromagnetic radiation that travels at the speed of light. They differ from X-rays only in their origin. X-rays originate in the orbital electrons of an atom, whereas gamma rays originate in the nucleus. They are caused by unstable nucleus. X-rays and gamma rays are also called 'photons' or packets of energy. As they have no mass, they have the greatest penetrating capability. Gamma rays are of primary interest in nuclear imaging systems. The energies of alpha and beta particles and gamma radiations are expressed in terms of the electron volt. One electron volt signifies the energy that an electron would acquire, if it were accelerated through a potential difference of one volt. Radioactive emissions have energies of the order of thousands or millions of electron volts. Alpha emission is characteristic of the heavier radioactive elements such as thorium, uranium, etc. The energy of alpha particles is generally high and lies in the range 2 to 10 MeV (millions electron volt). Due to the larger ionizing power of alpha particles, they can be distinguished from beta and gamma radiations on the basis of the pulse amplitude that they produce on a detector. Beta emissions have an energy range 0-3 MeV.

5.5. NUCLEAR MEDICINE IMAGING 5.5.1. Uptake Monitoring Equipment

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The clinical use of a radio-nuclide in medical investigations depends on obtaining a suitable distribution of the radio-nuclide in the patient. This is achieved by administering a suitable chemical substance tagged with a.radio-nuclide, emitting gamma radiations. The biological system under investigation selectively assimilates the administered dose to carry out its function. Since the gamma ray gets transmitted through the body tissues, an external monitoring system can be used to detect them and provide the measurement of the chemical substance. The fraction of the chemical present in the organ at any time would indicate the functional status or what is called the uptake of the organ. The most suited gamma energy range for uptake monitoring studies is from l00 keV to 500 keV.

Figure 5.16 shows a functional block diagram of a typical gamma counting system. It basically makes use of a NaI (TI) scintillation crystal for detection of gamma rays. This is followed by the photo-multiplier which converts the scintillations into an electrical signal. The output from the photo-multiplier is given to a pre-amplifier followed by a pulse shaping circuit and a pulse height analyser. The output of the analyser drives a counter/timer which displays the information on a digital counter and a strip chart recorder. A vital component in the system is the collimator the function of which is to exclude from the-, detector all gamma rays except those travelling in the preferred direction. A simple collimator, consists of a single tapered hole in a cylindrical lead

Fig. 5.16

Gamma counting system for in vivo measurements

block. The sides of the lead block must be sufficiently thick to absorb the majority of the gamma rays impinging obliquely, and the detector must be surrounded by an adequate thickness of lead to effectively shield it from all gamma rays except those entering through the aperture. The FOR MORE DETAILS VISIT WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554126

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shielded and collimated detector is called a probe. The probe is mounted on an adjustable support allowing it to be appropriately positioned in relation to the patient. The measurement can be carried out using the single probe or multi-probe counting system.

5.5.2. Radio-Isotope Rectilinear Scanner

The distribution of radioactive material within an organ or part of the body is studied by using radio-isotope rectilinear scanners. The scanner is a moving detector imaging system with a block diagram shown in Fig. 5.17. Heart of the system is the detector-collimator assembly. The detector is usually a three or five inch diameter NaI crystal, situated behind a focusing collimator. This is so mounted that it can travel in a regular scanning pattern back and forth across the area of interest, so that detected and amplified signals can be plotted to give a picture or contour map of radioactivity within the organ. Usually, the detector-collimator assembly, the photo-multiplier and the pre-amplifier are housed in a single unit, which is attached to a motor-driven device. This device defines the lateral and longitudinal limits of the scan.

A single probe scanner makes use of one detector that scans the area of interest. There are dual probe scanners that have two synchronously moving, axially opposite detectors with the patient between the two detectors. The scanning can be linear or one-dimensional. In the whole body counting applications, the detector is moved continuously over the body and the counts are integrated over the entire scan.

The recording may be done either by a photographic recorder or by dot recorders. In a photographic recorder, the light flashes can be photographed on a film, from the face of a cathode ray tube. The dot recorder is most commonly used. It produces a map (Fig. 5.18) of the distribution of activity within the area of interest by recording dots or slit-like marks on paper. The dot recording mechanism consists of an electrically heated stylus to burn a small spot on a sheet of electrically

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Fig. 5.17.

Block diagram of a typical rectilinear scanner

Fig. 5.18.

Typical scintogram using a dot recorder

118

conducting paper, each time a pulse passes through the stylus. The pulses to the stylus are delivered from the pulse height analyser after scaling down the counts by an adjustable scaling factor from 1 to 256. A scaling factor of 16, for example, would mean that for every 16 counts arriving at the input of the scaling circuit from the pulse height analyser, one dot appears on the paper. This reduction in counting rate is necessary, because extremely high counting rates will drive the stylus wild. A count-rate metre is also incorporated to display or record the average count rate.

Some scanners make use of colour printing. In this technique, the maximum count rate is first established by moving the detector on the patient's body. This is then divided into six ranges, each being associated with a different colour print. As each count rate is recorded, it is allocated

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to one of the groups and the corresponding colour is printed. In this way, a coloured map showing the distribution of the isotope is built up.

5.6. THE GAMMA CAMERA

Gamma cameras are used to produce images of the radiation generated by radiopharmaceuticals within a patient's body in order to examine organ anatomy and function, and to visualize bone abnormalities. The wide variety of radiopharmaceuticals and procedures used allows for evaluation of almost every organ system. In addition to producing a conventional planar image (a two-dimensional image of the three-dimensional radio-pharmaceutical distribution within a patient's body), most stationary gamma camera systems can also produce whole-body images (single head-to-toe skeletal profiles) and tomographic images (cross-sectional slices of the body acquired at various angles around the patient and displayed as a computer-reconstructed image).

The gamma camera was developed by Anger (1958). He used a large circular area of thin scintillation crystal and an array of closely packed photo-multiplier tubes to amplify and locate the gamma ray interactions in the crystal and to display the scintillations instantly on a cathode ray tube. The camera could then be used to study the rapidly changing distribution of activity, after which dynamic studies could be performed.

The gamma camera is a stationary imaging device as opposed to the rectilinear scanner in which the detector is made to move over the organ of interest. In the case of the gamma camera, the whole organ under study is viewed during the entire period of data collection. This enables fast dynamic function studies of various organs to be carried out conveniently.

Modern-day gamma cameras constitute extremely complex electronic equipment, consisting of the following functional components (Fig. 5.19).

Detector : This consists of a Collimator, crystal, photo-multiplier tubes, position localization circuitry.

Camera Electronics : This includes correction circuitry, energy analysis circuitry, counting circuit, image display and image recording device.

Briefly, the function of gamma camera is as follows:

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When a photon of the radiation leaves the patient's body, it passes through the collimator and interacts with a crystal wherein its energy is converted into light. The light from the crystal is received by photo-multiplier tubes and converted into an electrical signal. The electrical signal passes through the position localization circuitry whose output consists of X and Y positional signals, and a Z or energy signal. The X, Y and Z signals are processed by special correction circuits which compensate for errors in the detection and localization of photon.

The Z or energy signal is then analysed in the pulse height analyser circuit to determine if the detected photon is within a user-specified energy range; if it is registered in the counter. The X and Y signals are then sent to an image recording

Fig. 5.19.

Block diagram of a gamma camera

device where they are used to position the beam of a cathode ray tube. The Z pulse then turns the beam on, causing a bright dot to appear at a location on the face of the CRT corresponding to the location in the crystal where the photon deposited its energy. This bright dot in turn exposes a film in the image recording device.

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Hundreds of thousands of photons leave the patient's body and strike crystal, each causing a black spot to be formed on the film. Eventually, an image of the distribution of the radionuclide within the body, made up entirely of dots, is created on the film.

Since gamma photons cannot be bent by using lenses, unlike light, a collimator is used to selectively absorb unwanted radiation: only photons traveling along the desired path are allowed to pass through to the detector. The collimator is usually made up of a heavy metal absorber such as lead, with some tungsten or platinum parts. The basic types of collimators used in conventional gamma camera imaging are pinhole, parallel-hole, diverging and converging collimators.

The modern gamma camera employs a crystal of up to 500 mm diameter, typically 6.4 mm or 9.6 mm thick with an array of 61, 75 or 93 photo-multiplier tubes. The equipment has become a prime general purpose instrument for radio-nuclide imaging in routine nuclear medicine.

The collimator normally consists of a very large piece of lead with many small parallel through-holes of equal cross-section. The number of gamma rays received by any region of the crystal is directly proportional to the amount of nuclide located directly below the region. Since the gamma rays travel in all directions, only about 0.01% of the rays emitted by the labelled organ are detected and used for image formation.

A polaroid camera is mounted on the oscilloscope for photographing the build-up of about 50,000 dots on the screen. In this way, a map can be used to study distribution of activity.

5.7. EMISSION COMPUTED TOMOGRAPHY

Emission computed tomography, provides in vivo three-dimensional maps of a pharmaceutical labelled with a gamma ray emitting radio-nuclide. The three-dimensional distribution of radio-nuclide concentrations are estimated from a set of two-dimensional projectional images acquired at many different angles about the patient. Several of the reconstruction algorithms are derived from the mathematical approaches used for transmission computed tomography. However, appropriate modifications have to be made to account for attenuation and photon scatter within the patient.

Emission computed tomography has developed in two complementary directions based on the type of radiation that is detected. One approach, positron emission tomography (PET),

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consists of the detection of annihilation coincidence radiation from positron emitter such as C-11, N-13, O-15, and F-18. When a positron (i.e., a positively charged electron) is emitted within tissue, it rapidly loses its kinetic energy in the same way that beta rays (electrons) lose their energy. The distance that the positron travels from the emission site depends on its initial energy, and typically has a range between 1 and 3 millimeters. After slowing down, the positron interacts with an electron, and both are annihilated, resulting in the emission of two 511 keV photons. To conserve momentum, the two annihilation photons are emitted in very nearly opposite directions (180째). Typically, one or more rings of discrete scintillators are used to detect the two photons (Fig. 5.20). Fast coincidence timing circuits minimize the detection of randomly occurring single events. Furthermore, collimation within the plane

not

required

since

the

emission point essentially lies on a line determined by the two crystals that detected the two coincident

Fig.

5.20

Principle

photons. Collimation is usually required, however, perpendicularly to the transverse plane.

PET

scanner (after Jaszczak, 1988) The second approach to emission computed tomography involves the detection of gamma rays emitted singly by the radio-nuclidic tracer. This approach, referred to as single photon emission computed tomography (SPECT) requires collimation within the transverse plane as well as in the perpendicular direction. SPECT uses conventional radionuclides such as Tc99m (140 keV gamma photon) and TI-201 that are routinely used in all nuclear medicine departments.

SPECT detectors typically consist of Na(TI) scintillators mounted in a specially designed gantry. The system illustrated in Fig. 21.13 uses a conventional scintillation, or gamma camera that rotates about the patient to obtain a set of projectional views over 360째. These views are then used

reconstruct

the

regional

radio-

pharmaceutical concentrations within the body. Since the gamma camera obtains two-dimensional images, the entire organ of interest can be imaged

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with a single rotation of the camera about the patient. Although

presently most SPECT systems

are based on the Anger camera approach, discrete detector devices have also been developed.

5.7.1. Positron Emission Tomography (Pet Scanner)

Positron emission tomography is an imaging modality for obtaining in vivo cross-sectional images of positron-emitting isotopes that demonstrate biological function, physiology or pathology. In this technique, a chemical compound with the desired biological activity is labelled with a radioactive isotope that decays by emitting a positron, or positive electrons. The emitted positron almost immediately combines with an electron and the two are mutually annihilated with the emission of two gamma rays. The two gamma ray photons travel in almost opposite directions, penetrate the surrounding tissue and are recorded outside the subject by a circular array of detectors (Fig. 5.22). A mathematical algorithm applied by computer rapidly reconstructs the spatial distribution of the radioactivity within the subject for a selected plane and displays the resulting image on the monitor. Thus, PET provides a non-invasive regional assessment of many biochemical processes that are essential to the functioning of the organ being visualized.

Fig. 5.22 Principle of positron emission tomography (PET Scanner)

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The positron(  ) is emitted from a proton-rich nucleus with a variable amount of kinetic 

energy, the maximum amount being the endpoint energy (E  ), given for various isotopes in 

Table 5.2.

This energy is dissipated in the patient over a range of tissue of the order of a few millimeters. The 511-keV





combines with a free electron (  ) and the masses are transmuted to two 

rays which are emitted at 180° ± 0.25° to one another to satisfy conservation of

momentum as in Fig. 5.23. The variable finite range of the

Table 5.2

 are

Positron Emitters Commonly Used in PET

Isotope

E

1.74

2.07 m

0.96

20.39 m

1.19

9.96m

0.65

109.77 m

2.68

7.64 m

1.90

68.1 m

3.35

1.27 m

2.32

38.1 m



(MeV)

(min)

as well as the angular variation of about 180° fundamental

limitations

the

resolution

achievable with PET.

The compounds used and quantitated are labelled with proton-rich positron (  ) emitters 

Fig. 5.23 The basic decay process for a positron emitter

that are usually cyclotron-produced. The principal isotopes are

O, and

F. If the compound of interest is labelled in a

known position and it maintains this positron, a PET scan permits measurement of the positron concentration (  Ci / mL) in a small-volume element within an organ or region of interest. This 3

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A variation of 0.25Â° in an angular distribution of the back-to-back 511 keV

125

ď §

rays will

cause a degradation of 1.75 mm at the centre of an 85-cm whole-body tomograph. Neither of the basic limitations will cause a significant loss in resolution in present-day PET designs. They are simply fundamental to the method and cannot be eliminated.

Two design types of positron-emission tomographs have been introduced, one employing opposed large-area detectors which require rotation around the patient to provide the necessary degree of angular sampling, and the other, employing multiple individual crystal detectors surrounding the patient in a circular or hexagonal array. Conventional lead absorption collimators are not required because the coincident detection of two 511 keV photons indicates the line of origin along which the photons were emitted. However, in order to reduce the random coincidence count rate, some degree of collimation is normally employed. Pulse processing needs to be much faster than with single-photon systems, to keep random coincidences to manageable proportions. With fast-response detectors and suitably fast electronics, it may be possible to use the difference in the time of arrival of the annihilation photons at opposite detectors to locate the site of positron decay and improve spatial resolution.

Figure 5.24 illustrates gantry and detector components used by Hoffman et al (1985) in a PET system. The gantry has a large opening (diameter = 65 cm) and can image both the brain and torso of an adult patient. The entire detector assembly may be tilted to obtain oblique sections. Bismuth germanate (BGO) scintillation crystals, 5.6 mm wide, 30 mm high, and 30 mm deep, are used to

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126

Fig. 5.24. Gantry and detector modules used in PET Scanner (after Hoffman et al. 1985)

detect the 511 keV annihilation radiation. The detectors are arranged in a circular ring geometry, with 512 detectors per ring. The system has two rings and produces three scanning planes (two direct and one cross plane). In order to facilitate replacement, the detectors are arranged in modules or buckets containing 16 detector packages. Each package contains two crystals and two PMTs. The centre-to-centre spacing of the crystals is 6.1 mm. Axially, the two rings are separated by 36 mm. Besides containing the two BGO crystals and PMTs, the bucket also contains amplifiers / discriminators and other front-end processing electronics. In order to increase linear sampling, the entire detector assembly can wobble in a small circular orbit. This wobbling procedure is used to optimize spatial resolution (Jaszczak, 1988).

The original PET scanners were constructed using a thallium-doped sodium iodide [(NaI (TI)] detector. Its high efficiency at 511 keV, ease of fabrication, and low cost made it an obvious choice in a number of initial designs utilizing discrete crystals. Its principal disadvantage in PET work was the decreasing detection efficiency caused by the trend toward smaller crystals required for high resolution while maintaining a reasonably high total system efficiency. This limitation places a practical lower limit on the resolution that is attainable with NaI (TI)-based detection systems. With the development of a new scintillator material, bismuth germanate (BGO) (X = 79, p = 7.13 g/mL), with three times the stopping power of NaI(TI), a new generation of highresolution, high-efficiency PET scanners has become possible.

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A simplified block diagram of the data acquisition system is shown in Fig.5.25. Distributed processors are used throughout the system to maximize speed for simultaneous data collection.

Fig. 5.25. Data acquisition system for a PET Scanner (after Hoffman et. al. 1985)

Individual and analog detector signals are amplified and the time of interaction is then determined using a constant fraction discriminator. A time encoder converts the event into a 14 bit word containing the detector number and event time within 8 ns. This word is passed to the coincidence processor every 224ns. The energy window is controlled automatically by the microprocessor located in each detector bucket. A threshold of 200 keV is typically used to allow for detection of gamma rays that have been scattered within a detector and escaped.

The

system consists of a fan beam geometry with an angular sampling of 0.7 degrees. The linear sampling is 2.9 mm.

The main processor serves to monitor and control the various processing jobs. An array processor is used to perform the primary reconstruction. Several peripheral devices, including a display processor, are attached to the system computer.

The GE 4096 Positron Emission Tomography Camera System (Fig. 5.26) is a high resolution Whole Body PET Scanner. It uses a detector ring with a diameter of 101 cm. The 4096 individual crystals of the scanner are arranged in eight rings of 512 crystals each. Onto each set of 16 crystals, two dual photo-multiplier tubes are attached providing increased positional sampling. Each crystal is made up of Bismuth Germanate (Bi4Ge3O12) and is 6 mm trans-axial, 12

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128

mm axial and 30 mm radial in size. 64 individual detector cassettes allow for easy and fast servicing.

The system provides for interleaved imaging of 30 slices in a single acquisition interval either in wobble or stationary modes. The patient port is 57 cm which allows for large patient scanning. Patient positioning is accomplished with convenient operator controls on each side of the gantry. Enhanced patient positioning is provided via +20째 gantry pivot and tilt. Accurate and reproducible patient positioning is accomplished with a triple laser positioning system. The contoured, carbon fibre imaging table minimizes attenuation and additional padding allows for maximum patient comfort.

Horizontal or axial table positioning can be controlled manually from the gantry controls or by computer from the operator's console. The axial range of the table is 170 cm, the height is adjustable from 60 to 120 cm, and the maximum weight the table will support is 300 Ibs. Two, 6 mm diameter, pin-shaped

Ge sources (2 mCi and 10 mCi) extending over the entire axial field

of view are provided for transmission measurements, adjustment of gain and coincidence timing and normalization of detector efficiencies. The pin-source is continuously sampled providing accurate measurement of random and scattered events. The pin-sources are removable and are stored in a lead container away from the gantry when not in use.

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Fig. 5.26

129

PET Scanner from Mis GE Electrical, USA

The DAP (Data Acquisition Processor) contains a 68030 Processor and dual Intel i960 RISC Processors. The DAP controls the real-time acquisition of data for the system.

Radio-nuclide images are inherently very noisy and in comparison to most other types of images, they are of very poor quality. The radiation emitted by the object is X or gamma radiation, arising either directly from nuclear transitions or indirectly via positron emission or electron capture. In practice, surprisingly, little of radiation is used in the formation of the image, typical 3

-2

photon density is of the order of 10 cm as compared to about 10 cm in radiography and 10 cm

-2

in conventional photography. Statistical fluctuations in photon density, inherent in the

radioactive decay process, are usually apparent and spatial resolution is at best currently just under 1 cm in most clinical applications.

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5.8. RADIO PHARMACEUTICALS - AN EMERGING CURE

Radiation Risk Acceptable - The peaceful harnessing of atomic energy is evident in the form of radio pharmaceuticals which constitute a special class of drugs. Nuclear medicine covers the unique diagnostic, therapeutic and investigative procedures involving the use of radiopharmaceuticals.

These drugs can be successfully applied for the treatment of diseases. By "whole-body counter" 612 deficiency anaemia can be readily diagnosed. Mal absorption of other important nutrients such as fats, iron salts and carbolydrates etc. can be readily evaluated and several tropical diseases diagnosed. The response of individual patient to different therapeutic treatment can also be carefully monitored by these diagnostic investigations.

Imaging - Scientiscanning or imaging procedure enables evaluation of size, shape and location of organ in the body including thyroid, liver, kidney, spleen, lung, heart, stomach, gall bladder, brain and skeleton. Precise localization and delineation of cysts, abscesses, tumours, lesions and cancerous growth is of great help in their treatment by radiation therapy. Diseases of thyroid glands such as thyrotoxicosis, hypothyroidism and function disorders of kidney, heart and liver can be diagnosed by using radiopharmaceuticals. The dynamic recording of sequential images of heart following an intravenous 'bolus' injection provides a means of visualising both the anatomical and functional characteristics of heart and major blood vessels. Studies like cardiac out put, ejection fraction, pulmonary blood volume and diastolic ventricular volume etc. are useful to cardiologists.

'Renography' i.e. study of kidney functions is helpful in screening patients with high blood pressure to detect surgically correctable renovascular disorders and in monitoring kidney transplant. These investigations are non-traumatic and non-invasive causing minimum discomfort to the patient :

High energy radiations emitted by radio-isotops can be advantageously used for selective irradiation of certain tissues and for destruction of cancer cell. For example, sodium radio-iodide is successfully employed for the therapy of thyro-toxicosis and thyroid cancer while sodium radiophosphate is administered for the treatment of rare type of blood cancer called polycythemia vera.

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Recently in 1989 Dr. Darrell Fisher, Project Manager, USA applied the new technique for cancer treatment that involves the injection of radio-active antibodies into the body to destroy cancer cells. He also compared the effectiveness of different radionuclides. He reported that alpha emitters may be better against diffuse tumors and beta emitters against solid tumors. Tumors disappeared in three to six weeks from five patients terminally ill with cancer of the lymph glands.

Bone-crippling osteoporosis, a painful disease that afflicts millions of old women, was the isotope target of Dr. Robert Schenter. His promising programme uses the diagnostic properties of gadolinium-153, a radio-product of Handfordâ&#x20AC;&#x2122;s fast flux reactor. The latter produces gadolinium and other isotopes more efficiently than other reactors.

Dr. Joseph R, Castro, chief of radiotherapy, explained that not all radiations are harmful. In accelerator complex called "Wobbler hummed" - the accelerator particles can be used to cure malignant cells of chordoma, a massive tumor that engulfs a part of spinal cord without damaging other tissues or cells. Thus the fact that radiation burst peaks attack precisely on target makes the radiation risk acceptable.

Radio Immunoassay - It is a novel radio analytical technique which can be employed for the estimation of minute concentrations of important biological ingredients suck as harmones vitamins, drugs and viral proteins in body fluids. It offers an easy and reliable means of diagnosing the diseases. Radio-immunoassay of thyroid harmones can be used for the early diagnosis of thyroid diseases including hypothyroidism in infants. The incidence of neo-natal hypothyroidism is high in many developing countries and there is a large scope for low cost reliable technique such as radio immunoassay for mass screening of infant population. Assay of another hormone called human placental lactogen in blood serum of pregnant women enables early diagnosis of possible complications in pregnancy such as abortion and foetal distress.

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