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Essentials of Radiology: Common Indications and Interpretation 4th Edition Mettler Jr. Md Mph
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Child Abuse Medical Diagnosis and Management 4th Edition Antoinette Laskey Md Mph Mba Faap (Editor)
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Preface
Radiology continues to get little attention in most medical school curricula, expect perhaps as an elective. The classic gross anatomy lab has been dead (so to speak) for decades, and most health care providers learn human internal anatomy through radiology or electronic formats. This text is not meant you make you a radiologist, nor is it simply a book for a medical student elective, although it has been widely used for the latter. It is intended to be a text that will provide you with a basis for radiologic anatomy, imaging fundamentals, and appropriate imaging for most common clinical problems and be useful for years in your practice. The text is generally organized by clinical presentation (e.g., low back pain, headache) and discusses the imaging that is initially appropriate and why. As such, this text has found wide use among medical students, first year radiology residents, primary care physicians, nurse practitioners, physician’s assistants, and other health care professionals.
The fourth edition comes 13 years after the publication of the first edition. Why is a fourth edition needed?
There remains rapid transition in the imaging field, with changes in detector systems to solid state, development of new techniques (e.g., breast tomosynthesis), and continuing development of appropriateness criteria. In the last several years almost 100 new criteria have been developed, which are included in this text. Screening guidelines have been changing, and “rules” have been developed to minimize unnecessary examinations and radiation dose. These areas have all been updated in this edition without expanding the length of the text. While appropriate imaging software is included in some hospital image procedure ordering systems, this is neither widespread nor readily available to you on your smart phone or tablet, but this text and its images are.
I hope that this book fits your needs and wish you the best in your career.
Fred A. Mettler, Jr.
Acknowledgments
I thank my colleagues who have helped me with this edition and previous editions, including Blaine Hart, MD; Charles Hickam, MD; Peter Humphrey, MD; and Josh Robertson, MD. I also thank Gary Mlady, MD, and RuthAnne Bump for their encouragement and help. And a particular note of
gratitude goes out to all those who have worked very hard over the years on many task groups to compile information and recommendations for the American College of Radiology Appropriateness Criteria, which have been essential for this text.
1 Introduction
AN APPROACH TO IMAGE INTERPRETATION
The first step in medical imaging is to examine the patient and determine the possible cause of his or her problem. Only after this is done can you decide which imaging study is the most appropriate. A vast number of algorithms and guidelines have been developed, but no definite consensus exists on the “right” one for a given symptom or disease because a number of imaging modalities have similar sensitivities and specificities. In this text I provide tables of appropriate initial imaging studies for various clinical situations. When possible, these tables are based on the published literature and recommendations of professional societies. When this is not possible, I give you my opinion based on 45 years of clinical practice.
What should you expect from an imaging examination? Typically one expects to find the exact location of a problem and hopes to make the diagnosis. Although some diseases present a characteristic picture, most can appear in a variety of forms, depending on the stage. As a result, image interpretation will yield a differential diagnosis that must be placed in the context of the clinical findings.
Examination of images requires a logical approach. First you must understand the type of image, the orientation, and the limitations of the technique used. For example, I begin by mentally stating, “I am looking at a coronal computed tomography (CT) scan of the head done with intravenous contrast.” This is important, because intravenous contrast can be confused with fresh blood in the brain.
Next I look at the name and age on the image label to avoid mixing up patients, and this allows making a differential diagnosis that applies to a patient of that age and sex. You would not believe the number of times that this seemingly minor step will keep you from making dumb mistakes.
The next step is to determine the abnormal findings on the image. This means that you need to know the normal anatomy and variants of that particular part of the body, as well as their appearance on the imaging technique used. After this, you should describe the abnormal areas, because it will help you mentally order a differential diagnosis. The most common mistake is to look at an abnormal image and immediately name a disease. When you do this, you will
find your mind locked on that diagnosis (often the wrong one). It is better to say to yourself something like, “I am going to give a differential diagnosis of generalized cardiac enlargement with normal pulmonary vasculature in a 40-year-old man,” rather than to blurt out “viral cardiomyopathy” in a patient who really has a malignant pericardial effusion.
After practicing for 20 years or so, a radiologist knows the spots where pathology most commonly is visualized. Throughout this text, I point out the high-yield areas for the different examinations. Although no absolute rules exist, knowing the pathology and natural history of different diseases will help you. For example, colon cancer typically metastasizes first to the liver rather than the lungs, whereas sarcomas preferentially metastasize to the lungs rather than the liver.
After reviewing the common causes of the imaging findings that you have observed, you should reorder the causes in light of the clinical findings. At this point, you probably think that you are finished. Not so. Often a plethora of information is contained in the patient’s image files or in the hospital’s computer information system. This comes in the form of previous findings and histories supplied for the patient’s other imaging examinations. Reviewing the old reports has directed me to areas of pathology on the current image that I would have missed if I had not looked into the medical information system. A simple example is a pneumonia that has almost but not completely resolved or a pulmonary nodule that, because of inspiratory difference, is hiding behind a rib on the current examination.
You probably think that you are finished now. Wrong again. A certain number of entities could cause the findings on the image, but you just have not thought of them all. After I have finished looking at a case, I try to go through a set sequence of categories in search of other differential possibilities. The categories I use are congenital, physical/ chemical, infectious, neoplastic, metabolic, circulatory, and miscellaneous.
X-RAY
Regular x-rays (plain x-rays, also sometimes called radiographs) account for about 75% of imaging examinations. X-ray examinations, or plain x-rays, are made by an x-ray
beam passing through the patient. The x-rays are absorbed in different amounts by the various tissues or materials in the body. Most of the beam is absorbed or scattered. This represents deposition of energy in the tissue but does not cause the patient to become radioactive or to emit radiation. A small percentage of the incident radiation beam exits the patient and strikes a detector.
The historical imaging receptor was a film/screen combination. The x-ray beam would strike a fluorescent screen, which produced light that exposed the film, and then the film was developed. Newer systems are called computed radiography or digital radiography. In computed radiography, the x-rays strike a plate that absorbs the x-rays and stores the energy at a specific location. The plate is then scanned by a laser, which releases a point of light from the plate. The location is detected and stored in a computer. In digital radiography detector systems, the x-ray hits a detector and then is converted to light or an electrical charge immediately. Once either type of image is stored in the computer, it can be displayed on a monitor for interpretation or transmitted to remote locations for viewing.
Four basic tissue densities, or shades, are visible on plain x-rays. These are air, fat, water (blood and soft tissue), and bone. Air is black or very dark. On regular x-rays and CT scans, fat is generally gray and darker than muscle or blood (Fig. 1.1). Bone and calcium appear almost white. Items that contain metal (such as prosthetic hips) and contrast agents also appear white. The contrast agents generally used are barium for most gastrointestinal studies and iodine for most intravenously administered agents.
Remember that standard or plain x-rays are twodimensional presentations of three-dimensional information. That is why frontal and lateral views are often needed. Without these, mistakes can easily be made. You must remember that an object visualized on a specific view is somewhere in the path of the x-ray beam (not necessarily in the patient). If an object projects outside the patient on any view, it is outside the patient. However, even if an object
projects within the patient on two orthogonal views, it can still be located outside the patient (Figs. 1.2 and 1.3). Each additional view needed to make a diagnosis requires an additional x-ray exposure and therefore adds to the patient’s radiation dose. Radiation doses from various examinations are given in the Appendix.
The terminology used to describe images is usually quite straightforward. Chest and abdominal radiographs are referred to as upright or supine, depending on the position of the patient. In addition, chest x-rays are usually described as posteroanterior (PA) or anteroposterior (AP) (Fig. 1.4). These terms indicate the direction in which the x-ray beam traversed the patient on its way to the detector. PA means that the x-ray beam entered the posterior aspect of the patient and exited anteriorly. AP means that the beam direction through the patient was anterior to posterior. A left lateral decubitus view is one taken with the patient’s left side down.
Position is important to note, because it can affect magnification, organ position, and blood flow and therefore significantly affect image interpretation. For example, the heart appears larger on AP than on PA images because on an AP projection the heart is farther from the detector and is magnified more by the diverging x-ray beam. It also appears larger on supine than on upright images because the hemidiaphragms are pushed up, making the heart appear wider. Portable chest images are taken not only in the AP projection but also with the tube closer to the patient than on standard upright images. This magnifies the heart even more.
Use of contrast agents permits visualization of anatomic structures that are not normally seen. For example, intravenously or intra-arterially injected agents allow visualization of blood vessels (Fig. 1.5). If imaging is done with standard format, the blood vessels appear white. Digital imaging allows subtraction or removal of unwanted structures, such as the bones, from an image (see Fig. 1.5B). Often the computer manipulation is done in such a way that the
Soft tissue
Bone
Fat
Air
• Fig. 1.1 The Four Basic Densities on an X-Ray. A lateral view of the forearm shows that the bones are the densest, or white; soft tissue is gray; fat is somewhat dark; and air is very dark. The abnormality in this case is the fat in the soft tissue of the forearm, which is due to a lipoma.
Film
Anterior-posterior
Lateral
• Fig. 1.2 Spatial Localization on an X-Ray. On both anteroposterior (AP) and lateral projections, the square and round objects will be seen projecting within the view of the chest, even though the square object is located outside the chest wall. If you can see an object projecting outside the chest wall on at least one view (the triangle), it is outside the chest. If, however, an object looks as though it is inside the chest on both views, it may be either inside or outside.
A
B
• Fig. 1.3 What Is the Location of the Keys? On both the posteroanterior (PA) view of the chest (A) and the lateral view (B), the keys seem to be within the center of the chest. Actually, if you look carefully, you will notice that the keys do not change position at all, even though the patient has rotated 90 degrees. The keys are located on the receptor cassette and are not in the patient.
arteries may appear black instead of white, although this usually does not present a problem in interpretation.
Contrast agents are used to fill either a hollow viscus (such as the stomach) or anatomic tubular structures that can be accessed in some way (such as blood vessels, ureter, and common bile duct). When you see an abnormality on one of these studies, you must determine whether the location is intraluminal, mural, or extrinsic. This usually requires seeing the abnormality in perpendicular views (Fig. 1.6). Unless you are careful about this determination, you will make errors in diagnosis.
Contrast agents instilled orally, rectally, or retrograde into the ureter or bladder incur little or no risk unless
aspiration or perforation occurs. With the intravenously or intra-arterially administered agents, a small but real risk for contrast reaction exists. This is something that you should consider before ordering a contrast-enhanced CT scan. About 5% of patients will experience an immediate mild reaction, such as a metallic taste or a feeling of warmth; some experience nausea and vomiting, wheeze, or get hives as a result of these contrast agents. Some of these mild reactions can be treated with 50 mg of intramuscular diphenhydramine (Benadryl). Because contrast agents also can reduce renal function, they should not generally be used in patients with compromised renal function (estimated glomerular filtration rate [eGFR] < 50 to 60 mL/min).
About 1 in 1000 patients have a severe reaction to intravascular contrast. This may be a vasovagal reaction, laryngeal edema, severe hypotension, an anaphylactic-type reaction, or cardiac arrest. A vasovagal reaction can be treated with 0.5 to 1.0 mg of intravenous atropine. The most important initial therapeutic measures for these severe reactions are to establish an airway, ensure breathing and circulation, and give intravenous fluids. Other drugs
Detector
Anterior-posteriorPosterior-anterior
• Fig. 1.4 Typical X-ray Projections. X-ray projections are typically listed as anteroposterior (AP) or posteroanterior (PA). This depends on whether the x-ray beam passed through the patient from anterior to posterior or the reverse. Lateral (LAT) and oblique (OBL) views also are commonly obtained.
obviously also may be necessary. The risk for death from a study using intravenously administered contrast agents is between 1 in 40,000 and 1 in 100,000.
COMPUTED TOMOGRAPHY
CT is accomplished by passing a rotating fan beam of x-rays through the patient and measuring the transmission at thousands of points. The data are handled by a computer that calculates exactly what the x-ray absorption was at any given spot in the patient. The data can be manipulated in a number of ways, displayed on a screen, or photographed. Because the data points are in the computer memory, it is possible to “window” the data and obtain a number of images without additional radiation exposure (Fig. 1.7). The computers can even display the data as a threedimensional rotating image, although this is rarely necessary for diagnosis. Compared with plain x-rays, CT uses about 10 to 100 times more radiation.
On early CT scanners the x-ray tube rotated around the patient to obtain a single “slice,” and then the table was moved incrementally before another slice was obtained. Newer scanners allow the x-ray tube to stay on and rotate at the same time that the table is moving. This is called a spiral scanner or helical scanner. The most modern scanners not only have the helical motion but also have multiple rows of detectors and can obtain more than 100 image data slices at once.
The appearance of tissues on CT scan depends to some extent on the computer manipulation, but in general the
• Fig. 1.5 Pulmonary Angiogram. (A) A conventional view of blood vessels can be obtained by injecting iodinated contrast material into the vessels. On these images the vessels will appear white and the bones will be seen as you would normally expect (white). A digital subtraction technique with a computer may show the vessels either as black (B) or as white, but the bones will have been subtracted from the image.
Intraluminal lesion
Intramural lesion
• Fig. 1.6 Appearances of Different Lesions Depending on Their Location When Using Contrast. Contrast medium is used to visualize tubular structures, including the spinal canal, blood vessels, gastrointestinal tract, ureters, and bladder. (A) Intraluminal lesions, such as stones or blood clots within the lumen of the given structure, produce a central defect on both anteroposterior (AP) and lateral projections. On the AP and lateral views the contrast will show acute angles on both sides and in both projections. (B) Intramural lesions will produce a defect that indents the column of contrast. When seen tangentially, an acute angle will appear between the normal wall and the beginning of the indentation. (C) Extramural lesions also can indent the wall, but at the point of indentation, the angle will be somewhat blunted as compared with the intramural lesion.
• Fig. 1.7 Computed Tomography (CT). Images of the abdomen are presented here. (A) The image was made by using relatively wide windows during viewing, and no intravenous contrast was used. (B) The windows have been narrowed, producing a rather grainy image, and intravenous contrast has been administered so that you can see enhancement of the aorta, abdominal vessels, and both kidneys (K) In both images, contrast has been put in the bowel (B) to differentiate bowel from solid organs and structures. L, Left; R, right; Sp, spine.
basic four densities on CT images are the same as those in plain x-rays: air is black, fat is dark gray, soft tissue is light gray, and bone or calcium and contrast agents are white. One advantage of CT is that actual x-ray absorption of a specific tissue can be displayed. The units used are Hounsfield units. The Hounsfield density of water is zero. The greater sensitivity of CT compared with plain x-rays allows areas of tiny punctate calcification to be seen.
CT scans are presented as a series of slices of tissue. The method is similar in principle to slicing a loaf of bread and pulling up one slice at a time to examine it. Thus CT is a two-dimensional display of two-dimensional information, and objects appear where they really are in space. The scans or slices are shown as if you were viewing the patient from the foot of the patient’s bed. Thus the individual’s right side is on your left (Fig. 1.8). This also is the convention used for transverse images of ultrasound and magnetic resonance imaging (MRI).
Contrast agents, frequently used in CT scans, are usually the same water-soluble oral, rectal, or intravenous iodinated agents used in other imaging studies. Intravenous contrast agents are common, being used in probably 75% of all CT studies, and obviously carry the risk for contrast reactions discussed previously. Rapid acquisition of images allows the intravenously administered contrast to be displayed and images acquired in arterial, venous, or delayed phases with only a single injection.
The appeal of CT is that a large number of structures are visualized simultaneously. In a patient with abdominal pain, one CT examination shows the liver, adrenal glands, kidneys, spleen, aorta, pancreas, and other structures. This allows the clinician to identify macroscopic pathology quickly. A
• Fig. 1.8 Orientation of Computed Tomography (CT) and Magnetic Resonance (MR) Images. CT and MR usually present images as transverse (axial) slices of the body. As you stand and look at the patient from the foot of the bed, if you think of these images as slices lifted out of the body, you will have the orientation correct.
• Fig. 1.9 Ultrasound Examination of the Liver and Kidney. This is a longitudinal image, and you are essentially looking at the patient from the right side. The patient’s head is to your left. The liver has rather homogeneous echoes, and the kidney is easily seen as a bean-shaped object posterior to the right lobe of the liver.
ULTRASOUND
Ultrasound examination uses high-frequency sound waves to make images. The technology is that of sonar or a glorified fish finder used by fishermen. The image is made by sending high-frequency sound into the patient and assessing the magnitude and time of returning echoes. Echoes are the result of interfaces or changes in density. Typically a cyst has few if any echoes, because it is mostly water. Tissues such as liver and spleen give a picture with rather homogeneous small echoes caused by the fibrous interstitial tissue (Fig. 1.9). High-intensity echoes are caused by calcification, fat, and air.
The technology of ultrasound is attractive because it does not use ionizing radiation and the machines are relatively
Fig. 1.10 Color Doppler Ultrasound. In addition to displaying anatomy, ultrasound can analyze blood flow direction and velocity. In this longitudinal image of the liver, the red is blood in the portal vein flowing toward the transducer (located on the anterior aspect of the abdomen) and the blue represents blood flowing away from the transducer.
inexpensive. For these reasons, ultrasound has found widespread use in obstetrics. The use of so-called real-time ultrasound allows the images to be seen in sequential frames just as in a movie. This capability has proved popular for imaging rapidly moving structures, such as the heart. Ultrasound images can be quite dependent on operatorset parameters, and the field of view within the patient is limited. Thus unless clear labels are placed relative to orientation, the images can be difficult or impossible for the novice to interpret. Ultrasound images are usually presented as white echoes on a black background. In addition to using echoes to generate images, the ultrasound equipment can analyze the returning echo frequencies. This Doppler analysis allows for identification of moving blood, as well as its direction and velocity. Examples of its use are to identify and quantitate stenoses of the carotid arteries or the direction of blood flow in the portal vein (Fig. 1.10).
NUCLEAR MEDICINE
Nuclear medicine images are made by giving the patient a short-lived radioactive material. The most commonly used radionuclides decay rapidly and have half-lives of only minutes or hours. Most materials administered are not detectable within a day or so after administration. With the attachment of a radionuclide (such as technetium 99m) to specific carrier compounds, concentration of the radioactivity can be imaged and measured in a chosen organ or tissue, such as the thyroid, bone, lung, heart, abscess, or tumor. Few, if any, significant patient reactions are found to radiopharmaceuticals used for diagnosis.
Nuclear medicine images are made by a gamma camera or positron emission scanner that records radiation emanating from the patient and makes an image of the distribution
Head Feet Anterior Liver
Kidney
Posterior
Head Feet Anterior Liver Gallbladder
Portal vein
•
• Fig. 1.11 Nuclear Medicine
Bone Scan. Radioactivity has been introduced intravenously and localizes in specific organs. In this case a tracer makes the radioactivity localize in the bone and kidneys. Nuclear medicine can obtain images of a number of organs, including lungs, heart, and liver. ANT, Anterior view; POST, posterior view.
of the radioactive material (Fig. 1.11). The radiation dose to the patient is determined by the amount of radioactive material initially injected into the body. Therefore once the radiopharmaceutical has been given, additional images can be obtained without increasing the radiation dose. Images are usually obtained as planar images that, like plain x-rays, display three-dimensional data in two dimensions. These images are labeled as anterior, lateral, and so forth. Computer technology (similar to CT) has been applied to nuclear medicine and allows images to be displayed as slices of the tissue of interest. The major advantage of nuclear medicine is its ability to obtain an image of physiologic function. For example, virtually no other imaging technique can assess regional pulmonary ventilation or hepatobiliary function.
MAGNETIC RESONANCE IMAGING
MRI generates images by applying a varying magnetic field to the body. The magnetic field aligns atoms. When the field is released, radio waves are generated. The frequency of the emitted radio waves is related to the chemical environment of the atoms and their location. With computer analysis of
these data, magnetic resonance (MR) images (which are essentially hydrogen maps) can be generated.
Although many MRI techniques exist, the two basic types of images are T1 and T2. T1 images show fat as a white or bright signal, whereas water (or cerebrospinal fluid [CSF]) is dark. On a T2 image, fat is dark, and blood, edema, and CSF appear white (Fig. 1.12). Unfortunately, calcium and bone are difficult to see on MR images. What people think are the (white) bones is really visualization of fat in the marrow. Computer manipulation of MR images allows slices similar to those of CT orientation to be used. An intravenous contrast agent (gadolinium) is often used in conjunction with MRI. Significant patient reactions are rare with this agent, although nephrogenic systemic fibrosis has occasionally been reported in patients with severely impaired renal function (eGFR < 30 mL/min).
The primary advantages of MRI are that it obtains exquisite images of the central nervous system and stationary soft tissues (such as the knee joint). It also does not use ionizing radiation. Recent developments and shorter imaging times have allowed images of the heart and blood vessels to be generated without the need to inject anything into the patient (Fig. 1.13).
Disadvantages of MRI have been artifacts caused by patient motion, the inability to bring ferrous objects near the magnet, and cost. The major safety problem with these magnets is that they are so strong that if you bring a ferromagnetic object (such as a wrench) into the room, it can accelerate to 150 miles per hour as it is ripped out of your hand and flies into the bore of the magnet. Large floor polishers have been sucked into magnets (Fig. 1.14). If a patient is in the machine at the time, lethal consequences will result. Be aware that some “sandbags” used for neck stabilization actually contain small BBs and can destroy magnets.
HYBRID IMAGING
Increases in computer power and advances in equipment manufacturing have allowed data imaging sets from various modalities to be combined and the images coregistered. The most popular use of this has been integration of positron emission tomography (PET) functional nuclear medicine data with CT anatomic data (PET/CT) (Fig. 1.15). This currently has wide use in the imaging of cancer. Other forms of hybrid imaging exist, including PET combined with MRI.
NONINTERPRETATIVE SKILLS, QUALITY, AND PATIENT SAFETY
In addition to diagnosing and treating illness, physicians and other medical professionals need to have additional knowledge and skills to ensure that medical care is carried out in a safe, efficient, and high-quality environment. There are a number of methods to achieve these goals.
fat appears white, water and cerebrospinal fluid
and brain and muscle
In almost all MR images, bone gives off no signal and will appear black. (B) With T2 imaging, fat is dark, and water and CSF have a high signal and will appear bright or white. The brain and soft tissues still appear gray.
Informed Consent. All patients have a right to know what type of procedure is being suggested or ordered and to be able to ask questions regarding the procedure or examination. For simple procedures a short discussion or note in the chart may be all that is necessary. However, for
most complex procedures a written informed consent is obtained by a qualified assistant or by the physician (who is ultimately responsible in any case). The contents of an informed consent include expected benefits and risks, alternative procedures, and the risk of not having the recommended procedure. The consent can be granted by the patient or, if this is not possible due to mental issues or being underage, by a guardian or legal representative. If there is an emergency that may cause life-threatening conditions or serious disability and informed consent cannot be obtained, it may still be possible to proceed.
• Fig. 1.12 Magnetic Resonance (MR) Imaging of the Brain. A wide variety of imaging parameters can make tissues appear vastly different. (A) The two most common presentations are T1 images, in which
(CSF) appear black,
appear gray.
• Fig. 1.13 Magnetic Resonance Angiogram. An anterior view of the head showing intracerebral vessels, including the anterior cerebral artery (ACA) and the middle cerebral artery (MCA). These images were obtained without injection of any contrast agent.
• Fig. 1.14 Floor Polisher in a Magnet. The high magnetic field strength of a magnetic resonance machine is shown by a heavy floor polisher sucked into the scanner. The polisher was inadvertently brought into the room by cleaning personnel. (Courtesy T. Haygood, MD.)
• Fig. 1.15 Positron Emission Tomography (PET)/Computed Tomography (CT) Hybrid Imaging. PET nuclear medicine data (A) and CT data (B) can be coregistered to provide a single image combining both functional and anatomic information (C). In this case the patient has colon cancer with hepatic metastases, which were not easily seen on the CT scan alone.
Medical Errors and Adverse Events
The National Academy of Sciences has estimated that 44,000 to 98,000 deaths per year can be attributable to medical errors and that about half of these are preventable. A medical error can be defined either as the failure of a planned action to be completed as intended or as the use of a wrong plan to achieve an aim. Factors contributing to medical errors are multiple. They may involve human factors such as fatigue, ambient noise, poor lighting, confusing or nonstandardized controls on equipment, decentralized delivery, or poor systems design. Errors in radiology and nuclear medicine have been described as perceptual (60%–80%) versus cognitive/interpretative (20%–40%). A perceptual error is one where a lesion can be seen in retrospect but was not identified by the initial interpreter. Generally, the cause of such errors is not clear, but it is hypothesized that they might be due to a number of factors, including poor conspicuity of the lesion, reader fatigue, rapid pace in performing interpretations, distractions, and “satisfaction of search” (one lesion is seen and the interpreter is happy and stops looking). A cognitive error is when the lesion is identified but interpreted to be something that it is not (e.g., interpreting a lung mass as a cancer when it really is an infectious process).
Common tools for evaluating medical errors include the “root cause analysis” (RCA) to analyze events that have happened within the timeline. Very often, multiple causes are identified; some of these may be quickly fixed, but others may require system changes. Other tools include a fishbone (Ishikawa) or Pareto diagram.
The Institute of Medicine (IOM) has embodied six quality improvement (QI) aims for health care: safety, timeliness, effectiveness, efficiency, equity, and patient-centered care. There also are national patient safety goals issued by the Joint Commission. These include two patient identifiers, timely reporting of critical results, marking of procedure site, hand hygiene, and time-out before procedure. Medication reconciliation is a process to prevent unintended medication discrepancies and is a complete review of all medications at points of transition, including admission, transfer, and discharge. It is required for patients admitted to a hospital or who have changes made to their existing medications. The review process for medication administration includes information regarding the right patient, right medication, right route, right dose, right time, and right documentation.
Universal protocol is also a way to prevent errors. This involves a preprocedure verification process (preferably) involving the patient to determine the correct procedure
and site, as well as using a standardized list to determine the availability of items needed for the procedure and relevant documentation. The process also includes “time-outs,” where all team members agree on identification of the site and procedure to be performed. A time-out is usually performed separately for each procedure if multiple procedures are being performed. Attention to hand washing is crucial for most patient care and for invasive procedures (such as central venous catheter insertion). The use of a cap, mask, sterile gown, sterile gloves, large sterile sheet, hand hygiene, and cutaneous antisepsis is required.
Quality control (QC) is an ongoing review of the quality of all factors involved in producing an item. In radiology or nuclear medicine this would include a daily check on machine performance or review of images by the physician to make sure that they do not need to be repeated. Quality assurance (QA) is a term not often used today. It is a static process that typically is a reactive retrospective process to determine who was at fault after a medical error. Quality improvement (QI) involves both prospective and retrospective reviews and is a continuous process that attempts to avoid placing blame but rather to create systems that prevent errors from happening.
There are a number of methods used to measure and improve quality and to develop best practices. Benchmarking is used to compare portions of the system to results derived from peers or standards. An example of this would be comparing the exposure or dose parameters used in your practice. These can be compared to online recommendations of groups such as the American College of Radiology Image Wisely Program (http://www.imagewisely.org/) or the Society of Nuclear Medicine and Molecular Imaging Practice Guidelines (http://www.snmmi.org/ClinicalPractice/ content.aspx?ItemNumber=6414). Another tool is measurement of key performance (dashboard) indicators, which allows visual analysis of variation. A chart can be used to measure variability over time and set upper and lower control values (sometimes called an investigation level) to distinguish signal from noise and to enable reduction of unnecessary variation. An example might be analysis of the time it takes for a particular diagnostic test to be interpreted or complication rates from specific procedures.
Once an area for QI has been identified, a hypothesis is formed, changes are made, and a reanalysis is performed to
see if things have been fixed or if additional changes are necessary. This has been referred to as the Plan-Do-StudyAct (PDSA) cycle. Other methodologies for process improvement involve engaging all workers involved in the process. These include the “Lean” method, which in radiology and nuclear medicine can avoid inventory pileup and standardization of procedures. A “Six Sigma” method is based on analysis of standard deviation from a mean for a particular measure.
Special noninterpretative skills in radiology include being able to assess the appropriateness of an examination (justification) for a particular patient and managing to get the needed diagnostic information with the lowest radiation dose. Remember that too little a dose is a problem since you will not have enough image quality to make the diagnosis. You should be able to explain alternative nonradiation procedures to the patient as well as some concept of radiation risks. You need to know that there has been new equipment acceptance testing and periodic calibration (sometimes daily) as required. Of course, you also need to know how to manage emergencies, especially contrast reactions. Most of this material can be located on the website of the American College of Radiology.
Suggested Textbooks and Website
General Radiology
Brant WE, Helms C. Fundamentals of Diagnostic Radiology. Philadelphia: Lippincott, Williams & Wilkins; 2012.
Nuclear Medicine
Mettler F, Guiberteau M. Essentials of Nuclear Medicine and Molecular Imaging. 7th ed. Philadelphia: Elsevier; 2018.
Ultrasound
Rumack CM, Levine D. Diagnostic Ultrasound. 5th ed. Philadelphia: Elsevier; 2018.
Computed Tomography and Magnetic Resonance
Haaga JR, Boll DT. CT and MRI of the Whole Body. 6th ed. Philadelphia: Elsevier; 2016.
Appropriateness Criteria for Ordering Studies
American College of Radiology. ACR Appropriateness Criteria. Available at: http://acr.org/Clinical-Resources/ACR-Appropriateness-Criteria. Accessed May 7, 2018.
2 Head and Soft Tissues of Face and Neck
SKULL AND BRAIN
The appropriate initial imaging studies for various clinical problems are shown in Table 2.1.
Normal Skull and Variants
Normal anatomy of the skull is shown in Fig. 2.1. The most common differential problem on plain skull x-rays is distinguishing cranial sutures from vascular grooves and fractures. The main sutures are coronal, sagittal, and lambdoid. A suture also runs in a rainbow shape over the ear. In the adult, sutures are symmetric and very wiggly and have sclerotic (very white) edges. Vascular grooves are usually seen on the lateral view and extend posteriorly and superiorly from just in front of the ear. They do not have sclerotic edges and are not perfectly straight.
A few common variants are seen on skull x-rays. Hyperostosis frontalis interna is a benign condition of females in which sclerosis, or increased density, is seen in the frontal region and spares the midline (Fig. 2.2). Large, asymmetric, or amorphous focal intracranial calcifications should always raise the suspicion of a benign or malignant neoplasm. Occasionally, areas of lucency (dark areas) are found where the bone is thinned. The most common normal variants that cause this are vascular lakes or biparietal foramen. Asymmetrically round or illdefined holes should raise the suspicion of metastatic disease (Fig. 2.3).
Paget disease can affect the bone of the skull. In the early stages, very large lytic, or destroyed, areas may be seen. In later stages, increased density (sclerosis) and marked overgrowth of the bone, causing a cotton wool appearance of the skull, may be seen (Fig. 2.4). Always be aware that both prostate and breast cancer can cause multiple dense metastases in the skull and that both diseases are more common than Paget disease.
BRAIN
Normal Anatomy
Box 2.1 gives a methodology to follow or checklist of items to use when examining a computed tomography (CT) scan. Both CT and magnetic resonance imaging (MRI) are capable of displaying anatomic slices in a number of different planes. The identical anatomy of the brain can appear quite different on CT and magnetic resonance (MR) images (Fig. 2.5). The normal anatomy of the brain on CT and MR images is shown in Figs. 2.6 and 2.7. You should be able to identify some anatomy on these images. There are many very complex imaging sequences used during MRI, depending upon the clinical question or suspected pathology. You are not expected to be familiar with all of these, but you should realize that success in making a diagnosis depends on your indicating the clinical problem accurately so that the radiologist can prescribe the correct imaging sequences.
Intracranial Calcifications
Intracranial calcifications can be seen occasionally on a skull x-ray, but they are seen much more often on CT. Intracranial calcifications may be due to many causes. Normal pineal and ependymal calcifications may occur. Scattered calcifications can occur from toxoplasmosis, cysticercosis, tuberous sclerosis (Fig. 2.8), or granulomatous disease. Unilateral calcifications are very worrisome because they can occur in arteriovenous malformations, gliomas, and meningiomas.
Headache
Headaches are among the most common of human ailments. They can be due to a myriad of causes and should Text continued on p. 19
Imaging Modalities for Cranial Problems
Suspected Cranial Problem
Skull fracture
Major head traumaa
Mild head traumaa
Acute hemorrhage
Intracerebral aneurysm or arteriovenous malformation
Aneurysm (chronic history)
Hydrocephalus
Transient ischemic attack
Acute transient or persistent CNS symptoms or findings
Acute stroke
Suspected hemorrhagic
Suspected nonhemorrhagic
Ataxia (acute or chronic unexplained)
Cranial neuropathy
Multiple sclerosis
Tumor or metastasis
Carotid/vertebral dissection (ipsilateral Horner syndrome or unilateral headache)
Abscess
Preoperative for cranial surgery
Meningitis
Seizure
New onset or poor therapeutic response
New onset posttraumatic
Febrile or alcohol withdrawal without neurologic deficit
Focal neurologic deficit
Vertigo
If suspect acoustic neuroma or posterior fossa tumor
Episodic vertigo (peripheral) with hearing loss or other neurologic abnormalities or persistent vertigo (central)
Hearing loss
Sensorineural
Conductive
Mixed sensorineural and conductive, congenital, total deafness, or cochlear implant candidate
Vision loss
Adult sudden, or with proptosis, uveitis, scleritis, or ophthalmoplegia
Head injury
Child acute or progressive, proptosis, or orbital asymmetry
aSee text for description of low, moderate, and high risk after head trauma. ACTH, Adrenocorticotropic hormone; CNS, central nervous system; CT, computed tomography; FDG, fluorodeoxyglucose; MRI, magnetic resonance imaging; PET, positron emission tomography; TSH, thyroid-stimulating hormone.
Coronal suture
Vascular groove
Frontal sinus
Sphenoid sinus
Maxillary sinus
Parietal bone
Temporal bone
Lambdoid suture
Occipital bone
Sella turcica
Frontal sinus
Lesser wing of sphenoid
Greater wing of sphenoid
Maxillary sinus A
Superior orbital rim
Ethmoid sinus
Petrous bone
Maxilla
Mandible
• Fig. 2.1 Normal Skull. (A) Lateral, (B) anteroposterior (AP),
Mastoid air cells
Foramen magnum
Ethmoid sinus
Lambdoid suture
Petrous bone
Mandibular condyle
Nasal septum
Inferior orbital rim
Maxillary sinus
Mastoid air cells
Odontoid process of C2
Orbit
Zygoma frontal process
Zygomatic arch
Coronoid process of mandible
Angle of mandible
• Fig. 2.1, cont’d (C) AP Towne projection, and (D) AP Waters view.
• Fig. 2.2 Hyperostosis Frontalis Interna. A normal variant, most common in female patients, in which increased density of the skull occurs in the frontal regions. Notice that sparing of the midline is present (arrows).
• Fig. 2.3 Multiple Myeloma. Multiple asymmetric holes in the skull are seen only with metastatic disease. Metastatic lung or breast carcinoma can look exactly the same as this case of multiple myeloma.
• Fig. 2.4 Paget Disease. The fluffy cotton wool densities overlying the skull are caused by bone expansion. Note also that the calvaria is very thick (arrow). The base of the skull has become softened; the cervical spine and foramen magnum look as though they are pushed up, but in reality the skull is sagging around them.
• Fig. 2.5 Axial Images of the Brain on Computed Tomography (CT) and Magnetic Resonance Imaging (MRI). (A) On a noncontrast CT scan. the skull is easily seen, the brain is varying shades of gray, and cerebrospinal fluid (CSF) is dark. (B) On a T1 MRI scan. the skull is difficult to see, the brain is gray, and the CSF is dark. (C) On a T2 image, the CSF is white.
Frontal lobe
Parietal lobe Falx cerebri
Lateral ventricles
Frontal lobe
Parietal lobe Falx cerebri
Lateral ventricles
Frontal lobe
Lateral ventricles
Parietal lobe
Frontal lobe
Anterior horn of lateral ventricle
Sagittal sinus Superior cerebellar cistern
Frontal lobe
Genu corpus callosum
Caudate nucleus
Anterior horn of lateral ventricle
Putamen
Thalamus Third ventricle
Splenium corpus callosum
Occipital lobe
Anterior horn of lateral ventricle
Caudate nucleus
Putamen
Thalamus
Internal cerebral veins
Atrium of lateral ventricle
Occipital lobe
B1
B2
B3
A1
A2
A3
• Fig. 2.6 Normal Axial Images of the Brain at Three Different Levels. Noncontrast computed tomography (A1, B1, C1), T1 magnetic resonance images (MRI) (A2, B2, C2), and T2 MRI images (A3, B3, C3).
Temporal lobe
Pons
Mastoid air cells
Fourth ventricle
Cerebellum
Skull
Optic globe
Sphenoid sinus
Temporal lobe
Internal carotid
Mastoid air cells
Pons
Fourth ventricle
Cerebellum
Optic globe
Sphenoid sinus
Temporal lobe
Internal carotid
Mastoid air cells
Pons
Fourth ventricle
Cerebellum
C1
C2
C3
• Fig. 2.6, cont’d
Cingulate gyrus
Frontal lobe
Corpus callosum
Pons
Pituitary gland
Clivus
Nose
Tongue
Skull with marrow
Lateral ventricle
Third ventricle
Cerebral peduncle
External auditory meatus
Parietal lobe
Thalamus
Occipital lobe
Cerebellum
Medulla
Spinal cord
C2
Falx cerebri
Septum
pellucidum
Thalamus
Temporal lobe
Hippocampus
Pons Sylvian fissure
• Fig. 2.7 Normal T1 Magnetic Resonance Imaging Anatomy of the Brain in Sagittal (A) and Coronal (B) Projection.
• BOX 2.1 Items to Look for on Computed Tomography Brain Scana
Look for the following:
• Focally decreased density (darker than normal) due to stroke, edema, tumor, surgery, or radiation
• Increased focal density (whiter than normal) on a noncontrast scan
• In ventricles (hemorrhage)
• In parenchyma (hemorrhage, calcium, or metal)
• In dural, subdural, or subarachnoid spaces (hemorrhage)
• Increased focal density on a contrast scan
• All items above
• Tumor
• Stroke
• Abscess or cerebritis
• Aneurysm or arteriovenous malformation
• Asymmetric gyral patter n
• Mass or edema (causing effacement of sulci)
• Atrophy (seen as very prominent sulci)
• Midline shift
• Ventricular size and position (look at all ventricles)
• Sella for masses or erosion
• Sinuses for fluid or masses
• Soft tissue swelling over skull
• Bone windows for possible fracture
calcifications are seen about the ventricles in the posterior parietal regions. Other diseases that could show this appearance include intrauterine TORCH (toxoplasmosis, other agents, rubella, cytomegalovirus, herpes simplex) infections. ANT, Anterior.
• Fig. 2.8 Tuberous Sclerosis. Scattered
• BOX 2.2 Imaging Indications for Headachesa
CT without contrast is indicated for the following:
Sudden onset of the “worst headache of one’s life”
(thunderclap headache)
Posttraumatic headache
MRI is indicated for the following:
A headache that:
Worsens with exertion, cough, or sexual activity
Is associated with a decrease in alertness
Is positionally related and of skull base, periorbital, orbital, or trigeminal autonomic origin
Awakens one from sleep
Changes in pattern over time
A new headache:
In an HIV-positive individual or cancer patient
Associated with papilledema
Associated with focal neurologic deficit
Associated with mental status changes
In a patient > 60 years of age, with sedimentation rate > 55 mm/h and temporal tenderness
In a pregnant patient
A chronic headache with new features or neurologic deficit
Suspected meningitis or encephalitis
CTA or MRA of the head and neck is indicated for the following:
Sudden onset of unilateral headache
Suspected carotid or vertebral dissection or ipsilateral Horner syndrome
CT, Computed tomography; CTA, computed tomography angiogram; HIV, human immunodeficiency virus; MRA, magnetic resonance angiogram; MRI, magnetic resonance imaging.
aFor most of the above indications, CT is acceptable if MRI is not feasible or available. MRI is usually not indicated for sinus headaches or chronic headaches with no new features. See Box 2.3 for CT indications in sinus disease.
• BOX 2.3 Indications for Computed Tomography or Magnetic Resonance Imaging in Adult Sinus Disease
CT scanning is indicated in acute complicated sinusitis if the patient has the following:
• Sinus pain/discharge and
• Fever and
• A complicating factor such as the following:
• Mental status change
• Facial or orbital cellulitis
• Meningitis by lumbar puncture
• Focal neurologic findings
• Intractable pain after 48 hours of intravenous antibiotic therapy
• Immunocompromised host
• Sinonasal polyposis
• Possible surgical candidate
• Three or more episodes of acute sinusitis within 1 year in which the patient has signs of infection
CT scanning is indicated in chronic sinusitis if the following occurs:
• No improvement is seen after 4 weeks of antibiotic therapy based on culture or
• No improvement is seen after 4 weeks of intranasal steroid spray
CT or MRI scanning is indicated in cases of suspected sinus malignancy
MRI scanning with and without contrast is indicated in patients with suspected intracranial complications of sinusitis
CT, Computed tomography; MRI, magnetic resonance imaging.
For sinus imaging in children, see Chapter 9.
be characterized by location, duration, type of pain, provoking factors, and age and sex of the patient. In the primary care population, less than 0.5% of acute headaches are the result of serious intracranial pathology. Simple headaches, tension headaches, migraine headaches, and cluster headaches do not warrant imaging studies. A good physical examination is essential, including evaluation of blood pressure, urine, eyes (for papilledema), temporal arteries, sinuses, ears, neurologic system, and neck. In a patient with a febrile illness, headache, and stiff neck, a lumbar puncture should be performed. In only a few circumstances is imaging indicated (Box 2.2).
In general, imaging is indicated when a headache is accompanied by neurologic findings, syncope, confusion, seizure, and mental status changes or after major trauma. Sudden onset of the “worst headache of one’s life” (thunderclap headache) should raise the question of subarachnoid hemorrhage. Sudden onset of a unilateral headache with a suspected carotid or vertebral dissection or ipsilateral Horner syndrome should prompt a CT or MR angiogram. Sinus headaches can usually be differentiated from other causes because they worsen when the patient is leaning
forward or when pressure is applied over the affected sinus. Indications for CT use in sinus headaches are presented in Box 2.3.
Hearing Loss
Hearing loss is characterized as conductive, sensorineural, or mixed. Conductive loss results from pathology of the external or middle ear that prevents sound from reaching the inner ear. Sensorineural loss results from abnormalities of the inner ear, including the cochlea or auditory nerve. CT is the best technique for evaluating conductive loss and the bony structures of the middle ear. Not all patients with conductive loss need a CT scan. Indications include complications of otomastoiditis, preoperative and postoperative evaluation of prosthetic devices, cholesteatoma, and posttraumatic hearing loss. Sensorineural hearing loss may be sudden, fluctuating, or progressive and may also be associated with vertigo. It can be due to viral infection, eardrum rupture, acoustic neuroma, or vascular occlusive disease. Evaluation is best done by MRI, with and without intravenous contrast.
Fig. 2.9
Skull
Skull fractures (arrows) are usually dark lines that are very sharply defined and do not have white margins. (A) On the anteroposterior view, it cannot be determined whether the fracture is in the front or the back of the skull. (B) With a Towne view, however, in which the neck is flexed and the occiput is raised, this fracture can clearly be localized to the occipital bone.
Head Trauma
On skull x-rays, fractures are dark lines that have very sharp edges and tend to be very straight (Fig. 2.9). If a fracture is present over the middle meningeal area, an associated epidural hematoma may be found. If a depressed fracture is present, the lucent fracture lines can be stellate or semicircular (Fig. 2.10). In either of these cases, substantial brain injury may be present and a CT scan, including bone windows, is indicated.
Skull x-rays are ordered much too frequently. A skull fracture without loss of consciousness is very rare. Significant brain injury may be found without a skull fracture. The patient should be examined clinically and a decision made as to whether the physical findings and history
Note the very wiggly posterior suture lines and the normally radiating vascular grooves. (B) The anteroposterior view shows the amount of depression of the fracture (arrow), although this is usually much better seen on a computed tomography scan.
indicate a moderate to severe head injury or mild head injury. CT, MRI, or skull radiography is not needed for low-risk patients. Low-risk patients are defined as those who are asymptomatic or have only dizziness, mild headache, scalp laceration, or hematomas, are older than 2 years, and have no moderate- or high-risk findings.
Patients at moderate risk are those who have any of the following conditions: history of change in the level of consciousness at any time after the injury, progressive or severe headache, posttraumatic seizure, persistent vomiting, multiple trauma, serious facial injury, signs of basilar skull fracture (e.g., hemotympanum, “raccoon eyes,” cerebrospinal fluid [CSF] rhinorrhea or otorrhea), suspected child abuse, bleeding disorder, or age younger than 2 years (unless the injury is trivial).
High-risk patients are those with any of the following conditions: focal neurologic findings, a Glasgow Coma Scale score of 8 or less, definite skull penetration,
•
Linear
Fracture.
Vascular grooves
Sutures
• Fig. 2.10 Depressed Skull Fracture. This patient was hit in the head with a hammer. (A) The lateral view shows the central portion of the fracture, which is stellate (large arrows), and the surrounding concentric fracture line (small arrows).
metabolic derangement, postictal state, or decreased or depressed level of consciousness (unrelated to drugs, alcohol, or other central nervous system [CNS] depressants). If a moderate or severe injury is present and the patient is neurologically unstable, a CT scan should be done to exclude a hematoma. If the patient is neurologically stable, an MR scan is preferable to look for parenchymal shearing injuries. In mild head injury (with no loss of consciousness or neurologic deficit), the patient may be observed. There are three sets of prediction rules as to who does not need CT imaging: New Orleans Criteria (NOC), Canadian CT Head Rule (CCHR), and National Emergency X-Radiography Utilization Study (NEXUS II). If a patient has any of the following, a CT scan may be indicated; headache, vomiting, age older than 60 years, alcohol or drug intoxication, deficit in short-term memory, seizure, visible trauma above the clavicles, fall from more than three feet or five steps, coagulopathy, scalp hematoma, acute focal neurologic deficit, or skull fracture. If a persistent headache occurs after trauma, CT scanning should be performed.
Suspected Intracranial Hemorrhage
If the presence of acute intracranial hemorrhage is suspected, the study of choice is a CT scan without intravenous contrast. The scan is done without contrast because acute hemorrhage appears to be white on a CT scan (Fig. 2.11) and so does intravenously administered contrast. Hemorrhage into the ventricles is usually seen in the posterior horns of the lateral ventricles. Blood is denser than CSF and therefore settles dependently. This settling process is not seen with subarachnoid or intraparenchymal blood. The presence of hemorrhage is a contraindication to anticoagulation.
• Fig. 2.11 Gunshot Wound of the Head. A noncontrast computed tomography scan shows bilateral soft tissue swelling and a hemorrhagic track across the brain. Blood appears white and is also is seen within the lateral ventricles. Several small air bubbles are seen in the lateral ventricles along the track and along the anterior surface of the brain. ANT, Anterior
Intraparenchymal bleeding can result from a ruptured aneurysm, stroke, trauma, or tumor. Grave prognostic factors are large size and brainstem location. Most hypertensive bleeds (80%) occur in the basal ganglia, 10% occur in the pons, and 10% in the cerebellum. An associated mass effect may be present with compression of the ventricles or midline shift. The findings of acute hemorrhage on a noncontrast CT scan indicate increased density in the parenchyma (Fig. 2.12). Differentiation from calcification usually is easily made by clinical history and, if necessary, by having the area of interest measured on the scan in terms of density (Hounsfield units).
Subdural hematomas are seen as crescent-shaped abnormalities between the brain and the skull. They can cross suture lines, but they do not cross the tentorium or falx. In some cases, subdural hematomas can be quite difficult to see because new blood appears denser or whiter than brain tissue (Fig. 2.13A). As the blood ages (over a period of several weeks), it becomes less dense than the brain (see Fig. 2.13B). Obviously, it follows that a subacute phase occurs during which the blood is the same density as the brain (isodense). In this stage, sometimes the only clue that a subdural hematoma is present is effacement of the gyral pattern on the affected side, a midline shift away from the affected side, or ventricular compression on the affected side.
• Fig. 2.12 Intracerebral Hemorrhage. In this hypertensive patient with an acute severe headache, the noncontrast computed tomography scan shows a large area of fresh blood in the region of the right thalamus. Blood also is seen in the anterior and posterior horns of the lateral ventricles. Because blood is denser than cerebrospinal fluid (CSF), it is layered dependently. ANT, Anterior
• Fig. 2.13 Subdural Hematomas. (A) A noncontrast computed tomography scan of an acute subdural hematoma shows a crescentic area of increased density (arrows) in the right posterior parietal region between the brain and the skull. An area of intraparenchymal hemorrhage (H) also is seen; in addition, mass effect causes a midline shift to the left (open arrows). (B) A chronic subdural hematoma is seen in a different patient. An area of decreased density appears in the left frontoparietal region effacing the sulci, compressing the anterior horn of the left lateral ventricle, and shifting the midline somewhat to the right. ANT, Anterior
Epidural hematomas follow the same changing pattern of density as do subdural hematomas. The major differential point from an imaging viewpoint is that they are lenticular rather than crescentic (Fig. 2.14) and tend not to cross suture lines of the skull. Epidural hematomas are associated with temporal bone fractures that have resulted in a tear of the middle meningeal artery.
Subarachnoid hemorrhage is usually the result of trauma or a ruptured aneurysm. It is most often accompanied by a very severe sudden-onset headache. Subarachnoid
Fig. 2.14 Epidural Hematoma. In this patient, who was in a motor vehicle accident, a lenticular area of increased density is seen in the right parietal region (arrows) on a noncontrast axial computed tomography scan. These typically occur over the groove of the middle meningeal artery. Areas of hemorrhage also are seen in the left frontal lobe. ANT, Anterior
hemorrhage can really be visualized only in the acute stage, when the blood is radiographically denser (whiter) than the CSF. The most common appearance is increased density in the region around the brainstem in a pattern sometimes referred to as a Texaco star (Fig. 2.15). Increased density due to the presence of blood also can be seen as a white line in the sylvian fissures, in the anterior interhemispheric fissure, or in the region of the tentorium. In the absence of trauma, a ruptured aneurysm should be suspected. As discussed in Chapter 9, in infants, both intraventricular and intraparenchymal hemorrhage can be visualized and monitored by using ultrasound. This can be done only if the fontanelles have not closed.
Pneumocephalus
Air within the cranial vault is almost always the result of trauma. Even tiny amounts of air are easily seen on CT as decreased density (blackness; see Fig. 2.11). It is preferable to do a CT scan instead of an MRI examination because of the superior ability of CT to localize skull fractures and fresh hemorrhage. It also is easier to manage an unstable patient in a CT scanner than in an MRI machine.
Hydrocephalus
Dilatation of the ventricles can be obstructive or nonobstructive. The ventricles are easily seen on a noncontrast CT or MRI study. If the cause is obstructive, both modalities have a good chance of finding the site of obstruction.
•
• Fig. 2.15 Acute Subarachnoid Hemorrhage. A noncontrast axial computed tomography scan shows the blood as areas of increased density. (A) A transverse view near the base of the brain shows blood in the “Texaco star” pattern (arrows), formed by blood radiating from the suprasellar cistern into the sylvian fissures and the anterior interhemispheric fissure. (B) A higher cut shows blood as an area of increased density in the anterior and posterior interhemispheric fissures, as well as in the sulci on the right (arrows). ANT, Anterior.
Transient Ischemic Attack
A transient ischemic attack (TIA) is defined as a neurologic deficit that has an abrupt onset and from which rapid recovery occurs, often within minutes, but always within 24 hours. The imaging indications for patients with a new neurologic deficit are shown in Box 2.4. A TIA indicates that the patient may be at high risk for stroke. In the acute setting, the initial test of choice is a CT scan to differentiate an ischemic event from a hemorrhagic one. A second CT scan can be obtained in 24 to 72 hours if the diagnosis is in doubt, but an MRI is more sensitive in identifying early ischemic damage and may establish the cause of the TIA. If initial vertebrobasilar findings are seen, an MRI provides
• BOX 2.4 Imaging Indications With a New Neurologic Deficit
Acute onset or persistence of the following neurologic deficits is an indication for computed tomography or magnetic resonance imaging:
• New vision loss
• Cranial neuropathy
• Aphasia
• Mental status change (e.g., memory loss, confusion, impaired level of consciousness)
• Sensory abnormalities (e.g., hemianesthesia/hypesthesia including single limb)
• Motor paralysis (e.g., hemiparesis or single limb)
• Vertigo with headache, diplopia, motor or sensory deficit, ataxia, dysarthria, or dysmetria
better evaluation of the posterior fossa than a CT scan. Regardless of whether a carotid bruit is present in this setting, a duplex Doppler ultrasound examination of the carotid arteries is indicated if the patient would be a surgical candidate for endarterectomy. MR angiography can be used to visualize carotid stenosis.
Stroke
A stroke may be ischemic or associated with hemorrhage. An acute hemorrhagic stroke is most easily visualized on a noncontrast CT scan because fresh blood is quite dense (white). A diagnosis of stroke cannot be excluded, even with normal results on a CT scan taken within 12 hours of a suspected stroke. A purely ischemic acute stroke is difficult to visualize on a CT scan unless mass effect is present. This is noted as compression of the lateral ventricle, possible midline shift, and effacement of the sulci on the affected side. One key to identification of most strokes is that they are usually confined to one vascular territory (such as the middle cerebral artery). An acute ischemic stroke is very easy to see on an MRI study, because the edema (increased water) can be identified as a bright area on T2 images. In spite of this, an MRI scan is not needed for a patient with an acute stroke. Because anticoagulant therapy is often contemplated, a noncontrast CT scan can be obtained to exclude hemorrhage (which would be a contraindication to such therapy).
After about 24 hours, the edema associated with a stroke can be seen on a CT scan as an area of low density (darker than normal brain). If a contrast CT scan is done 1 to several days after a stroke, enhancement (increased density or whiteness) may be seen at the edges of the area (so-called luxury perfusion). During the months after a stroke, atrophy of the brain occurs, which can be seen as widened sulci and a focally dilated lateral ventricle on the affected side (Fig. 2.16). Different specific MRI imaging sequences are performed when an acute ischemic, hemorrhagic, or chronic stroke is suspected (Fig. 2.17).
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composed of Parmelia, Gyrophora, Cetraria, Acarospora, Lecanora, Lecidea, Buellia, etc. The stratum itself is physically very different and constitutes a distinct habitat. These groups are really small formations, which are quite distinct from the surrounding forest or meadow. This is proven conclusively in many places in the mountains where areas of the characteristic lichen formations of cliffs are carried by the fall of rock fragments into forest and meadow, where they persist without modification. This also shows clearly that the groups on scattered rocks in the same area are to be regarded as examples of the same cliff formation, except where the differences are evidently to be ascribed to development and not to alternation. Where these rock formations can not be traced to cliffs or magmata with certainty, they must be considered as antedating the vegetation in which they occur. Often, indeed, especially in igneous areas, they are relicts of the initial stage of a primary succession. Finally, they prove their independence of the forest or meadow formation by initiating a distinct succession within these. Crustaceous groups or formations yield to foliose ones, and these in turn give way to formations of mosses, particularly in the forest where the effect of the diffuse light is felt. From the above, the following rule of formational limitation is obtained: any area, which shows an essential difference in physical character, composition, or development from the surrounding formation is a distinct formation.
347. The parts of a formation. All the parts which make up the structure of a formation are directly referable to zonation and alternation, alone or together, or to the interaction of the two. The principles which underlie this have already been discussed under the phenomena concerned. It is necessary to point out further that the structure may be produced in several ways: (1) by zonation alone, (2) by alternation alone, (3) by zonation as primary and alternation as secondary, (4) by primary alternation and secondary zonation, (5) by the interaction of the two, as in layered formations. Though all these methods occur, the first two are relatively rare, and the resulting structure comparatively imperfect. The typical structure of formations can best be made clear by the consideration of a prairie which belongs to the fourth group, and a forest which represents the last.
Fig. 75. Relict lichen formation in a spruce forest, invaded by rock mosses.
Fig. 76. Early (prior) aspect of the alpine meadow formation (Carex-Campanulacoryphium), characterized by Rydbergia grandiflora.
The major divisions of prairie and forest formations are regularly due to alternation. There is an inherent tendency to the segregation of facies, arising out of physical or historical reasons, or from a combination of both. Not all formations show this, but it is characteristic of the great majority of them. The primary areas which thus arise have been called associations: they are naturally subordinate to the formation. To avoid the confusion which inevitably results from using the word association in two different senses, it is proposed to term this primary division of the formation, a consociation, or better, a consocies. This term is applied only to an area characterized by a facies, or less frequently, by two or more facies uniformly commingled. The consocies of grassland are determined by grasses, those of forests by trees, etc. From the different position of the facies in these two types of vegetation such areas are readily seen at all times in the forest, but they are often
concealed in grassland by the tall-growing principal species of the various aspects. When definite consocies are present, they are often found to mingle where they touch, producing miniature transition areas, and, very rarely, they sometimes leave gaps in which no facies appears.
Fig. 77. Late (serotinal) aspect of the alpine meadow, characterized by Campanula petiolata, Rydbergia in fruit.
The seasonal changes of a formation, which are called aspects, are indicated by changes in composition or structure, which ordinarily correspond to the three seasons, spring, summer, and autumn. The latter affect the facies relatively little, especially those of woody vegetation, but they influence the principal species profoundly, causing a grouping typical of each aspect. For these areas controlled by principal species, but changing from aspect to aspect, the term society is proposed. They are prominent features of the majority of
herbaceous formations, where they are often more striking than the facies. In forests, they occur in the shrubby and herbaceous layers, and are consequently much less conspicuous than the facies. A close inspection of the societies formed by principal species shows that they are far from uniform. Since they usually fail to exhibit distinct parts, it becomes necessary to approach the question of their structure from a new standpoint. Such is afforded by aggregation, which yields the simplest group in vegetation, i. e., that of parent and offspring. This is so exactly a family in the ordinary sense that there seems to be ample warrant for violating a canon of terminology by using the word for this group, in spite of its very different application in taxonomy. It has already been shown that aggregation further produces a grouping of families, which may properly be called a community. As they are used here, family and community become equally applicable to the association of plants, animals, or man. Both families and communities occur regularly in each society of the formation, and they represent its two structures. In some cases, all the families are grouped in communities, two or more of which then form the society. Very frequently, however, families occur singly, without reference to a community, and the two then constitute independent parts of the same area. This is typically the case wherever gregarious species are present, since these are merely family groups produced by aggregation.
78. Calthetum (Caltha leptosepala), a consocies of the alpine bog formation.
Fig.
Fig. 79. Iridile (Iris missouriensis), a society of the aspen formation.
Objection may be made that this analysis of formational structure has been carried too far, and that some of the structures recognized are mere interpretations, and not actual facts. Such a criticism will not come from one who has got beyond the superficial study of formations, for he will at once recognize that certain probable features of structure have not been considered. On the other hand, the ecologist or the botanist who has not made a careful investigation from the standpoints of development and structure will naturally refrain from expressing an opinion, until he has obtained an acquaintance at first hand with the facts. Over-refinement is the usual penalty of intensive work. The unbiased investigator, however, will not be misled by the suddenness with which new concepts appear. It seems plausible that the structure of a formation, if not as definite, is at least nearly as complex as that of an individual plant. Few botanists will insist that the refinement of tissues and tissue systems has been carried further than the differentiation of the plant
warrants. Yet, if these had been defined within a period of a few years rather than slowly recognized during more than a century, they would have been called seriously in question. As a matter of fact, the consocies, under the term association, and the society, under various names, have been recognized by ecologists for several years. They are definite phenomena of alternation which can be found anywhere. The family and the community, though the latter is less distinct in outline, are equally valid structures, the proof of which anyone can obtain by thorough methods of study.
348. Nomenclature of the divisions. The suffix -etum is used to designate a consocies of a formation, e. g., Picetum, Caricetum, etc. When two or more species characterize the area, the most important, or more rarely, the two are used. The termination used to designate a society is -ile, as Asterile, Sedile, Rosile. The suffix which denotes the community is -are, and for the family, it is -on, viz., Giliare, Bromare, Bidenton, Helianthon, etc. Layers are indicated by the affix -anum, as Opulasteranum, Verbesina-Rudbeckianum, etc. It is evident that these suffixes, like the terms to which they refer, must be used always for the proper divisions if they are to have any value at all. There has been a marked tendency, for example, to useetum in connection with the names of groups of very different rank. It is hardly necessary to point out that such a practice does not promote clearness. The following tabular statement will illustrate the application of both terms and suffixes:
349. The investigation of a particular formation. A comprehensive and thorough study of a formation should be based upon as many examples of it as are accessible. The example which is at once the most typical and the most accessible is made the base area. This plan saves time and energy, reduces the number of instruments that are absolutely necessary, and establishes a common
basis for comparison. The inquiry should be made along four lines, all fundamental to a proper knowledge of the formation. These lines are: (1) the determination of the factors of the habitat, (2) a quadrat and a transect study of the structure of the formation, (3) a similar investigation of development, (4) a floristic study of the contiguous formation, with special reference to migration. The sequence indicated has proven to be the most satisfactory, and is to be regarded as all but absolutely essential. Naturally, this applies only to the order in which the various lines are to be taken up, as they are carried on together when the work is fully under way. Since instrument and quadrat methods have already been given in detail, it is unnecessary that they be repeated. Similarly, the questions which pertain to structure and development and to the surrounding vegetation are considered in detail in the pages which precede.
Fig. 80. Eritrichiare (Eritrichium aretioides), a community of the alpine meadow formation.
CLASSIFICATION AND RELATIONSHIP
350. Bases. Formations may be grouped with reference to habitat or kind, development or position. Classification upon the basis of habitat places together formations which are similar in physiognomy and structure. Developmental classification is based upon the fact that the stages of a particular succession are organically connected or related, though they are normally different in both physiognomy and structure. Grouping with respect to position is made solely upon occurrence in the same division of vegetation. The formations thus brought together usually possess neither similarity of kind or structure, nor do they have any necessary developmental connection. Habitat and developmental classification are of fundamental value; regional arrangement is more superficial in character. All serve, however, to emphasize different relations, and, while the developmental system expresses the most, they should all be used to exhibit the vegetation of a region, province, or zone.
351. Habitat classification. In arranging formations with reference to habitats, the direct factors, water and light, can alone be used to advantage. Such a system is fundamental, because it is founded upon similarity of habitat and of structure. Proposed groupings based upon nutrition-content, or upon the division of factors into climatic and edaphic, have elsewhere[42] been shown to be altogether of secondary importance, if not actually erroneous. The basis of the habitat grouping is water-content, which is supplemented by light whenever the factor is decisive. The primary divisions thus obtained are water, forest, grassland, and desert, which are characterized respectively by associations of hydrophytes, mesophytes, hylophytes, poophytes, and xerophytes respectively. Within these, formations are arranged according to the type of habitat, i. e., pond, meadow, forest, dune, etc. These divisions
Fig. 81. Pachylophon (Pachylophus caespitosus), a family of the gravel slide formation.
comprise all formations which belong to the type by virtue of their physiognomy and structure. Such formations differ from each other very considerably or completely in the matter of floristic, i. e., component species, but they still belong to the same type. A dune formation in the interior and one on the coast may not have a single species in common, and yet they are essentially alike in habitat, development, and structure.
352. Nomenclature. The names of formations are taken from the habitats which they occupy. Each formation should have a vernacular and a scientific name. The latter is especially important since it ensures brevity and uniformity, and obviates the obscurity and confusion that arise from vernacular terms in many tongues. Scientific names have been made uniformly from Greek words of proper meaning by the addition of the suffix -ium (εῖον), which denotes place.[43] The following list gives the English and the scientific name of the various habitats, and their corresponding formations, and indicates the primary divisions into which these fall.
I.
Hydrophytia: water plant formations
ocean: oceanium: oceanad,[44] oceanophilous, etc.
sea: thalassium
surface of the sea: pelagium
deep sea: pontium
lake: limnium, limnad
pond, pool, tiphium, tiphad
stagnant water: stasium: stasad
salt marsh: limnodium, limnodad
fresh marsh: helium
wet meadow: telmatium
river: potamium
creek: rhoium
brook: namatium
torrent: rhyacium
spring: crenium
warm spring: thermium
ditch: taphrium
sewer: laurium
swamp forest: helohylium
swamp open woodland: helodium
meadow thicket: helodrium
bank: ochthium
rock bank: petrochthium
sand bank: ammochthium
mud bank: pelochthium
rocky seashore: actium
sandy seashore: agium
sandbar: cheradium
tank: phretium
Mesophytia: middle plant formations
Sciophytia: shade plant formations
forest: hylium
grove: alsium
orchard: dendrium
canyon: ancium
open woodland: orgadium
thicket: lochmium
Heliophytia: sun plant formations
meadow: poium
pasture: nomium
culture land: agrium
waste place: chledium
III.
Xerophytia: dry plant formations
desert: eremium
sand-hills, sandy plain: amathium
prairie, plains: psilium
dry, open woodland: hylodium
dry thicket: driodium dry forest: xerohylium
gravel slide: chalicium
sandbar: syrtidium
sand draw: enaulium
blowout: anemium
strand: psamathium
dune: thinium
badlands: tirium
hill, ridge: lophium
cliff: cremnium
rock field: phellium
boulder field: petrodium
rock, stone: petrium
humus marsh: oxodium
alkali area: drimium
heath, dry meadow: xeropoium
moor: sterrhium
alpine meadow: coryphium
polar barrens: crymium
snow: chionium
wastes: chersium
Particular formations are indicated by means of floristic distinctions. Thus, Populus-hylium is the aspen forest as distinguished from the Picea-Pseudotsuga-hylium, or the balsamspruce forest; and the Bulbilis-psilium, or buffalo-grass prairie, from the Bouteloua-Andropogon-psilium, or grama-bluestem prairie. Similarly, the aspen formation of the Old World and of the New may be distinguished as Populus-tremula-hylium and Populustremuloides-hylium, respectively. In all formational names, the facies alone should be used. Frequently, a single facies will suffice for clearness. As a rule, however, the two most important facies should be employed; in rare cases only is it necessary to use the names of three. When it is desirable to refer to two or more examples of the same formation, a geographical term is added, e. g., (1) Populushylium (Crystal Park), (2) Populus-hylium (Cabin Canyon).
353. Developmental classification. This is based upon succession as the record of development. Upon the basis of development, all the formations which belong to the same succession are classed together. They are arranged within each group in the sequence found in the particular succession. From its nature, developmental classification is of primary importance in exhibiting the history of vegetational changes. It has less value than the habitat system for summarizing the essential structure of a vegetation, inasmuch as it places the emphasis upon historical rather than structural features. It is evident that both deal with the same formations, and that the difference is merely one of viewpoint. The habitat classification is simpler in that it considers only those formations actually on the ground, while development has regularly to take into account stages which have disappeared. The groups of the developmental system, and the arrangement of formations
within them have already been indicated under the nomenclature of succession (sections 326 and 327).
354. Regional classification. The grouping of formations with respect to the divisions of vegetations is chiefly of geographical value. It indicates a certain general relationship, but its principal use is to summarize the structure of the vegetative covering of a region. The arrangement of formations in the various divisions is made with reference to the outline of North American vegetation (section 341). This is naturally based upon the identity of altitude and latitude zones. In the study of mountain countries, it is often desirable to group formations with reference to altitude alone. In this case, the grouping is based upon the following divisions: (1) bathyphytia, lowland plant formations; (2) mesiophytia, midland formations; (3) pediophytia, upland formations; (4) pagophytia, foot-hill formations; (5) orophytia, subalpine formations; (6) acrophytia, alpine formations; (7) chionophytia, niveal formations.
355. Mixed formations. These are mixtures of two, rarely more, adjacent formations, or of two consecutive stages of the same succession. Mixed formations are really transitions in space or in time between two distinct formations. Theoretically, they are to be referred to one or the other, according to the preponderance of species. Actually, however, they often persist in an intermediate condition for many years, and it becomes necessary to devote considerable attention to them. In some cases, there is good reason to think that the species of two contiguous formations have become permanently associated, and thus constitute a new formation. This is often apparently true in succession, when the change from one stage to the next requires a long term of years, but it is really true only of the very rare cases in which a succession becomes stabilized in a transition stage. When the mixture is due to development, the formations concerned are often quite dissimilar, e. g., grassland and thicket, thicket and forest. If it is the result of position, the formations are usually similar, i. e., both are grassland, thicket, or forest, since the plants of the lower level are regularly assimilated or destroyed, when invasion occurs at two levels. The term mictium (μικτόν, mixture) is here proposed for the designation of all mixed formations, whether they arise from succession or from juxtaposition. Thus, the Mentzelia-Elymus-mictium is the transition
between the Mentzelia-Pseudocymopterus-chalicium and the Elymus-Muhlenbergia-chalicium. Similarly, the Populus-Piceamictium and the Pinus-Pseudotsuga-mictium are transition stages in the development of the Picea-hylium. On the other hand, the Andropogon-Bulbilis-mictium is a mixture produced by the mingling of two contiguous prairie formations. In the future development of this subject, it will probably become desirable to name mixed formations on the basis of origin, but at present this is unnecessary. Both in classification and in description they should be considered between the formations which give rise to them, and this will at once indicate their origin.
Fig. 82. A mixed formation of aspens and spruces (Populus-Picea-mictium), preceding the final spruce forest of a burn succession.
Puzzling cases of mixture resulting from position occur toward the limits of facies which occupy extensive areas. Bouteloua
oligostachya, and Andropogon scoparius extend from the prairies through the sand-hills and plains, and into the foot-hills of the Rocky mountains. Their abundance at once raises a question as to the validity of the prairie, sand-hill, plain, and foot-hill formations. If these two grasses were controlling, and equally characteristic throughout, then the entire stretch would have to be regarded as a single formation. Since they are often absent, or mixed with other facies of greater importance, they can not be considered the sole tests of the formation. This view is reinforced by the fact that prairie, sand-hill, plains, and foot-hill all have their characteristic principal and secondary species, in addition to facies that are more or less typical. In certain formations, doubtless, Bouteloua and Andropogon are relicts, in others invaders, while in the formations actually constituted by them they are dominant. The final solution of such problems is quite impossible, however, until the comparative study of large areas can be based upon the accurate detailed investigation of the component formations.
E V
356. Scope and methods. The experimental study of the formation as a complex organism rests upon methods essentially similar to those discussed under experimental evolution. The scope of the two fields is practically the same, moreover, in that both deal with the experimental development of an organism and the structures that result. The actual problems are naturally very different, since the formation is a complex of individual plants, but the fundamental basis of habitat, function, and structure is common to both. However, the functions now to be considered are aggregation, invasion, competition, etc., and the structures, zones, consocies, societies, communities, and families. The latter may properly be regarded as adaptations called forth by the adjustment, i. e., aggregation, migration, ecesis, etc., of the formation to the physical factors of the habitat. As consequences of measured factors, formational adjustment and adaptation must themselves be carefully measured and recorded. For these purposes, the methods of quadrat and transect, of chart, photograph, and formation herbarium are used. Invaluable as they are for any scientific inquiry into vegetation, such methods form the very foundation of experimental study in which accuracy is the first desideratum.
It has already been shown that nature’s own experiments in the production of new forms furnish the best material for experimental evolution. This statement is equally true of experimental vegetation. The formation of new habitats by weathering and transport, and the denuding of old ones, yield experimental plots of the greatest value. This is likewise the case in the great majority of formations, where invasion or competition is active. These are the phenomena that must be considered in any careful study of vegetation, but in taking them up from the experimental standpoint, greater attention must be paid to detail, and the changes must be followed closely for a longer time. The method that makes use of existing changes in vegetation is designated the method of natural habitats. In contrast with this is the method of artificial habitats, in which the habitat itself is definitely modified, or a group of species actually transferred to a
different habitat. Many problems of vegetation can be attacked with greater success under control than in the field. This is particularly true of competition, in which results can be obtained most readily by means of the method of control habitats, as carried on in the plant house.
