The Physical Basis of

Figure 1-1. Standard posteroanterior chest radiograph demonstrated the striking contrast between the heart (H) and lungs (L). A tumor (T) is seen at the left hilum.

Computed radiography (CR) or digital radiography is presently replacing conventional screen-film combination techniques.The most common CR technique,photostimulable phosphor computed radiography (PPCR),uses a phosphor-coated plate to replace the film-screen combination.When a cassette containing the phosphor-coated plate is exposed to x-rays,the phosphor stores the absorbed x-ray energy.The exposed cassette is then placed in a PPCR reader that uses a laser to stimulate release ofelectrons,resulting in the emission ofshort-wavelength blue light.The brightness ofthe blue light is dependent on the amount of absorbed x-ray photon energy.This luminescence generates an electrical signal that is reconstructed into a gray-scale image,which may be displayed on a monitor or printed as a hard copy.Digital images generated from PPCR are capable ofbeing transmitted through a picture archiving and communications system (PACS),similar to other digital images acquired from CT or MR facilities.PPCR is better than plain radiography in linear response to a wide range ofx-ray exposure.However,PPCR provides less spatial resolution than plain radiography.Another CR technique that is being developed uses an amorphous selenium-coated plate,which directly converts x-ray photons into electrical charges.

Fluoroscopy uses a fluorescent screen instead ofradiographic film to view real-time images generated when an x-ray beam penetrates through a certain part ofthe body.An image intensifier absorbs x-ray photons and produces a quantity oflight on the monitor.The brightness ofthe image is proportional to the number ofincident photons received. Fluoroscopy is a major modality used to examine the gastrointestinal tract.For example,fluoroscopy can be used to follow the course ofcontrast materials through the gastrointestinal tract,allowing the evaluation ofboth structure and function.Spot filming or video recording may be used synchronously with fluoroscopy to optimally demonstrate pathology.Fluoroscopy is also used to monitor catheter placement during angiography and to guide interventional procedures.In recent years,digital detectors (such as chargecoupled devices,CCDs) have begun to replace video cameras on fluoroscopy units.

Conventional tomography produces an image ofone intended area by blurring structures superimposed on both sides ofa focus plane.This technique,however,has been largely replaced by CT.

Mammography uses a film-screen combination technique to evaluate breast lesions for the early detection of breast carcinoma.A mammographic unit is installed with a special x-ray tube and a plastic breast-compression device.A standard mammogram obtains views in two projections, producing craniocaudal (CC) and mediolateral oblique (MLO) images ofthe breast.Additional images ofthe breast in other projections,such as mediolateral (ML) views,and