Ultrasound and Computed Tomography



Ultrasound (US) and computed tomography (CT) markedly extend the diagnostic capacity of the radiologist far beyond that which was possible with barium studies alone. Indeed, organs such as the pancreas, which were previously radiologically invisible, are now readily imaged. The in­formation provided by US and CT is complemen­tary to that provided by barium studies: US and CT are most useful in examining the solid abdom­inal organs (liver, spleen, pancreas, kidney, retro­peritoneal lymph nodes] and gallbladder, whereas barium studies excel in depicting the tubular gas­trointestinal tract. outlines the relative merits of US and CT.

Ultrasound, which employs high-frequency sound waves rather than x-rays, allows the ex­amination of solid and fluid-filled structures non-invasively. Air or gas obscures the ultrasound beam; thus, structures under gas-filled loops of bowel cannot be visualized. US requires great skill on the part of the operator; however, real­time images allow examination of dynamic as well as static processes. Images, like those of CT, are displayed in cross-section. US has the ability (1) to detect abdominal masses as small as 2 cm in diameter, particularly in the right and left upper quadrants and pelvis; (2) to differentiate fluid-filled cysts from solid masses; (3j to identify gallstones rapidly and more efficiently than is possible with oral cholecystography or CT; (4] to identify dilated bile ducts; and (5) to detect ascites and some vascular abnormalities such as abdom­inal aortic aneurysms.

Computed tomography uses multiple x-ray beams and detectors in conjunction with com­puter analysis to identify and display small dif­ferences in tissue density. Anatomical definition is more precise with CT than with US and imaging quality is not affected by bowel gas, but the equip­ment is very expensive. CT is. primarily used for the detection of mass lesions (tumors, cysts, ab­scesses], although it also detects dilated bile ducts, pancreatic inflammation, and some gall­stones. Some diffuse hepatic parenchymal lesions that alter liver density, such as fatty liver or hem­ochromatosis, can also be identified by CT. Fi­nally CT is better than US in detecting retroper­itoneal lesions and parenchymal processes such as ruptures or hematomas of the liver, spleen,, and kidney.
Both US and CT also offer the option of per­forming guided thin needle aspiration of lesions virtually anywhere in the abdomen to obtain cells or fluid for cytological examination and culture.