Renal Computed Tomography
Renal computed tomography (CT) provides a useful image of the kidneys made from a series of tomograms or cross-sectional slices, which are then translated by a computer and displayed on a monitor. The image density reflects the amount of radiation absorbed by renal tissue and permits identification of masses and other lesions. An I.V. contrast medium may be injected to accentuate the renal parenchyma's density and help differentiate renal masses. This highly accurate test is usually performed to investigate diseases found by other diagnostic procedures, such as excretory urography.
Procedure And Posttest Care
Normally, the density of the renal parenchyma is slightly higher than that of the liver, but is much less dense than bone, which appears white on a CT scan. The density ofthe collecting system is generally low (black), unless a contrast medium is used to enhance it to a higher (whiter) density. The position of the kidneys is evaluated according to the surrounding structures; the size and shape of the kidneys are determined by counting cuts between the superior and inferior poles and following the contour of the renal outline.
Renal masses appear as areas of different density than normal parenchyma, possibly altering the kidneys' shape or projecting beyond their margins. Renal cysts, for example, appear as smooth, sharply defined masses, with thin walls and a lower density than normal parenchyma. Tumors such as renal cell carcinoma, however, are usually not as well delineated; they tend to have thick walls and nonuniform density. With contrast enhancement, solid tumors show a higher density than renal cysts but lower density than normal parenchyma. Tumors with hemorrhage, calcification, or necrosis show higher densities. Vascular tumors are more clearly defined with contrast enhancement. Adrenal tumors are confined masses, usually detached from the kidneys and from other retroperitoneal organs.Renal CT scanning may also identify other abnormalities, including obstructions, calculi, polycystic kidney disease, congenital anomalies, and abnormal accumulations of fluid around the kidneys, such as hematomas, lymphoceles, and abscesses. After nephrectomy, CT scanning can detect abnormal masses, such as recurrent tumors, in a renal fossa that should be empty.
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