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.

Purpose

• To detect and evaluate renal abnormalities, such as tumor, obstruction,calculi, polycystic kidney disease, congenital anomalies, and abnormal fluid accumulation around the kidneys
• To evaluate the retroperitoneum
• To guide needle placement before percutaneous biopsy
• To determine the kidney's size and location in relation to the bladder following a kidney transplant
• To localize renal or perinephric abscesses for drainage

Patient Preparation

• Explain to the patient that this test permits examination of the kidneys.
• If contrast enhancement isn't scheduled, inform the patient that he needn't restrict food or fluids. If contrast enhancement is scheduled, instruct him to fast for 4 hours before the test.
• Tell him who will perform the test and where and that the procedure takes about an hour, depending on the purpose of the scan.
• Inform the patient that he'll be positioned on an X-ray table, and that a scanner will take films of his kidneys.
• Warn him that the scanner may make loud, clacking sounds as it rotates around his body.
• Tell him that he may experience transient adverse effects, such as flushing, metallic taste, and headache, after injection of the contrast medium.
• Check the patient's history for hypersensitivity to shellfish, iodine, or contrast media. Mark any sensitivities clearly on the patient's chart.
• Just before the procedure, instruct the patient to put on a gown and to remove any metallic objects that could interfere with the scan.
• Administer prescribed sedatives.

Procedure And Posttest Care

• The patient is placed in a supine position on the X-ray table and secured with straps.

• The table is moved into the scanner.
• Instruct the patient to lie still.
• The scanner then rotates around the patient, taking multiple images at different angles within each crosssectional slice.
• When one series of tomograrns is complete, contrast enhancement may be performed. Another series of tomograms is then taken.
• After the I.V. contrast medium is administered, monitor for allergic reactions, such as respiratory difficulty, urticaria, or other skin eruption.
• Information from the scan is stored on a disk or on magnetic tape, fed into a computer, and converted into an image for display on a monitor. Radiographs and photographs are taken of selected views.
• After the test, tell the patient who received a contrast medium that he may resume his usual diet.

Precautions

• Watch for signs of hypersensitivity to the contrast medium if contrast enhancement is required.

Normal Findings

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.

Abnormal Findings

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.

Interfering Factors

• Patient's inability to remain still during the procedure
• Presence of contrast media from other recent tests or of foreign bodies such as catheters or surgical clips (possible poor imaging)