Anatomical Imaging of the Urinary



The plain film of the abdomen, or KUB, is a simple way of determining renal size and shape. The normal kidney shadow will approximate the length of three and onehalf vertebral bodies or about 12 cm. Bilaterally small kidneys in a patient with renal insufficiency implies a chronic, irre­versible process, whereas the presence of enlarged kidneys suggests obstructive, inflammatory, infil­trative, or cystic disease. Radiopaque renal calculi composed of calcium, magnesium ammonium phosphate (struvite), or cystine are often apparent on a plain film of the abdomen. Renal ultrason­ography is another noninvasive method of ob­taining an anatomical image of the kidney and the collecting system. Renal ultrasonography is par­ticularly useful in the detection of renal masses, renal cysts, and dilation of portions of the urinary tract (hydronephrosis). An accurate determina­tion of renal size and shape can also be discerned from the sonogram.

The radioisotopic renal scan provides infor­mation about renal blood flow and tubular func­tion. The test involves the intravenous adminis­tration of radiolabeled compounds that are excreted by the kidney. An external scintillation camera provides an image of the kidneys and cal­culates the rate of uptake and excretion of the la­beled compound. Technetium-99 DPTA is the compound used to assess renal vascular perfu­sion qualitatively. Impaired renal perfusion, as in the setting of unilateral renal artery stenosis or renal infarction, is characterized by asymmetric uptake of technetium. Generalized renal hypo­perfusion, as in the setting of acute glomerulo­nephritis or renal transplant rejection, can be rec­ognized also. An evaluation of renal tubular function may be obtained by the use of hippuran 131I, a compound eliminated by tubular secretion. Impaired hippuran excretion in the face of a nor­mal technetium perfusion is commonly observed with acute tubular necrosis or chronic renal dis­ease.

The intravenous pyelogram involves the intra­venous injection of iodinated radiographic contrast medium that is excreted by glomerular fil­tration. The contrast medium is concentrated in the renal tubules as water is reabsorbed, and this produces a nephrogram image within the first few minutes after injection. As the medium passes into the collecting system, the calyces, renal pel­vis, ureters, and bladder are visualized. This study is useful in the identification of lesions such as renal calculi, pyelonephritic scars, cysts, or renal tumors. The computerized axial tomograph (CAT} scan of the kidney provides similar information about renal anatomy, but, in contrast to the intravenous pyelogram, provides more precise differ­entiation of solid from cystic lesions. In both stud­ies, the uptake and excretion of contrast by the kidney is prolonged in patients with renal insuf­ficiency. In these patients, useful information can still be obtained from these tests by examining radiographs taken 24 to 48 hours after the injec­tion of contrast medium. The risk of contrast me­diuminduced nephrotoxicity is the major lim­iting factor to these tests and for this reason they should be used with caution in highrisk patients. Individuals with underlying renal dysfunction, multiple myeloma, or diabetes mellitus should be well-hydrated to reduce this risk.

Retrograde pyelography is performed by the in­jection of radiocontrast material directly into the ureters at the time of cystoscopy. It is useful in the definition of obstructing lesions within the ureter or renal pelvis, particularly in the setting of a nonvisualizing kidney on intravenous pye­lography. If an obstruction is identified, it can often be removed or bypassed by the placement of a ureteral catheter at the time of the procedure. Cystoscopy is often indicated in the evaluation of unexplained hematuria when bladder lesions are suspected.

Renal arteriography involves the direct injec­tion of radiographic contrast medium into the aorta and renal arteries and is used to define the renal vasculature. Renal arteriography is partic­ularly useful in the evaluation of patients with suspected renal artery stenosis or thrombosis and in those with a renal mass. Because renal arteri­ography is a more invasive test, its use should be limited to those situations in which a strong clin­ical indication exists and the patient is considered a candidate for surgical intervention. Renal ven­ography is used to confirm the diagnosis of renal vein thrombosis.