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 one-half 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 (Table 1).
Renal ultrasonography is another non-invasive method of obtaining an anatomical image of the kidney and the collecting system. This technique is particularly useful for the detection of renal masses, cysts, and dilation of portions of the collecting system (hydronephrosis). Ultrasonography can also be used to assess the patency of the renal veins in cases of suspected renal vein thrombosis.
The radio-isotopic renal scan provides important 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. Technetiurn-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 asymmetrical uptake of technetium. Generalized renal hypo perfusion, 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 I-131, a Compound eliminated by tubular secretion. Impaired hippuran excretion in association with normal technetium perfusion is commonly observed in acute tubular necrosis or chronic renal disease.
The intravenous urogram involves the intravenous administration of iodinated radiographic contrast medium that is excreted through the kidney by glomerular filtration. The contrast medium concentrates in the renal tubules and 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 renal calculi, pyelonephritic scars, cysts, or renal tumors and in defining various congenital anomalies of the urinary tract. The computed tomographic (CT) scan of the kidney provides more precise information regarding renal masses as well as a definition of the perinephric space and other retroperitoneal structures. In both studies, the uptake and excretion of contrast by the kidney are prolonged in patients with renal insufficiency. Delayed views obtained 24 to 48 hours after injection of contrast may provide useful information. However, the risk of contrast medium-induced nephrotoxicity limits the utility of these studies in certain high-risk patients. CT scanning of the kidney can be performed without intravenous contrast material in selected cases. The role of magnetic resonance imaging (MRI), the newest imaging technique in radiology, is uncertain at present.
Retrograde pyelography is performed by the injection of radio contrast material directly into the ureters at the time of cystoscopy. This technique 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 assess the renal vasculature. Renal arteriography is particularly useful nation 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 has been limited to those situations in which a strong clinical indication exists and the patient is candidate for surgical intervention. Renal vein catheterization is used to confirm the diagnosis of renal vein thrombosis or to obtain blood samples renal vein, particularly in the setting of no-vascular hypertension.
Table 1. Imaging studies of the urinary tract: comparative aspects
Dr. Afsaneh Jeddi