Poor flow along with poor function is consistent with acute tubular necrosis or end-stage renal disease. Decreased flow to one kidney suggests arterial occlusion of that kidney. To establish the possibility of renal artery stenosis, the test is done both with and without captopril

Ultrasonography noninvasively images the kidney. It can identify the renal cortex, medulla, pyramids, and a distended collection system or ureter. Kidney size can be determined; a kidney less than 9 cm in length indicates significant irreversible renal disease. A difference in size of more than 1.5 cm between the two kidneys is observed in unilateral renal disease. Renal ultrasound is also performed to screen for hydronephrosis, characterize renal mass lesions, screen for autosomal dominant polycystic kidney disease, evaluate the perirenal space, localize the kidney for a percutaneous invasive procedure, and assess postvoiding bladder residual.

The intravenous pyelogram (IVP) has been for many years the standard imaging procedure for evaluating the urinary tract since it provides an assessment of the kidneys, ureters, and bladder. The dye is filtered and secreted by the renal tubules in normal kidneys, resulting in a nephrogram formed by opacification of the renal parenchyma. The density of the nephrogram is dependent on the GFR. Filling of the pelvicaliceal system produces the pyelogram. The IVP can demonstrate differential function between the right and left kidneys by the rate of appearance of the nephrogram phase.

An IVP necessitates the injection of contrast and is relatively contraindicated in patients with an increased risk for developing acute renal failure (eg, diabetes mellitus with serum creatinine > 2 mg/dL, severe volume contraction, or prerenal azotemia); chronic renal failure with serum creatinine greater than 5 mg/dL; and multiple myeloma. IVP is performed to obtain a detailed view of the pelvicaliceal system, assess renal size and shape, detect and localize renal stones, and assess renal function. Ultrasonography has replaced it in many clinical situations.

Computed tomography (CT) is required for further investigation of abnormalities detected by ultrasound or IVP. Although the routine study requires radiographic contrast administration, no contrast is necessary if the reason for the study is to demonstrate hemorrhage or calcifications in the kidneys. Since contrast is filtered by the glomeruli and concentrated in the tubules, there is enhancement of parenchymal tissue, making abnormalities such as cysts or neoplasms easily identified and allowing good visualization of renal vessels and ureters. CT is especially useful for evaluation of solid or cystic lesions in the kidney or the retroperitoneal space, particularly if the ultrasound results are suboptimal.

MRI can easily distinguish renal cortex from medulla. Loss of corticomedullary function, which can be seen in a variety of disorders (glomerulonephritis, hydronephrosis, renal vascular occlusion, and renal failure) will be evident on MRI. Renal cysts seen on a CT scan can also be identified by MRI. For some solid lesions, MRI may be superior to CT scanning. MRI is indicated as an addition or alternative to CT for staging renal cell cancer and as a substitute for CT in the evaluation of a renal mass, especially for patients with tumors in whom contrast is contraindicated; in addition, the adrenals are well imaged.

Renal arteriography is useful in the evaluation of atherosclerotic or fibrodysplastic stenotic lesions, aneurysms, vasculitis, and renal mass lesions. Venography is useful to diagnose renal vein thrombosis.