Ultrasonography can characterize the thickness and echogenicity of the renal cortex, medulla, and pyramids and identify a distended urine collecting system. Kidney size can be measured; normal kidney size, which can vary depending on age and size of adults, is usually 9–12 cm in length. A kidney less than 9 cm in length in an adult suggests (but does not confirm) significant, irreversible kidney disease. A difference in size of more than 1.5 cm between the two kidneys may be observed in unilateral kidney disease. Kidney ultrasound is also performed to assess for obstruction and hydronephrosis (eFigure 22–2), to characterize kidney mass lesions, to screen for autosomal dominant polycystic kidney disease (see Figure 22–6), to evaluate the perirenal space, to localize the kidney for a percutaneous invasive procedure, and to assess postvoiding residual urine volume of the bladder. Assessing kidneys in markedly obese patients can be difficult due to body habitus; a CT scan may be required.
Hydronephrosis. A: Longitudinal scan from a patient with hydronephrosis due to chronic reflux. Numerous dilated fluid-filled spaces (calices) are seen as well as characteristic atrophy of the upper pole parenchyma (arrows). B: Longitudinal scan from a patient with moderate to marked hydronephrosis. Only a thin rim of renal parenchyma (arrows) remains. C: Longitudinal scan from a patient with chronic obstruction and marked hydronephrosis. No cortical tissue remains in this kidney. (Courtesy of Peter W. Callen, MD.)
CT scanning may be required to further investigate abnormalities detected by ultrasonography. Although routine CT requires radiographic contrast administration, no contrast is necessary if the study is performed to identify renal parenchymal hemorrhage or calcifications in suspected kidney stone disease (nephrolithiasis). Noncontrast helical CT scanning is 95% sensitive and 98% specific for diagnosing nephrolithiasis in patients with acute flank pain, and is considered the test of choice in diagnosis of nephrolithiasis. Because 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 scanning is especially useful for evaluation of solid or cystic lesions in the kidney or the retroperitoneal space, particularly if ultrasound results are suboptimal.
MRI can easily distinguish renal cortex from medulla. Loss of the corticomedullary junction in a variety of disorders (eg, glomerulonephritis, hydronephrosis, renal vascular occlusion, and renal failure) is evident on MRI. Renal cysts can also be identified by MRI. For some solid lesions, MRI may be superior to CT scans. MRI is indicated as an addition or alternative to CT scanning for ...