Sonography of the Kidney
by W. Charles O’Neill of Emory University
Sonography of the kidney can reveal critical information of 1) renal parenchyma, 2) the urinary space, 3) masses, and 4) renal vasculature.
Renal Parenchyma: Renal ultrasonography can elucidate kidney size, shape, cortical thickness and echogenicity. Size is directly proportional to the height of the patient, but is conventionally thought to be between 10–12 cm. Width of the kidney, while measured often, is clinically not relevant. Most adult kidneys have a uniform shape. Unlike neonates that have a lobular or globular kidney shape (normal), adults lose this shape. If, however the lobular appearance is seen in an adult kidney, it would suggest scar tissue and chronic disease. Cortical thickness is measured from the renal capsule to the base of the renal pyramid; normal is about 5–9 mm. Although it is difficult to measure (because the pyramids are sometimes hard to identify), cortical thickness is the best assessment for the presence of CKD. Finally, echogenicity is a qualitative measure of how much sound is reflected. The more sound reflected, the more scar tissue (CKD) of inflammatory cells (ARF) in the kidney and the whiter it appears.
Urinary Space: Filling defects are commonly due to kidney stones. Stones are easily identified by their hyperechoic nature and the resulting lack of sound transmission deep to the stone (resulting in a shadow). The subsequent proximal hydronephrosis can also be seen with ultrasound, but keep in mind that 1) not all stones are seen, especially if there is overlying bowel, and 2) not all dilated ureters are due to hydronephrosis (pregnancy, diuresis, stents all cause dilation of the ureter w/o mechanical obstruction).
Masses: Ultrasound is excellent at discriminating between fluid-filled (e.g., cysts) and solid masses. Simple cysts are thin walled, w/o internal septations or echoes, and w/o calcifications. Complex cysts are characterized by the opposite. Whether simple or complex, all cysts cause distal enhancement. This feature is not seen in solid masses, which makes it a good discriminating feature. Nevertheless, ultrasonography should not be used as a screening tool for solid masses.
Renal Vasculature: Peak systolic velocities of the renal artery are difficult to calculate with ultrasonography unless you have an experienced ultrasonographer. That’s because the angle of the sound plays a large role in this calculation. On the contrary, the resistive indices of the interlobular arteries can be calculated easily regardless of the angle of the sound because you are comparing velocities, and not measuring an absolute velocity.