Kidney Size

The kidney is imaged in transverse and sagittal planes and is normally 9 to 12 cm in length in adults. Differences in kidney size can be detected with all imaging modalities. Figure 5-1 diagrams the common causes of enlarged and shrunken kidneys.

The normal renal cortex is hypoechoic compared with the fat-containing echogenic renal sinus (Fig. 5-2, A). The cortical echotexture is defined as isoechoic or hypoechoic compared with the liver or spleen. In children, the renal pyramids are hypoechoic (Fig. 5-2, B), and the cortex is characteristically hyperechoic compared with the liver and the spleen.² In adults, an increase in cortical echogenicity is a sensitive marker for parenchymal renal disease but is nonspecific (Fig. 5-3). Decreased cortical echogenicity can be found in acute pyelonephritis and acute renal vein thrombosis.

The normal renal contour is smooth, and the cortical mantle should be uniform and slightly thicker toward the poles. Two common benign pseudomasses that can be seen with ultrasound are the dromedary hump and the column of Bertin. The column of Bertin results from bulging of cortical tissue into the medulla; it is seen as a mass with an echotexture similar to that of the cortex, but it is found within the central renal sinus (Fig. 5-4). The renal pelvis and proximal ureter are anechoic. An extrarenal pelvis refers to the renal pelvis location outside the renal hilum. The ureter is not identified beyond the pelvis in nonobstructed patients.

Obstruction can be identified by the presence of hydronephrosis (Fig. 5-5). Parenchymal and pelvicalyceal nonobstructing renal calculi as well as ureteral obstructing calculi can be readily detected (Fig. 5-6). The upper ureter will also be dilated if obstruction is distal to the pelviureteral junction (see Fig. 5-5, C). False-negative ultrasound examination findings with no hydronephrosis occasionally occur in early obstruction. Obstruction without ureteral dilation may also occur in retroperitoneal fibrosis and in transplanted kidneys as a result of periureteral fibrosis.

Cysts can be identified as anechoic lesions and are a frequent coincidental finding during renal imaging. Ultrasound usually readily identifies renal masses as cystic or solid (Figs. 5-7 and 5-8). However, hemorrhagic cysts may be mistakenly called solid because of increased echogenicity. Differentiation of cysts as simple or complex is required to plan intervention.

Bladder

Real-time imaging can be used to evaluate for bladder wall tumors and bladder stones. Color flow Doppler evaluation of the bladder in well-hydrated patients can be used to identify a ureteral jet, produced when peristalsis propels urine into the bladder. The incoming urine has a higher specific gravity relative to the urine already in the bladder (Fig. 5-9). Absence of the ureteral jet can indicate total ureteral obstruction.

Color Doppler investigation of the kidneys provides a detailed evaluation of the renal vascular anatomy. The main renal arteries can be identified in most patients (Fig. 5-10). Power Doppler imaging is a more sensitive indicator of flow, but unlike color Doppler imaging, power Doppler provides no information about flow direction and cannot be used to assess vascular waveforms. However, power Doppler imaging is exquisitely sensitive for detection of renal parenchymal flow and has been used to identify cortical infarction.

Renal Artery Duplex Scanning

The role of gray-scale and color Doppler sonography in screening for renal artery stenosis is controversial. The principle is that a narrowing in the artery will cause a velocity change commensurate with the degree of stenosis as well as a change in the normal renal artery waveform downstream from the lesion. The normal renal artery waveform demonstrates a rapid systolic upstroke and an early systolic peak (Fig. 5-11, A). The waveform becomes dampened downstream from a stenosis. This consists of a slow systolic acceleration (tardus) and a decreased and rounded systolic peak (parvus) (Fig. 5-11, B). It also results in a decrease in the resistive index, defined as the end-diastolic velocity (EDV) subtracted from the peak systolic velocity (PSV) divided by PSV: (PSV − EDV)/PSV. The normal resistive index is 0.70 to 0.72.

The entire length of the renal artery should be examined for the highest velocity signal. The origins of the renal arteries are important to identify because this area is often affected by atherosclerosis, but the arteries are often difficult to visualize because of overlying bowel gas. Within the kidney, medullary branches and cortical branches in the upper, middle, and lower thirds should be included to attempt detection of stenosis in accessory or branch renal arteries.

Proximal and distal criteria exist for diagnosis of significant renal artery stenosis, usually defined as stenosis greater than 60%. The proximal criteria detect changes in the Doppler signal at the site of stenosis and provide sensitivities and specificities ranging from, respectively, 0% to 98% and 37% to 98%.^9,10 Technical failure rates are typically 10% to 20%.¹¹ Renal artery stenosis may also be missed if PSV is low because of poor cardiac output or aortic stenosis. False-positive results can occur when renal artery velocity is increased because of high-flow states, such as hyperthyroidism or vessel tortuosity.¹² The distal criteria are related to detection of a tardus-parvus waveform distal to a stenosis; sensitivities and specificities of 66% to 100% and 67% to 94%, respectively, have been reported.^13,14 Technical failure with distal criteria is much lower than with proximal evaluation (<5%). False-negative results can occur from stiff poststenotic vessels, which will decrease the tardus-parvus effect.¹⁵ The tardus-parvus effect may also be a result of aortic stenosis, low cardiac output, or collaterals in complete occlusion, giving a false-positive result.

Combining the proximal and distal criteria improves the detection of stenoses. Sensitivity of 97% and specificity of 98% can be achieved when both the extrarenal and the intrarenal arteries are examined.¹⁶ When it is technically successful, Doppler ultrasound has a negative predictive value of more than 90%.¹⁶ However, reliable results require a skilled and experienced sonographer and a long examination time. Despite these limitations, Doppler studies also have several advantages. Noninvasive, inexpensive, and widely available, Doppler studies also allow structural and functional assessment of the renal arteries and imaging without exposure to radiation or nephrotoxic agents.

Some physicians prefer CT angiography (CTA) or magnetic resonance angiography (MRA) as a faster and more reliable screening tool than ultrasound, but at present the choice should depend on local expertise and preference. For further discussion of the diagnosis and management of renovascular disease, see Chapters 39 and 66.

Contrast-Enhanced and Three-Dimensional Ultrasound

Ultrasound contrast agents, initially introduced to assess cardiac perfusion, are now being used to evaluate perfusion to other organs, such as the kidney. These intravenous agents are microbubbles 1 to 4 µm in diameter (smaller than erythrocytes) that consist of a shell surrounding the echo-producing gas core. The microbubbles oscillate in response to the ultrasound beam frequency and give a characteristic increased echo signal on the image. Preliminary studies evaluating renal perfusion in dysfunctional kidneys show reduced flow compared with normal kidneys as well as improved lesion detection (Fig. 5-12). However, the clinical adoption of microbubble imaging in the kidney remains uncertain, particularly with the general availability and robustness of CT and MRI, and its use is off-label in the United States and not presently reimbursed.

Most renal calculi are radiodense, although only about 60% of urinary stones detected on CT are visible on plain films.¹⁷ CT demonstrates nonopaque stones, which include uric acid, xanthine, and struvite stones. However, neither CT nor plain radiography may detect calculi associated with protease inhibitor therapy.¹⁸ Oblique films are sometimes obtained to confirm whether a suspicious upper quadrant calcification is renal in origin. Calculi that are radiolucent on plain films are usually detected as filling defects on IVU. IVU has higher sensitivity than radiography but lower sensitivity than CT. If available, CT is the imaging modality of choice for detection of urinary calculi.¹⁹

Nephrocalcinosis may be medullary (Fig. 5-13, A and B) or cortical (C) and is localized or diffuse. The common causes of nephrocalcinosis are discussed in Chapter 59 (see Box 59-7).

The pelvicalyceal system is best evaluated on the early postcontrast films. Normally, there are about 10 to 12 calyces per kidney. The calyces drain into the infundibula, which in turn empty into the renal pelvis (Fig. 5-14). The infundibulum and renal pelvis should have smooth contours without filling defects. In a common variant, vessels can cross the pelvicalyceal system or ureters, causing extrinsic compression defects that should not be mistaken for tumors or other urothelial lesions. When more than one calyx drains into an infundibulum, it is known as a compound calyx, most frequently seen in the poles. The normal calyx is gently cupped. Calyceal distortion occurs with papillary necrosis and reflux nephropathy.