13.1.1 Kidneys

The normal sonographic appearances of the kidneys vary with age (Table 13.1). In the neonate, the glomeruli occupy twice as much of the cortical volume as in adults, and medullary volume is larger in children. Therefore, the renal cortex in the neonate is isoechoic to the liver and spleen, and the relatively high echogenicity of the renal cortex contrasts with the prominent, hypoechoic medullary pyramids (Fig. 13.1 a, b). In premature infants, the renal cortex is even hyperechoic to the liver and spleen. The echogenicity of the renal cortex decreases gradually in the first year of life, and the cortex usually becomes hypoechoic to the liver when the child is 12 to 15 months old (Fig. 13.2). Persistent fetal lobulation is a common finding in infants (Fig. 13.1 a). In infants and young children, the renal sinus lacks the echogenicity seen in adults and older children because of the paucity of fat (Fig. 13.3).

In the neonate, protein deposits may cause transient increased echogenicity at the tip of the medullary pyramids (Fig. 13.4). In children with normal urine output, this is a self-limiting finding that disappears in a few days and does not require further work-up.

A renal pelvis up to 10 mm in anteroposterior diameter is a normal finding in children and should not automatically be labeled as pathologic dilatation.

Fetal lobulation of the kidneys may persist through adulthood and can be differentiated from renal scarring by the lack of parenchymal thinning. Renal scars are most often located over the calices, whereas the lobules in fetal lobulation contain a pyramid, so the lobulation lies between the pyramids.

Cortical fusion defects may be seen as interrenuncular septa, which are hyperechoic lines extending from the cortex to the medulla of the kidney (Fig. 13.5). There is no associated parenchymal thinning.

A column of Bertin is a normal structure and represents the extension of renal cortical tissue that separates the pyramids. Occasionally, a prominent or hypertrophic column of Bertin may be seen in the middle part of the kidney and should not be mistaken for a renal tumor. The hypertrophic Bertin column is continuous with and similar in appearance to the renal cortex. The column may splay the calices, but the outline of the kidney is preserved (Fig. 13.6).

Compound calices can be seen in the upper and lower poles of the kidney as small, elongated, fluid-filled calices (Fig. 13.7). They occur when more than one papilla drains into a single calyx. Compound calices have no mass effect, there is no dilatation of the calyx neck, and there is no thinning of the overlying renal cortex.

Duplex systems are a common finding, with a reported incidence of 0.8%. Duplications range from a split renal pelvis to a complete duplication, with two separate ureters and two bladder ostia. The ureters often fuse somewhere distal to the renal pelvis to form one ureter (Fig. 13.8). Duplicated systems are more often associated with urinary obstruction or vesicoureteral reflux. In the absence of complications, such as recurrent or atypical urinary tract infections or dilatation, a duplex system may be regarded as a normal variant and does not require further work-up or treatment.

The normal waveform of the renal artery on pulsed Doppler sonography is shown in Fig. 13.9. Both peak systolic velocity and the resistive index (RI) of a normal kidney are age-dependent. Peak velocity in the main renal artery varies from 30 to 50 cm per second in the neonate to 100 to 110 cm per second in the adolescent. An RI up to 0.85 is normal in a neonate and decreases to less than 0.7 during the first year of life. The renal arteries course posterior to the renal veins and reach the renal hilum, where they branch to form segmental arteries. It is not uncommon to have accessory polar arteries to the kidney; therefore, Doppler curves must be obtained in all segments of the kidney.

13.1.2 Ureters

Nondilated ureters can be difficult to assess throughout their entire length; however, even normal ureters can often be visualized behind the bladder in children (Fig. 13.10). The ureters should be thin-walled and have a maximum diameter of 5 mm. Normal peristalsis of the ureters and the presence of a urine jet from the ureter ostia within the bladder are useful parameters for the evaluation of ureteral function and patency.

13.1.3 Bladder

The bladder can be properly assessed only when the bladder is full; therefore, proper hydration and filling of the bladder before the scan are important. The bladder wall is smooth, and the musculature at the trigone is slightly thicker than in the rest of the bladder. The bladder wall thickness should not exceed 3 mm when the bladder is full and 5 mm when it is empty. Bladder emptying and residual urine should be correlated with the patient’s sex, age, and bladder volume before micturition.

13.2.1 Renal Hypodysplasia

Although the term dysplasia is based on a histologic evaluation, it is often used to define cases with abnormal renal parenchymal structure—unilateral or bilateral, diffuse or focal—associated with a reduction of the kidney size. It is characterized by inhomogeneous hyperechoic parenchyma with less evident or absent corticomedullary differentiation. A common finding is the presence of isolated cysts, usually small (diameter < 1 cm) in the outer, often subcapsular, cortex (Fig. 13.11 and Fig. 13.12).

13.2.2 Ureteropelvic Junction Stenosis

Ureteropelvic junction stenosis is the most common type of obstruction of the upper urinary tract in childhood. It is often bilateral and may differ in degree between the two kidneys; it is frequently demonstrated in prenatal US scans. The stenosis may be intrinsic or due to external compression, often as the result of a supernumerary renal artery.

US features are often specific, with dilatation of the pelvicaliceal system in which the renal pelvis is more enlarged than the calices and in which a clear “stop” is seen at the ureteropelvic junction (Fig. 13.13) without dilatation of the ureter.

In case of a suspected crossing vessel on color Doppler US (Fig. 13.14), a more detailed anatomical study by means of magnetic resonance (MR) angiography is often necessary to plan the surgical approach in terms of risk and scheduled time in the operating theater.

13.2.3 Ureterovesical Junction Stenosis

Ureterovesical junction stenosis is a defect of the ureterovesical junction, usually unilateral. It is also associated with vesicoureteral reflux (VUR) and urinary tract infections (UTI). The whole collecting system on the affected side is dilated, with the lower part more markedly affected than the upper. It is sometimes possible to visualize the premural stenotic ureteral tract (Fig. 13.15). The ureter is often abnormally dilated and tortuous, and uroepithelial thickening can be seen (Fig. 13.16).

13.2.4 Ureterovesical Reflux

Ureterovesical reflux is a consequence of dysfunction in the ureterovesical junction. It is the most common urological anomaly associated with recurrent UTI in childhood and may cause renal scarring leading to reflux nephropathy (Fig. 13.17). US can show various degrees of hydroureteronephrosis, which tends to increase during crying, micturition, or other maneuvers that increase abdominal pressure. Thickened uroepithelium is often seen in reflux systems.

It may be possible to diagnose VUR on color Doppler US when backward flow of urine toward the ureter and the pelvis is detected (Fig. 13.18). However, normal US findings do not exclude the presence of VUR, even if it is moderate or severe. Patients with UTI must be selected for further work-up for VUR with voiding cystourethrography (VCUG) based on the clinical presentation of the infection, the patient’s age and family history, and the US findings (see section on UTI). Severe and early antenatal VUR in male fetuses is often associated with high-degree renal dysplasia, which can lead to progressive loss of renal function.

13.2.5 Duplicate Collecting System

This is a quite common, often bilateral, urologic anomaly. Duplicate collecting systems (also known as duplex collecting systems) can be defined as renal units containing two pyelocaliceal systems. The US feature is the presence of a midrenal parenchymal septum dividing the two moieties. Indirect US findings are increased renal size (elongated “bread stick” appearance) and contour notches (Fig. 13.19).

The collecting systems may fuse anywhere along their course to form a single ureter, or they may empty separately into the bladder (Fig. 13.20). Dilatation of one or both ureters is a frequent finding. VUR is common, involving the lower collecting system, while ureterocele is commonly associated with the upper pole ureter (Fig. 13.21). Ureterocele is a cystic dilatation of the intravesical submucosal ureter that changes in size with ureteral peristalsis/ micturition.

US shows an echogenic membrane arising from the bladder wall. This membrane is more or less rounded, owing to differences in internal tension (Fig. 13.22). Ureteroceles may have an ectopic orifice in the bladder neck or urethra (Fig. 13.23 and Fig. 13.24). Ectopic ureters and ureteroceles may also be encountered within a single excretory system.

13.2.6 Horseshoe Kidney

This anomaly is included in a group of so-called fusion anomalies, in which the two kidneys are fused in early embryonic life. Fusion anomalies of the kidneys can generally be grouped into two categories: (1) horseshoe kidney and its variants; and (2) cross-fused ectopy.

Horseshoe kidney is the most common fusion anomaly. It consists of two distinct functioning kidneys on each side of the midline, connected at the lower poles by an isthmus of functioning renal parenchyma or by fibrous tissue that crosses the midline of the body. Horseshoe kidneys are usually ptotic, meaning that the distance between the posteroinferior pilaster of the diaphragm and the upper renal pole is 3 cm or more. The isthmus is located in front of the inferior cava vein, aorta, and rachis (Fig. 13.25 a). VUR and/or ureteropelvic junction stenosis is more frequently seen in this anomaly (Fig. 13.25 b).

Horseshoe kidney is generally distinguished from cross-fused ectopia, in which the two fused kidneys lie on one side of the spine and the ureter of the crossed kidney crosses the midline to enter the bladder on the opposite side. Therefore, the presence of a ureter behind the bladder and the absence of an ipsilateral kidney combined with an elongated contralateral kidney is highly suggestive of cross-fused ectopia.