Kidneys : The kidneys are imaged in both longitudinal and transverse planes with the child primarily in the supine position. If the child is able to follow commands, the kidneys are best imaged with the child making a deep inspiration pushing the kidney caudally below the ribs. If the child cannot follow commands, the images might be better obtained with a bolster placed under the flank or with the child turned on the side and the arm raised above the head to open up the space between the ribs and the iliac crest allowing better access to the kidney. Images must be taken with the child in the prone position for better visualization of the kidney between the ribs. The right kidney is imaged along the anterior axillary line using the liver as an acoustic window while the left kidney is imaged along the posterior axillary line using the spleen as an acoustic window. Imaging is done with a curved linear array probe. For children under age 18 months or so, a 6–12 MHz probe is used while older children are better imaged using a 3.5–5 MHz probe.

Bladder : The bladder is imaged with the child in the supine position using a curved array probe (7.7–11 MHz for children under 18 months or 3.5–5 mHz for older children) which makes small skin contact and is easy to angle to gather the most information. The sonographer performs the study in both the transverse and longitudinal directions with the bladder at least partially filled looking for a normal bladder and surrounding organs and documenting any luminal, intraluminal, or extraluminal abnormalities. The transverse images are taken moving from the pubis to the umbilicus in 1–2 cm intervals while the longitudinal images are taken moving from the midline in either direction in 1–2 cm intervals. Measurements are made in the transverse plane measuring the height and width of the urine within the bladder and then the length in the sagittal plane. These three measurements are multiplied together and then multiplied by 0.65 [2] to calculate the bladder volume. Efflux of urine from the ureteral orifices can be demonstrated by applying the Doppler mode over the appropriate location on the bladder floor.

Scrotum : Ultrasound imaging of the scrotum is performed with the child supine and the scrotum supported by a folded towel placed between the thighs. In older children and adolescents, the penis position is maintained suprapubically with a second towel or held in position by the patient. Scanning should be performed with 7–18 MHz high-frequency linear array transducer. Both testes and epididymides should be compared for size, echogenicity, masses, and vascularity and imaged in both the transverse and longitudinal axes. Spectral Doppler analysis of the intratesticular vasculature of both testes is performed when indicated. In patients presenting with acute scrotum, the asymptomatic side should be scanned first so that grayscale and color Doppler gain settings are set optimally to obtain a baseline for comparison with the affected side. In the presence of a suspected varicocele, having the child perform the Valsalva maneuver or placing the child in the upright position can improve the evaluation of venous distention.

The documentation of the ultrasound examination should be as complete and systematic as the examination itself. Mention should be made of normal and abnormal findings. All measurements that were taken should be documented. An example of some components of documentation of the kidneys, bladder, and scrotum is shown in Table 12.2.

Unilateral renal agenesis is most commonly detected in children during imaging performed (1) prenatally, (2) for a urinary complaint, (3) during screening for associated anomalies, or (4) incidentally during evaluation of unrelated conditions. On ultrasound, the renal fossa is empty of a reniform structure and may instead be occupied by bowel. Otherwise the surrounding organs are normal in appearance. It is important that the person performing the study then scans the pelvis to determine if the kidney is ectopic in position. In infants, the adrenal will be prominent and in the normal anatomical position. The remaining kidney will be large reflecting compensatory hypertrophy. The diagnosis can be confined by nuclear scintigraphy (DMSA), MR, or CT; although Krill et al. found that measuring the polar size of the kidney on ultrasound offers sufficient accuracy in predicting a solitary kidney [4]. VCUG is indicated because of the significant incidence of vesicoureteral reflux in the remaining renal unit [5].

Kidneys begin their embryologic journey in the bony pelvis with the renal pelvis oriented anteriorly and then reach the renal fossa and finish their medial rotation by the ninth week of gestation. However, this process may abort anywhere along the course of ascent. Similarly, the metanephric blastema may cross over to the contralateral side and may attach to the other metanephric blastema (crossed-fused ectopia ).

A normal appearing reniform kidney may be seen in the pelvis behind the bladder (Fig. 12.3). However, because of the associated malrotation of a pelvic or crossed (fused, non-fused) kidney, the cortex may appear variable in echogenicity and in corticomedullary differentiation. Also, or for the same reason, measurement of polar length may be artificially small.

The horseshoe kidney is the most common fusion anomaly in children (Fig. 12.4). This may be suspected when both malrotated kidneys are positioned caudal to their usual position. Horseshoe kidneys are at risk for UPJ obstruction and Wilms’ tumor and these may be detected easily by ultrasound. The isthmus between the two kidneys may be thick on ultrasound; function of the isthmus can be evaluated by nuclear scintigraphy.

Ultrasound is well suited for evaluating the kidney with suspected renal vein thrombosis . Risk factors for renal vein thrombosis include dehydration, traumatic delivery, and sepsis. During the acute phase, during which there is considerable edema, ultrasound will demonstrate an enlarged kidney with loss of corticomedullary differentiation (Fig. 12.5). The parenchyma is hypoechoic early in the process due to edema but becomes hyperechoic as fibrosis sets in. Thrombi in smaller vessels are seen as radiating linear echogenic bands in the parenchyma. At later phases, atrophy will be seen as well as calcifications in the area of the thrombi. Venous flow is absent when Doppler is applied .

Urinary tract infections are a leading indication for ordering renal ultrasound in children. The spectrum of the clinical presentation of UTIs is variable and often matches the ultrasound findings. In many cases, the sonogram of the kidneys and bladder are completely normal. In some cases, all is normal except for some self-limited mild hydronephrosis or hydroureter as a result of the dilating effect of bacteria-released endotoxins. When there is diffuse pyelonephritis, the kidney will be enlarged and hypoechoic (either diffusely or multifocal) with loss of corticomedullary differentiation.

Following a pyelonephritic episode, particularly when associated with vesicoureteral reflux, the healing parenchyma may leave a scar. The scar needs to be large to be detected on ultrasound: the indented contour in the area(s) of scar reflecting cortical loss. This contraction of parenchyma may be focal or in severe cases involves the entire kidney. In such cases, the size of the kidney will also contract. One needs to avoid confusing these scars with persistent fetal lobulations. Scarring typically occurs over calyces (the point of retrograde entry of bacteria to the parenchyma) while the contour indentations associated with fetal lobulations are between the calyces (reflecting the adjacent lobes of renal development) (Fig. 12.7).