A renal US examination typically includes sonography of both kidneys, the perirenal areas, the aorta, inferior vena cava (IVC), and the urinary bladder. The American Institute of Ultrasound in Medicine provides guidelines for a satisfactory examination [1]. A sector or curved array transducer using frequencies between 2.25 and 5.0 MHz is recommended. The right kidney is examined from an anterolateral or direct lateral approach with the patient supine or in left lateral decubitus position, using the right lobe of the liver as a sonographic window. The left kidney is often more difficult to visualize because the spleen provides a more limited sonographic window. The left kidney is often obscured by bowel when scanned from the anterior abdomen. Placing the patient in right lateral decubitus position provides access to a more lateral and posterior approach through the spleen and psoas muscle. Multiple images of both kidneys are documented in longitudinal and transverse planes including the upper pole, middle section, and lower pole. Maximum renal lengths are recorded. Renal volumes may be calculated by measuring transverse and anteroposterior (AP) diameters from a mid-kidney transverse image. The standard formula (volume = length × width × AP diameter × 0.52) is utilized. The bladder lumen, wall thickness, and mucosal surface are documented. The retroperitoneum is examined for ureteral dilatation, adenopathy, or other abnormality.

Color flow and spectral Doppler are frequently utilized to confirm patency of the renal artery and vein, parenchymal blood flow, and the vascularity of any lesion detected. Power Doppler allows the most sensitive demonstration of renal vascularity [2,3].

For US examination, the bladder should be moderately distended to stretch the wall and to better visualize the mucosal surface and bladder lumen. Images are obtained in transverse and longitudinal planes. Attention should be directed to the trigone area, ureteral orifices,
and prostate gland. Transvaginal scanning will improve evaluation of the bladder in women. Large cystic pelvic masses may be mistaken for the bladder. When this is suspected, the bladder should be emptied by voiding or Foley catheter and rescanned.

Although small and tucked away in the retroperitoneum normal and abnormal adrenal glands can be visualized by US in most patients with a little effort and attention to anatomic landmarks. Normal adrenal glands are prominent and easily seen in the third-trimester fetus and the newborn, but require exacting technique to visualize in most adults. Adrenal abnormalities may be detected during routine renal sonography. Careful attention to the location of the adrenal glands is the key to recognizing them on US. The right adrenal gland is truly suprarenal and is best visualized by a posterolateral approach in transverse plane looking for the adrenal gland just above the right kidney, between the liver and right crus of the diaphragm posterior to the IVC as it enters the liver. The left adrenal gland is imaged by a posterolateral approach in coronal plane through the long axis of the left kidney. The left adrenal gland is seen between the upper pole of the left kidney and the aorta. High-resolution, real-time sonography allows visualization of normal adrenal glands in 71-92% of adults [4].

The normal adult kidney is bean-shaped with a smoothly convex, often lobulated, outer border (Fig. 3.1). The kidney is well defined by a thick fibrous capsule outlined by echogenic perirenal fat. This echogenic fat continues into the renal sinus, entering at the anteromedially oriented renal hilum and filling the middle of the kidney. The outer renal cortex is equal to, or slightly less than, the liver and spleen in echogenicity. Renal cortical echogenicity distinctly greater than liver parenchymal echogenicity is highly indicative of impaired renal function [5]. Liver parenchymal echogenicity distinctly greater than renal cortical echogenicity is highly indicative of diffuse hepatocellular fatty infiltration [6]. The cortex that extends centrally into the kidney is called septal cortex as it surrounds and separates the less echogenic medullary pyramids. Fusion of septal cortex from adjacent lobes produces a normal, but often bulbous, mass of cortex referred to as a column of Bertin (see Fig. 3.3) [7]. Distinct differentiation of cortical and medullary echogenicity is a sign of a
normal kidney. In infants up to 24 months of age, the echogenicity of the cortex is significantly greater than the renal cortex in adults and older children (Fig. 3.2). Its echogenicity commonly exceeds liver parenchymal echogenicity. The medullary pyramids remain echolucent and prominent. This appearance has been mistaken for hydronephrosis.

Persistent fetal lobation is a common renal anatomic variant [8]. The normal kidney is made up of 12-18 separate lobes consisting of a medullary pyramid surrounded by subcapsular and septal cortex [9]. A single lobe drains into a simple calyx. When several lobes drain into a single calyx, the calyx is considered to be a compound calyx. The fetal lobes act as tiny independent kidneys early in fetal life but progressively fuse into a single kidney with maturation. Incomplete lobular fusion is common and results in V-shaped indentations on the renal surface (Fig. 3.3). A prominent indentation between the upper and lower portion of the kidney has been called the junctional defect [7]. These normal V-shaped defects should not be mistaken for parenchymal scars.

Calyces unite to form the renal pelvis, which exits the renal sinus at the hilum. The calyces and pelvis are usually collapsed and are not seen as discrete structures within the renal sinus. When patients are well hydrated, high urine output causes mild normal dilatation of the calyces and pelvis. The renal pelvis may also be seen as a prominent fluid-filled structure when it is “extrarenal,” that is, primarily outside of the renal sinus. Lower surrounding tissue pressure allows the pelvis to be chronically dilated. These normal variations must be differentiated from hydronephrosis.

The main renal arteries are solitary in 60% of individuals and multiple and smaller in the remainder. The renal arteries arise from the lateral aspects of the aorta approximately 1 cm below the origin of the superior mesenteric artery. The right renal artery passes behind the IVC causing a small but prominent indentation on the posterior wall of the IVC (see Fig. 2.11). The left renal artery has a short direct course to the left kidney. Renal arteries are more commonly multiple when the kidney is malpositioned or malrotated. Supplemental renal arteries may course directly into the polar regions of the kidney without coursing through the renal hilum. Renal veins are usually solitary. The right renal vein is anterior to the right renal artery and courses anterosuperiorly to enter the IVC usually somewhat above the level of the right renal hilum. The left renal vein courses transversely anterior to the aorta to enter the IVC. The left gonadal vein enters the left renal vein just outside the left renal hilum. The right gonadal vein enters the IVC directly just below the junction of right renal vein and IVC.

The Doppler spectrum of the normal renal artery shows continuous forward flow into the kidney with relatively high velocities maintained throughout diastole indicating low intrarenal vascular resistance. Maximum normal peak systolic velocity is 180 cm/sec. Normal resistive index (RI) is below 0.70. The Doppler spectrum of the renal vein shows continuous venous flow with slight respiratory undulation.

Doppler spectra can be obtained from tiny intrarenal arteries (interlobar and arcuate arteries), whether they are actually visualized or not, by placing a Doppler sample volume
at the corticomedullary junction along the borders of the pyramids. The RI, calculated from the spectral display, has diagnostic value in a number of disease states. Like the main renal artery, RI of 0.70 is the upper limit of normal for intrarenal arteries. A number of representative readings should be averaged to determine a single representative value [10].

Characteristics of the normal kidney are summarized in Table 3.1.

The exact shape and appearance of the bladder varies with the degree of distension. When empty or nearly empty, the bladder wall is thick and somewhat irregular due to contraction of the wall musculature and wrinkling of the mucosa. With filling, the wall thins and the mucosa flattens to a smooth, well-defined surface. The normal bladder wall does not exceed 4 mm in thickness when the bladder is distended. The trigone is defined by the slight protuberances of the ureteral orifices and the more inferior urethral opening. The ureteral orifices are most easily located by identifying ureteral jets. Periodic ureteral peristalsis (average rate, 6 waves per minute) squirts jets of urine into the bladder lumen. These jets are seen on gray-scale US as tiny streams of microbubbles, and on color flow US as flashes of color projecting into the bladder lumen (Fig. 3.4) [11]. The urethral orifice is seen as a slight V-shaped depression at the bladder base. Enlargement of the prostate elevates the bladder base. Postvoid bladder volume does not normally exceed 22 mL. Normal
urine is anechoic. Reverberation artifact commonly projects into the bladder from the anterior bladder wall. Residual urine after voiding is quantified by the standard formula: volume = width × height × length × 0.52 (Table 3.2).

Adrenal glands are composed of cortex and medulla, which have different embryologic origin, function, and US appearance. The cortex secretes the steroid hormones cortisol, androgens, and aldosterone, whereas the medulla secretes catecholamines. The cortex is hypoechoic to liver parenchyma, whereas the medulla is near isoechoic with fat (Fig. 3.5).
Adrenal glands are relatively larger in the fetus and newborn because of the presence of the prominent fetal cortex that atrophies by 1 of year of age.