The adult kidney contains several lobules, each consisting of a rim of cortex surrounding a medullary pyramid that terminates in a papilla protruding into a minor calyx. The lobules fuse in utero or shortly after birth in most individuals. The cortex between two pyramids is called a column of Bertin. Minor calyces converge into major calyces that, in turn, converge to form the renal pelvis. The portion of the kidneys that is not parenchyma or urinary space is the sinus, which, in adults, is filled with adipose tissue (Fig. 10.1). Longitudinal views of the kidney are obtained in the supine position with the probe positioned so that the upper pole appears on the left side of the image. In the longitudinal plane, the normal kidney has a characteristic oval shape with a hypoechoic (dark) rim of cortex and medulla surrounding the echogenic (bright) sinus fat²,³,⁴ that obscures the calyces and blood vessels (Fig. 10.2). The medullary pyramids are slightly less echogenic than the cortex² and can often be discerned. They are particularly prominent when cortical echogenicity is increased. Transverse images are obtained perpendicular to the longitudinal axis and the kidney appears circular at each pole and C-shaped through the center due to the break in the parenchyma where the ureter and vessels enter, thereby providing the best views of the renal pelvis and hilum.

The appearance of the kidneys is subject to normal variations related primarily to incomplete fusion of the ranunculi. These include complete duplication of the collecting system, which appears as a band of cortex separating the sinus fat into two compartments (Fig. 10.3) and is present in 5% of kidneys, and hypertrophied columns of Bertin, where the cortex extends into the sinus but does not completely bridge
it (Fig. 10.4) and is present in 15% of kidneys.⁵,⁶ Junctional parenchymal defects are the most subtle manifestation of incomplete fusion, presenting as a wedge-shaped defect in the outer cortex filled with echogenic fat that is continuous with the renal sinus fat by a thin strand.⁶ In some individuals, the lobules fail to completely fuse and persist into adulthood. These so-called fetal lobulations appear as regularly spaced convexities, each containing a pyramid.⁷ Lobulation may reappear in chronic kidney disease due to atrophy of the columns of Bertin (Fig. 10.5).

Interpretation of the renal sonogram is based on kidney size and shape, cortical thickness and echogenicity, and the appear ance of the medullary pyramids, renal sinus, and the urinary space.

Glomerular or tubulointerstitial disorders usually present with diffuse changes in the kidneys. However, the changes can be similar in different disorders and the kidneys may appear normal. Thus, interpretation is very dependent on the clinical findings.

The most common parenchymal disorder that causes acute kidney injury is acute tubular necrosis (ATN). The renal sonogram can be normal in ATN²⁹,³⁰,³¹ but increased cortical echogenicity and cortical expansion have been observed in both animal and human studies,³²,³³,³⁴,³⁵ especially with nephrotoxic ATN,³²,³⁵,³⁶ whereas an enlarged hypoechoic cortex may be more typical of ischemic ATN.⁶,³² The cortical enlargement presumably represents edema whereas increases in cortical echogenicity may be due to cellular and proteinaceous casts and debris within the tubules. The degree of renal enlargement has been shown to inversely correlate with recovery time from ATN.³⁴ In general, sonography is rarely useful in the workup of acute renal failure when the clinical picture suggests ATN and urinary obstruction is unlikely. However, it may be helpful in identifying underlying chronic kidney disease in this setting.

Cysts are fluid-filled structures with an epithelial lining usually originating from renal tubules. The typical sonographic features of cysts (anechoic structures with distal enhancement) make them very easily discernible by ultrasonography. The most common type of cyst is a sporadic acquired cyst, which can be present without any kidney disease. Cysts can be simple (Fig. 10.12) or complex (Fig. 10.13), with the criteria for complexity including thickening of the wall, calcifications, more than two septations, and luminal echogenicity. Even though the great majority of complex cysts are benign, any complex cyst should be closely followed up with ultrasound or additional imaging studies (computed tomography [CT] and magnetic resonance imaging [MRI]), given the possibility of cystic renal cell carcinomas.⁵⁴

Acquired cystic kidney disease (ACKD) and autosomal dominant polycystic kidney disease (ADPKD) are the most common types of multicystic renal disease. ACKD is often encountered in patients with advanced chronic kidney disease (CKD) or end-stage renal disease (ESRD). Cysts are
typically of smaller size and the kidney is usually echogenic and small⁵⁵ (Fig. 10.14). ADPKD is the most common inherited cause of renal failure, and sonography is the cornerstone of diagnosis using specific criteria established by Ravine et al.⁵⁶ The performance of these criteria was found to be suboptimal in the PKD-2 genotype and revised unified criteria have recently been published by an international group of investigators (Table 10.1).⁵⁷

The pathognomonic sonographic appearance of ADPKD includes presence of multiple bilateral renal cysts (Fig. 10.15) and liver cysts (in 83%-90% of patients)⁵⁸ with significant renal enlargement. Other multicystic diseases such as medullary cystic kidney disease (MCKD), juvenile nephronophthisis, ACKD, or medullary sponge kidney⁵⁹,⁶⁰ do not present with renomegaly. Multiple cysts with renal enlargement can be seen in von Hippel-Lindau disease, tuberous sclerosis, and multicystic dysplastic disease. Complex cysts are also commonly seen in ADPKD and the main etiologies are intracystic hemorrhage and infection. Even though renal cell carcinoma can be seen in this ADPKD, these patients are not at increased risk.⁶¹

Urinary obstruction typically results in hydronephrosis: a dilatation of the collecting system which may be predominantly seen in the minor calyces, the major calyces, or both. More atypical cases may present with minimal dilatation of the collecting system, particularly in the acute setting.⁶² Hydronephrosis is only an anatomic diagnosis and may not indicate urinary obstruction. Brisk diuresis (such as in diabetes insipidus), papillary necrosis, and pregnancy can all result in nonobstructive calyceal dilatation.²⁸,⁶²,⁶³,⁶⁴ Grading systems for severity of the hydronephrosis have proven to be of limited clinical utility because the degree of hydronephrosis may correlate poorly with the extent of obstruction. When obstruction is the primary cause of renal failure, it is always associated with hydronephrosis. In acute obstruction, the cortex is intact (Fig. 10.16) whereas chronic obstruction can lead to marked thinning of the cortex (Fig. 10.17). Failure to visualize the proximal ureter suggests obstruction at the ureteropelvic junction, whereas a dilated proximal ureter (hydroureter) (Fig. 10.16) indicates either obstruction at the level of the ureter or bladder. A large postvoid bladder indicates urinary retention, where sometimes the distal ureters can also be visualized (Fig. 10.18), whereas an empty bladder with dilatation of the distal ureters suggests obstruction
at the bladder inlet. Recognition of hydronephrosis can be difficult in polycystic kidney disease (PKD), where massive cyst formation can prevent or obscure calyceal dilatation and more subtle signs should be sought (Fig. 10.19). This is an important cause of acute renal failure and can be due to stones or blood clots from cyst rupture. Radioisotope scanning may confirm obstruction when sonographic findings are not characteristic.

Occasionally, peripelvic cysts may also mimic hydronephrosis (Fig. 10.20). These are actually dilated lymphatics and, because they track with the blood vessels, can sometimes have a branching pattern. A rim of sinus fat separating the “cysts” from the parenchyma and the absence of a dilated ureter are useful hints toward peripelvic cysts. Another differential diagnosis for hydronephrosis is venous engorgement often seen in cases of volume overload or renal vein thrombosis. In general, the renal vein branches before entering the renal sinus (bush appearance) whereas the collecting system branches in the sinus (pruned tree appearance). The presence of venous pulsations and the tracking of the vein medially to the vena cava, as well as the
use of Doppler sonography, can differentiate blood vessels from the urinary tract. Occasionally, the renal pelvis is situated outside the sinus (extrarenal pelvis) appearing as a dilated proximal ureter but without any calyceal enlargement (Fig. 10.21).

The urinary bladder should be carefully examined by sonography in all patients with hydronephrosis. Additional indications are anuria, hematuria, pain, and urinary tract infections. The bladder is located in the midline, posterior to the symphysis pubis, and contains no luminal structures. When full, it appears as an anechoic fluid collection that is oval in transverse plane and becomes more elongated in the sagittal plane. The bladder volume is calculated by using the following formula for ellipsoid structures⁶⁵:

Volume = 0.523 × length × width × depth

A normal bladder is usually empty after a complete void and should not contain more than 10 mL of urine. A postvoid residual volume of more than 50 mL is associated with a threefold risk of urinary retention in males and is often used as a threshold for the diagnosis of urinary retention.⁶⁶ Hyperechoic structures such as stents, Foley catheters, bladder stones (Fig. 10.22), tumors, and blood
clots (Fig. 10.23) may be visible in the bladder. The presence of urinary jets from a ureter proves its patency. Ureteral stents may transmit bladder pressure back to the kidney, resulting in hydronephrosis. Thus, the diagnosis of stent obstruction requires an empty bladder.

Stones typically reflect most of the sound, rendering them echogenic with a distal shadow and very easily discernible by ultrasonography no matter what the composition. However, both findings may not be present and small kidney stones
and ureteral stones may not be visible. Stones are seen in the urinary space (Fig. 10.24) and may be associated with urinary obstruction, hydronephrosis, and urinary tract infection. Staghorn calculi typically fill the entire calyceal system but can appear as multiple stones on single images (Fig. 10.25).

In nephrocalcinosis (Fig. 10.9) and papillary necrosis, the calcification is in the renal medulla and not the urinary space but this distinction can sometimes be difficult. The differential diagnosis also includes ureteral stents, which typically are not as echogenic and yield less distinct shadows.

Renal neoplasms are usually discovered incidentally or during the workup for pain or hematuria. Special attention should be given to rule out cysts, transmission artifacts, and normal variants such as lobulation and hypertrophied columns of Bertin.⁶⁷ Although the absence of blood flow by Doppler ultrasound may point to a cyst, additional imaging is almost always indicated because sonography can rarely identify the cause of a solid lesion.

Sonography is not indicated in most routine cases of pyelonephritis. Indications include male gender, children, failure to resolve, complications, and frequent recurrence. Pyelonephritis is usually lobular, appearing as a poorly defined hypoechoic area corresponding to a lobule, but it occasionally is diffuse. This can progress to abscesses which are usually single or multiple, heterogenous masses. The differential diagnosis includes complex cysts and neoplasms, which may require CT or MR for diagnosis.

Renal hemorrhage usually appears as perirenal or subcapsular hematomas and usually results from percutaneous kidney biopsy, surgery, or trauma. Initially they may appear as anechoic fluid collections but typically they are heterogeneous with fluid and solid components.

Although many aspects of sonography are similar in native and transplanted kidneys, important differences exist due to anatomic considerations. The allograft is usually placed in the right pelvic fossa, aligned along the incision with the hilum oriented inferiorly and posteriorly, but a variety of other orientations can be encountered especially in obese patients, repeat transplantations, and combined kidney-pancreas transplantation. The donor artery and vein are anastomosed to the external iliac (or the common iliac) vessels and the ureter is anastomosed to the superolateral wall of the bladder (Fig. 10.28). When two kidneys are transplanted en bloc, portions of the donor aorta and vena cava are retained and anastomosed to the recipient vessels.⁷⁸,⁷⁹ The kidney is often placed within the peritoneum when combined with a pancreas transplant. In very young children, the kidney may be placed posterior to the cecum with anastomosis of the donor vessels to the great vessels.⁷⁹ The anatomic relationships are readily apparent on sonograms. The psoas muscle and iliac vessels lie posteriorly and are usually imaged transversely on longitudinal scans of the allograft. The former can be identified by its contraction when the leg is flexed at the hip, and the latter are frequently pulsatile. The ureter and renal vessels are often visible even in normal allografts. The ureter courses medially, usually lying directly under the lower (medial) pole, whereas the renal vein travels posteriorly. The bladder lies medially and can easily be mistaken for a perirenal fluid collection. The peritoneum is superior and occasionally anterior to the allograft, typically appearing as a
beaklike projection over the allograft, and is easily identified by the peristalsis of the bowel loops.

Sonography of the transplanted kidney is indicated in most cases of acute renal failure⁸⁰ and can easily diagnose thrombosis of the renal artery or vein, urinary obstruction, and urine leaks in the immediate postoperative period. Other, nonspecific findings can suggest acute rejection. Sonography can be helpful in guiding percutaneous biopsy, diagnosing fluid collections, measuring residual bladder volume, and identifying ureteral stents.