9

Kidneys

looking caudally to the “normal location” along the line of ascent (pelvic, lumbar, abdominal dysplasia). Renal agenesis in one-third of patients is associated with cystic seminal vesicles or unilateral seminal vesicle agenesis.

Anomalies, Malformations

Hypoplasia

A hypoplastic kidney appears sonographically as an abnormal development of the renal tissue, which is usually of less than normal size, but with a normal structure. In dysplasia there are additional disturbances of normal renal architecture, with the kidney presenting as small or very small: some hypoplastic kidneys are too small to be defined with ultrasound. Differentiation from an atrophic kidney is not always possible, but most hypoplastic kidneys exhibit normal parenchyma and normal central hilar echoes. Calcifications or multicystic changes may be found (Fig. 9.7).

Dysplasia

Because of the common association of dysplastic kidneys with a urinary tract obstruction and other extrarenal syndromes, they are found more often in children than in adults. In ultrasound imaging the kidney is small and shows a dysplastic parenchyma with single or multiple small or large cysts and a dissolved corticomedullar differentiation without a regular architecture.

Fig. 9.7 Renal hypoplasia. The apparent “missing left kidney” is probably a tiny hypoplastic kidney (cursors).

9

Kidneys

Fig. 9.8 Polycystic renal degeneration (Potter type III adult polycystic disease). Cystic renal degeneration (N). The parenchyma is difficult to identify because of the many anechoic spaces.

a Macrocystic-bullous form, 72-year-old man: macrocystic conglomerate with still detectable residual parenchyma (N).

b Predominantly microcystic form in a 21-year-old man: multiple fine cysts (Z) with relatively well preserved parenchyma.

Fig. 9.9 Multiple renal cysts (Z). The kidney is still of normal size, and the parenchyma is clearly visible; dialysis.

renal parenchyma. This pattern requires differentiation from multiple renal cysts. In this case the parenchyma is still clearly visible and generally the kidneys are not enlarged (Fig. 9.8, Fig. 9.9; see also “Circumscribed Changes”, p. 344).

Kidneys

Anomalies, Malformations

Aplasia, Hypoplasia

Cystic Malformation

Anomalies of Number, Position, or Rotation

Fusion Anomaly

Anomalies of the Renal Calices Vascular Anomaly

Diffuse Changes Circumscribed Changes

Duplex Kidney

Ectopic Kidney

Malrotation

Duplex kidney is the most common renal anomaly, occurring in 1% of the general population. It is characterized by duplicated renal pelvises and two ureters that unite somewhere between the kidney and bladder. The duplex kidney is also enlarged. When the ureters have different insertions, the ureter with the more distal insertion belongs to the upper moiety, according to the Meyer–Weigert rule. The ureter belonging to the lower moiety consistently empties proximate to that site. In a duplex kidney with hydronephrosis, then, the upper

ureter (e. g., megaureter, see Chapter 11) and associated pelvis will be obstructed owing to the ectopic insertion (caution: upper-pole cyst). But if the lower ureter is affected, the obstruction is due not to an ectopic insertion but to some other obstructive process.

It is common to find an ectopic insertion of the lower moiety with an associated ureterocele, leading to vesicorenal reflux or ureteral obstruction that again affects the upper moiety. (In the case of a ureterocele, it cannot be de-

termined with ultrasound whether the ureter runs within or outside the detrusor muscle.)

The ultrasound features of a duplex kidney are (Figs. 9.10, 9.11, 9.12, 9.13):

●Longitudinal enlargement with a normal organ thickness

●A parenchymal bridge separating the upper and lower collecting systems

●Usually, a notch at the level of the parenchymal bridge

●Duplication of the renal hilum

Fig. 9.10 Duplex kidney: 12 cm long kidney with two pelvises separated by a band of parenchyma.

Fig. 9.12 Duplex kidney. The two moieties are separated by a parenchymal band (B), where a notch is visible in the renal outline (arrows; the lower arrow marks the hilum of the lower renal segment).

Fig. 9.11 Duplicated renal hilum. VC = vena cava; AO = aorta; N = kidney; VR = renal vein.

a Upper renal vein.

Fig. 9.13 CDS image showing two renal pelvises. N = kidney; VC = vena cava.

a Upper renal artery (A).

b Lower renal vein, demonstrated by a lower transverse scan.

b Lower renal artery (AR).

Ectopic Kidney

A lumbar kidney is the most common form of renal ectopia, with ultrasound revealing the ectopic kidney in the anterior iliac fossa. The key sonographic landmark is the iliac artery— the ectopic kidney will generally be found anterior to that vessel. The less common pelvic kidney lies anterior to the sacrum and below the aortic bifurcation. Abdominal ectopia is the easiest form to detect, as the affected kidney is markedly lower than its counterpart; but in contrast to the lumbar and pelvic forms, usually the kidney is not small or malrotated (Fig. 9.14).

Fig. 9.14 Ectopic right kidney in the lesser pelvis (renal pelvic dysplasia).

a The right kidney is not found at its usual site posterior to the liver (LE) (no renal impression). M = lumbar muscle.

b The dysplastic right kidney (N) is located in the lesser pelvis on the right side along the iliac artery (AI). P = renal pelvis.

Malrotation

Malrotation is an anomaly in which the renal hilum faces anteriorly (caution: this makes the vessels vulnerable to a percutaneous needle). Otherwise difficult to detect, a malrotated kidney can be positively identified by the location of its hilum (Fig. 9.15).

Fig. 9.15 Malrotated kidney in a slightly ectopic location (cursors). The renal hilum faces anteriorly.