Kidney: Ultrasound Anatomy and Scanning Methods

Fig. 1.1

B-mode shows longitudinal section of a left kidney, with normal volume and echostructure and bipolar diameter of 11.3 cm

In axial scans, acquired at the renal hilum, the transverse diameter measures between 5 and 6.5 cm (median 5.7 cm) (Fig. 1.2).

Fig. 1.2

B-mode shows axial section of a left kidney, with normal volume and echostructure and transverse diameter of 5.28 cm

Three different areas in the context of the renal parenchyma may be distinguishable, according to their echogenic characteristics: the cortex, medulla (constituted by the pyramids), and renal pelvis.

The cortex is located between the base of the pyramids and the surface of the organ, but it goes even deeper between the pyramids themselves, separating them from each other and forming the kidney Bertin columns.

Parenchymal thickness (calculated as the total of cortical and medullary thickness), measured from the outer edge to the border of the renal sinus, varies in normal subjects between 16 and 20 mm (Fig. 1.3). Parenchymal thickness <13 mm is an absolute contraindication to renal biopsy.

Fig. 1.3

B-mode shows longitudinal section of a left kidney, with normal volume and echostructure and with cortical thickness of 1.75 cm

Cortical thickness (or corticomedullary) is measured between the outer contour of the kidney and the base of the pyramid, considered normal between 8 and 11.5 mm.

Determining cortical echogenicity is also important; it is measured in relation to renal and extrarenal structures, as it can allow to make evaluations about eventual kidney damage.

The cortex, as already mentioned, is the outermost portion of the renal parenchyma and proves to be hyperechoic compared to the medulla and hypoechoic compared to the pelvis.

A useful tool is represented by the attempt to quantify the alteration in terms of grades or numbers expressing the relative cortical echogenicity, using the scale of Hrikak et al.

This scale, constituted by four grades, compares the normal liver and/or renal sinus echogenicity, considered as the two opposed terms of comparison:

0 – normal hypoechoic compared to the liver (Fig. 1.4)

Fig. 1.4

B-mode image of the right kidney with hypoechoic echostructure compared to the liver parenchyma (Hrikak scale 0)
1 – isoechoic compared to the liver
2 – moderately hyperechogenic compared to the liver but less than the renal sinus
3 – hyperechogenic compared to the liver but equivalent to that of the renal sinus

The renal capsule is not easily distinguishable from the perirenal band and from the perirenal fat, but normal kidney outline shows a regular trend. In coronal scans, the regularity of the profile of the upper pole can be broken by a hyperechoic triangular formation, called hyperechoic renal triangle.

The hyperechoic renal triangle is formed by an incomplete fusion of two adjacent lobules in the metanephric kidney, with penetration of the renal sinus fat tissue into the external capsule fat.

The continuity between the triangle and the renal sinus should be carefully researched as a distinctive sign, which can be distinguished by a scar or a parenchymal angiomyolipoma.

The renal pyramids (the medulla) are hypoechoic; they may not be well delineated and also in normal conditions, their display can be modest or absent (except in children and thin patients) in relation to the hydration; in some ultrasound scans, they can be easily confused with microcysts or chaliceal diverticula. Corticomedullary differentiation can be well defined when pyramidal margins are net. In conditions where there is interstitial edema (acute tubular necrosis, tubulointerstitial nephropathy), the upper boundary of the pyramidal base can be nuanced, making it difficult to distinguish between the two areas.

The renal sinus is the oval hyperechoic area between the inner profile of the parenchyma and the renal hilum; the echogenicity is high for the presence of numerous interfaces and components with different acoustic impedance (collector, vessels, nerves, lymphatics, fibrous tissue, fat).

A variable amount of fibroadipose tissue surrounds the renal pelvis, the chalices, and their infundibula. In cases where the sinus fibroadipose tissue is exuberant, the renal pelvis is compressed, and infundibula are elongated and ironed. This condition is known as “renal sinus sclerolipomatosis,” a benign alteration of physiological echostructure due to a fat proliferation in relation to old age, obesity, and corticosteroids. The proliferation of fat causes a mass effect that compresses the intrarenal collecting system without causing chaliceal obstruction. Obesity in the elderly is the main cause of sinus sclerolipomatosis.

Sometimes in young the sinus fibroadipose tissue can have medium-low echogenicity and simulates a pelvis urothelioma, a hydronephrosis, or a cluster of peripelvic cysts.

Sometimes, even the scars of a chronic infection, especially if this is associated with lithiases, can simulate an asymmetric lipomatosis.

On the other side, a venous sinus ectasia or a peripelvic cyst cluster can simulate an expansion of the pelvis.

The use of color Doppler allows the differential diagnosis in the first case, while the absence of communications between the single cysts is diriment in the second one. An extrarenal pelvis can simulate a dilation and/or a ureteropelvic junction anomaly (Fig. 1.5).

Fig. 1.5

B-mode image of extrarenal pelvis without evidence of hydronephrosis

In elderly patients with diffuse atherosclerotic disease, the arcuate arteries, located at the corticomedullary junction, may present as thin hyperechoic interfaces and be confused with microcalculi or medullary fibrotic calcifications. Occasionally, small hyperechoic foci, picturesquely called “unidentified bright objects” (UBOs), can be found in the context of the parenchyma. These “spots” are frequently associated with reverberation artifacts, while the acoustic rear cone is usually absent. The possible causes of the “UBOs” may actually be represented by small lithiases foci at the apex of the papilla (Randall bodies), but also by microcysts, small chaliceal diverticula with calcified wall or intraluminal calcium milk, small calcified angiomyolipomas, or arteriole fibro-calcifications.

The pelvis and major and minor chalices are usually virtual and not detectable by B-mode ultrasound. Their profile can stand out clearly, into the hyperechoic sinus; in relation to physiological causes such as hyperhydration, a hyper-repletion of the bladder, pregnancy, antispasmodic abuse, and elastic hypotonia; or for an intersection with abnormal vessels. In these cases, the intrarenal excretory system appears, and the hypo-anechoic cavity running through the renal sinus, and corresponding to the urinary tract, has a very thin wall. The presence of complex chalices arising from the fusion of several smaller chalices is a frequent anomaly of the polar areas (particularly of the upper pole) and can simulate a pseudo-mass [1–3].

1.3 Anatomic Variations

The knowledge of anatomic variants that come into differential diagnosis with various pathological conditions is very important; the morphological variants of renal ultrasonography are more commonly found in:

Lobulations and fetal incisures, the hyperechoic line, and triangle
Dromedary humps
Hypertrophy of the tubercles and renal rims
Mesorenal column hypertrophy or mesorenal septa

1.3.1 Lobulations and Fetal Incisures

Normally the renal outer profile is smooth, while the inner one is jagged, for the presence of columns and pyramids which protrude in the renal sinus. It may happen, however, that also the outer profile is irregular for the presence of prominent areas, bounded by incisions and expression, respectively, of the lobulations and the fetal incisures (Fig. 1.6). The fetal lobulations coincide with a lobar unit (consisting of the cortical mantle with his pyramid): fetal sulci split two contiguous lobar units that correspond to the cortical columns. They must be differentiated from the segmental pyelonephritis scars that are located in correspondence of the renal chalices and not of the Bertin columns; another sign of benignity, which facilitates the differential diagnosis, is the presence of homogeneous parenchyma below the capsular incision and the absence of distorted and dilated chalices [4–7].