The Roentgen Appearance of the Central Fat Tissue of the Kidney: Its Significance in Urography

Abstract

THE CENTRAL OR peripelvic fat tissue of the kidney is a normal anatomical constituent of the renal hilus and sinus. It enters the hilus with the renal pedicle, passes upward and into the central cavity (sinus) of the kidney, and extends to the inner aspect of the renal parenchyma at the floor of the sinus. The kidney pelvis and the major and minor calyces, branches of the renal artery, and tributaries of the renal vein are embedded in this fat tissue, which entirely covers their surfaces (Fig. 1A). Sometimes the extrarenal pelvis, too, may be surrounded by fat lobules. Little attention has been paid to this central fat tissue by anatomists and pathologists, and there has been no previous radiological study of its appearance and significance. Due to the difference in x-ray absorption between the renal parenchyma and the fat tissue (Fig. 1B), the latter is not infrequently recognizable in roentgenograms of the kidneys. The translucency produced by the fat usually occupies the central and medial areas of the kidney shadow. It is fan-shaped or trapezoid in outline, irregular or spotty, according to the anatomic distribution of fat lobules. If the translucent area is fan-shaped, its apex lies near or in the hilus, while its outer border lies along the inner aspect of the kidney parenchyma (Fig. 2A). If the kidney is rotated about its long axis, the translucency becomes narrow and tends to lie in the central area of the kidney shadow (Fig. 2B). The irregular saw-tooth appearance frequently seen on the inner border of the parenchymal shadow at the lateral margins of the translucent area is due to the alternating projection of the renal papillae into the sinus and the columns of Bertin dipping between them. The borders of the kidney shadow itself and the outlines of the central fat tissue together demonstrate the kidney parenchyma (cortex and medulla). The fat accompanying the calyces to their particular pyramids may produce extensions far out into the cortex. Here it appears on excretory urographs as a narrow translucent halo surrounding the major calyx on all sides. The variation in distribution of fat is great and often differs between the two kidneys of one individual. Visualization and recognition of the central fat tissue are often disturbed by air-containing intestinal loops. The sawtooth margins of the air-distended descending duodenum may be confused with the lateral margins of the central fat tissue. In such instances a comparison of films taken in supine and erect positions and, specifically, laminagraphs will aid interpretation. The translucency of the fat and the inner outlines of the kidney substance are clearer in the early stages of excretory urography than they are in plain or retrograde films, for dye excreted into the parenchyma increases its opacity without influencing the radiolucency of the fat.