Medullary rays. Visualization during excretory urography.

The medullary rays are well-defined anatomic structures consisting of bundles of renal tubules which form in the renal cortex and continue through the renal medulla as the medullary striations. These structures are occasionally visualized during excretory urography in abnormal physiologic states characterized by tubular stasis and hyperconcentration of contrast material. Under these conditions, fine striations are visualized which correspond in position and orientation to these structures. The work of others has shown that in tubular stasis, contrast material is hyperconcentrated in the tubules. On microradiography in normal and obstructed kidneys, the highest concentrations of contrast material are seen in the lumen of collecting ducts. Lastly, retrograde injection of contrast material into the medullary rays produces a roentgen image similar to our illustrated cases.