Figure 13b. Horseshoe kidney and long-standing hydronephrosis of the left kidney in a 33-year-old man. (a, b) Axial nephrographic-phase (a) and excretory-phase (b) CT scans demonstrate hydronephrosis of the left kidney (arrows) with atrophy of the kidney. Note the contrast material layering in the renal collecting system on the excretory-phase image (arrowhead in b) due to the lack of mixing with the urine in the dilated renal pelvis. (c) Excretory-phase VR image demonstrates a horseshoe kidney. Note that the left renal collecting system is not well opacified (arrows) due to the layering of contrast material in the hydronephrotic kidney.