Hydronephrosis is an abnormal collection of urine within the renal pelvis. It usually indicates some obstruction to urine drainage. Since urine is a uniform liquid it generates no echoes on ultrasound. That is why it appears as a black (anechoic) area on the ultrasound image. Notice the large black area in the hilum of the kidney on the right. This is the renal pelvis. The black areas projecting into the parenchyma (top and sides of left image) are dilated infundibulae and calyces of the kidney. Notice that there are no such hypoechoic (black) areas seen in the kidney on the left.

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©David A. Hatch, M.D., 1996