Kidney, X-ray
Contrast media (CM) that reaches the kidneys will, instead of being returned to the heart, be gradually filtered from the blood and washed out with the urine through special thin-walled blood vessels that look something like a ball of yarn. The CM contained in the urine is collected and transported through the renal pelvis to the ureter and the bladder. Also, some of the CM that went to other parts of the body during the first round of circulation will reach the kidneys on the second round, and be filtered. Thus, the concentration of CM in the blood is reduced for every round of circulation. More and more CM will be washed out with the urine.
After as little as 2-3 minutes the concentration of CM in the kidneys will be so much higher than in the surrounding tissue that the kidneys can clearly be seen as two fist-sized, bean-shaped pale shadows on the X-ray image. After another 5-10 minutes the renal pelvis will be visible, looking like a plump hand with short, stubby fingers. If the kidneys are diseased, the diseased part will often not secrete the CM as efficiently as the healthy parts, and thus appear darker on the picture. If one entire kidney is damaged, it will hardly be visible at all, or it will only become visible at a much later time than the undamaged kidney.
GE Healthcare Glossary