A renal ultrasound, sometimes referred to as a Doppler ultrasound, is used in order to detect a mass, an obstruction or a stone located in the kidney. It can also be performed in order to determine the size of the kidney. During the procedure, a physician will pass a transducer across the kidneys (from over the skin) emitting sound waves like as sonar. The waves bounce off the kidney, transmitting a picture of the organ onto a TV video monitor that allows the screener to measure the size and determine if a mass exists. The process works very similar to a pregnancy sonogram and is a short, painless procedure.