An ultrasonic transmission technique is described which images the velocity variation within metal parts. The arrival time of the first ultrasonic pulse to traverse the object is detected and displayed as a gray scale on a cathode ray tube while the part is being scanned horizontally and vertically. The velocity variation imaged within several cast turbine parts will be shown. The system can detect voids as small as 0.020 inch. With a more complex computer program and knowing the dimensions of the part, tomographic reconstruction of the velocity variation within parts with curved surfaces should be possible.