The use of ultrasonics in medicine to produce images of the inside of the body. This is achieved by observing the timing and strength of the various echoes that are returned as the ultrasound interacts with the varying surfaces and density of the organs. The technique is sufficiently rapid to be able to capture motion within the body, including that of blood flow. It is safe for living tissue, hence its common use during pregnancy for scans of the developing foetus.

http://www.ob-ultrasound.net/history1.html A short history of the development of medical ultrasonography