Doppler ultrasonography [Christian J. Doppler] , a technique used in ultrasound imaging to monitor moving substances or structures, such as flowing blood or a beating heart. The frequency of ultrasonic waves reflected by a moving surface is slightly different from that of the incident waves. The detected frequency shift yields information about the moving surface. The technique can be used to locate vessel obstructions, observe fetal heart sounds, localize the placenta, and image heart functions. Also called Doppler scanning.
