The Doppler effect is the physical phenomenon in which the perceived frequency of a wave changes when the source and observer are moving relative to each other. In everyday life, this is experienced when a siren sounds higher-pitched as an ambulance approaches and lower-pitched as it moves away. In medical ultrasound, the Doppler effect is used to detect and measure blood flow: when ultrasound waves bounce off moving red blood cells, the frequency of the returning echoes shifts depending on whether the blood is moving toward or away from the transducer and how fast it is flowing. This frequency shift is processed by the ultrasound machine to calculate blood flow velocity and direction.
Sonography Term