Christian Johann Doppler first described the effect of motion of sound sources and the frequency change of the sound to the observer.
Doppler ultrasound uses this effect to detect and measure blood flow, and the major reflector is the red blood cell. Doppler ultrasound depends on the fact that if the reflecting surface is moving in relation to the transducer (blood flowing in a vessel) the frequency of the received ultrasound wave will be different from that of the transmitted wave. If blood cells are moving towards the transducer, they increase the frequency of the returning signal. As cells move away from the transducer, the frequency of the returning signal decreases.

See also Quadrature Detection and Doppler Techniques.

(CPA) Color power angio is a Doppler measurement, which employs the Doppler effect to assess whether blood is moving towards or away from the probe. By calculating the frequency shift of a particular sample volume, its speed and direction can be determined and visualized. This is particularly useful in cardiovascular studies and essential in many areas such as determining reverse blood flow in the liver vasculature in portal hypertension. The CPA information is displayed as a color image superimposed on the 2D gray scale image.

See also Color Power Doppler.

The Doppler angle (theta) is the angle of incidence between the ultrasound beam and the estimated flow direction (parallel to the long axis of the vessel). If the beam is parallel to the flowing blood, the Doppler angle is zero, and the determination of flow is most accurate.

See also Beam Vessel Angle, Doppler Effect and Doppler Ultrasound.

Doppler fluximetry is a method to study the hemodynamic of fetus and placenta. Fluximetry is based on Doppler ultrasound. The resistance of blood flow in the vessels can be determined by quantitative or qualitative analyzing of sound waves depending on the pulsating blood flow.
Doppler fluximetry can be theoretically applied to vessels in every area of the body, but in practice insufficient size of some vessels is a limitation.

See also Doppler Effect.

The Doppler interrogation frequency is the frequency of the transmitted acoustic energy relative to the measured Doppler frequency shift. This frequency is most but not necessarily, the nominal transducer frequency.

See also Doppler Effect and Doppler Ultrasound.