Color Doppler ultrasound is the device that is able to detect and measure blood flow. Doppler ultrasound depends on the Doppler effect, a change in the frequency of a wave resulting here from the motion of a reflector, the red blood cell. his device depicts the amplitude, or power, of Doppler signals rather than the frequency shift. This allows detection of a larger range of Doppler shifts and thus better visualization of small vessels, but at the expense of directional and velocity information. Color Doppler depicts blood flow in a region and is used as a guide for the placement of the pulsed Doppler gate for more detailed analysis at a particular site.

Doppler ultrasound has many applications including, for example, the detection and measurement of decreased or obstructed blood flow to the legs. Color Doppler ultrasound is done first to evaluate vessels rapidly for abnormalities and to guide placement of the pulsed Doppler to gain sample volume for detailed analysis of velocities.