Doppler ultrasound: A form of ultrasound that can 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 . There are several kinds of Doppler ultrasound:

• Color Doppler -- This technique estimates the average velocity of flow within a vessel by color coding the information. The direction of blood flow is assigned the color red or blue, indicating flow toward or away from the ultrasound transducer .

• Pulsed Doppler -- This method allows a sampling volume or "gate" to be positioned in a vessel visualized on the gray-scale image, and displays a graph of the full range of blood velocities within the gate versus time. The amplitude of the signal is approximately proportional to the number of red blood cells and is indicated, not in color, but simply as a shade of gray.

• Power Doppler -- This 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.

Named for Christian Johann Doppler (1803-1853), the Austrian physicist, who discovered the effect that now bears his name.

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