Doppler Shift is the change in the perceived frequency relative to the transmitted frequency. The Doppler shift is dependent on the insonating frequency, the velocity of moving blood, and the angle between the sound beam and direction of moving blood.
Doppler equation:
Doppler shift frequency: fD = fr - f0 = 2f0v/c
Where fD is the Doppler shift frequency = the difference between transmitted and received frequencies.
Ultrasound system use the following equation:
Doppler shift frequency with incident angle: fD = 2f0v/c cosØ
Where f is the transmitted frequency, v is the blood velocity, c is the speed of sound in tissue, cosØ is the Cosine of the blood flow to beam angle.
The Doppler angle (theta) is the angle of incidence of the beam upon the object. If the beam is parallel to the flowing blood, the angle theta is zero, and the determination of flow is most accurate. If the angle of incidence is greater, the results are less reliable. Doppler shift results with an angle greater than 20° should not be used for the calculation.

See also Doppler Interrogation Frequency, Zero Crossing Detector, Doppler Effect, Doppler Ultrasound and Motion Discrimination Detector.

The Doppler spectrum indicates how the echo power is distributed according to the Doppler shift frequency. The Doppler shift frequency is directly related to the radial velocity of the scatterer.

See also Modal Velocity, Doppler Effect and Doppler Ultrasound.

In a color map, colors are allocated to Doppler shift frequencies corresponding to flow. Color (colour, Brit.) maps may also display Doppler amplitude, signal power, variance, or the gray levels of the B-mode image.

See also Color Amplitude Imaging, Color Priority, and Color Saturation.

The color priority determines which signal of B-mode and Doppler, detected from the same location will be displayed in a given pixel. Low color priorities will allow solid tissue to conceal Doppler shifts at the same location; a high priority ensures that color will overwrite the gray scale image.
Also called color-echo write priority or angio write priority.

See also Directional Indicators.

(CWD) Continuous wave (CW) Doppler is an ultrasound imaging mode, which records blood flow velocities along the length of the beam. Continuous wave Doppler uses different crystals to send and receive the signal. The transducer operating in continuous wave mode utilizes one half of the elements and is continuously sending sound waves of a single frequency while the other half is continuously receiving the reflected signals.
The advantages of a continuous wave transducer are a high sensitivity and no Nyquist limit. CW Doppler does not alias but has no depth precision and large gate. The beat frequency is the Doppler shift. CW Doppler echocardiography employs this technique to record the flow of blood through the cardiovascular system.

See also Cross Talk, Periorbital Doppler, and Mirror Artifact.