Fetal doppler ultrasonography is medical ultrasonography that employs the doppler effect to generate imaging of the movement of tissues and body fluids (usually blood), and their relative velocity to the probe. By calculating the frequency shift of a particular sample volume, for example, flow in an artery or a jet of blood flow over a heart valve, its speed and direction can be determined and visualized. Color Doppler or color flow Doppler is the presentation of the velocity by color scale. Color Doppler images are generally combined with grayscale (B-mode) images to display duplex ultrasonography images, allowing for simultaneous visualization of the anatomy of the area.

The Doppler data is displayed graphically using spectral Doppler, or as an image using color Doppler (directional Doppler) or power Doppler (non directional Doppler). This Doppler shift falls in the audible range and is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, pulsating sound.

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All modern ultrasound scanners use pulsed Doppler to measure velocity. Pulsed wave instruments transmit and receive series of pulses. The frequency shift of each pulse is ignored, however the relative phase changes of the pulses are used to obtain the frequency shift (since frequency is the rate of change of phase). The major advantage of pulsed wave Doppler (PW Doppler) over continuous wave (CW Doppler) is that distance information is obtained (time between transmitted and received pulses multiplied by sound velocity equals distance) and gain correction is applied. The disadvantage of pulsed Doppler is that the measurements can suffer from aliasing. The terms Doppler ultrasound and Doppler sonography have been accepted to apply to both pulsed and continuous Doppler systems, despite the different mechanisms by which the velocity is measured.