8 Doppler Imaging
The Doppler shift is the change in frequency of sound when the sound wave strikes a moving object. This means the frequency of the transmitted and reflected sound waves is not the same. Doppler shifts in clinical imaging are in the audible range (±10 KHz). Red blood cells are the primary reflectors that produce Doppler shifts. Ultrasound machines can color-encode the mean velocity (color Doppler), variance within the sample volume (variance Doppler), and power spectrum of the frequency shift (power Doppler).1
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