" ...... The basis of the autocorrelation detector is that the echo wavetrains from stationary targets have corresponding changes with time, whereas sequential echo wavetrains from moving targets have corresponding changes in the relative phase. The autocorrelaton detector produces an output signal that depends on the relative phases of consecutive pairs of received echo wavetrains. Thus, the echo wavetrains themselves are their own references for phas comparison. The autocirrelation detector functions by multiplying two echo wavetrains, one currently being received by the transducer and the other, having been received from the immediately preceding pulse transmission and delayed for a time exactly equal to the interval between pulse transmissions. The output from the autocorrelator has constant amplitude except where consecutive wavetrains have phase differences. In color doppler processors, a parrellel and separate process of velocity and velocity variance are made. The value of the velocity variance can be considered to be a measure of the width of the doppler frequency spectrum, which increases with the degree of flow disturbance. The final processor in the circuitiory, the color processor, assigns luminance, hue and saturation to the display, following one of the designated color-coding schemes. ..... "
From "Ultrasonic Imaging of the Human Body" by PNT Wells, in Rep. Prog. Phys. 62, 671-722. 1999.
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