Figure 9. Schematic of the color flow imaging algorithm. A delay is applied to subtract the prior echo signals from the current echo signals. This subtraction reduces undesirable stationary echo signals. The autocorrelator compares the prior echo signals with the current echo signals to determine the mean velocity and the variance in each pixel. Gating circuits and a digital scan converter map these values to the image. Color processing applies a color map of velocities to the data.