Comparison of the performance of the RF cross correlation and Doppler autocorrelation technique to estimate the mean velocity of simulated ultrasound signals.

In pulsed Doppler systems the received RF (radio frequency) signal is multiplied by a quadrature reference signal and subsequently averaged over a short depth range to obtain a sample of the complex Doppler signal. The mean frequency of the sampled Doppler signal, obtained with the autocorrelation function, reflects the mean velocity of the scatterers moving through the sample volume. An alternative is to evaluate the two-dimensional cross correlation function of a short segment of the RF signals over subsequent lines, giving the mean velocity of the scatterers. Both methods of velocity estimation were applied to computer-generated RF signals with varying RF bandwidth, signal-to-noise ratio, and mean and width of the imposed velocity distribution. The length of the RF signal segment and the number of lines for velocity estimation (package length) affects the accuracy of the velocity estimate. It can be concluded that the cross correlation technique behaves superiorly especially for a low velocity dispersion. Furthermore, the standard deviation of the velocity estimate decreases for an increasing sample volume length and package length, while the performance of the conventional Doppler technique is rather independent of the length of the sample volume. The difference between both techniques decreases for a greater package length or for signals simulating a wide velocity distribution.