Subsample volume processing of Doppler ultrasound signals

Processing of Doppler signals produced by pulsed Doppler systems is based on the assumption that the phase of the received high frequency ultrasound signals changes linearly with depth. However, the random spatial distribution of scatterers is not in accordance with this basic assumption. Consequently, averaging of the demodulated signal over an observation window, covering a few periods of the received signal, does not improve the estimate for the instantaneous quadrature components of the Doppler signal originating from a given depth. Hence, the accuracy of the Doppler velocity estimate is independent of the length of the observation window employed. However, splitting the observation window in subsample volumes, each with a length of one period at the emission frequency, and combining the Doppler signals of the subsample volumes at the last stage of signal processing, i.e., mean Doppler frequency estimation using the autocorrelation technique, results in a considerable reduction of the variance of the velocity estimate. Using a computer simulation of the signal processing involved, it is demonstrated that with subsample volume processing the variance of the velocity estimate attains the same variance as is expected for the RF cross correlation technique.