Interpolation method for time delay estimation in the RF-signal crosscorrelation technique for blood velocity measurement

The cross correlation method (CCM) for blood flow velocity is based on the time delay estimation of the echoes from pulse-to-pulse. The sampling frequency is usually kept low in order to reduce computation complexity, and the peak in the correlation function is found by interpolating the correlation function. The parabolic-fit interpolation method introduces bias at low ratio of sampling rate to ultrasound center frequency. In this work, 4 different interpolation methods are suggested to improve the estimation accuracy. 1. Parabolic-fit with bias-compensation, derived from a theoretical signal model. 2. Parabolic-fit combined with linear filter interpolation to the correlation function. 3. Parabolic-fit to the correlation function of the complex signal envelope. 4. Matched filter applied to the correlation function interpolation. The new interpolation methods are analyzed both by computer simulated signals, and RF-signals recorded from patient data with velocities up to 4 times the Nyquist velocity. The results show that these methods have similar estimation accuracy when signal to noise ratio(SNR) is high and the matched filter interpolation gives the better performance when SNR is low