NONSTATIONARITY BROADENING REDUCTION IN PULSED DOPPLER SPECTRUM MEASUREMENTS USING TIME-FREQUENCY ESTIMATORS

The spectral width of Doppler signals is used as measure of lesion-ind uced flow disturbance. Its estimation accuracy is compromised using th e conventional short-term Fourier transform (STFT) since this method i mplicitly assumes signal stationarity during the signal window while t he Doppler signals from arteries are markedly nonstationary. The Wigne r-Ville (WVD), Choi-Williams (CWD) and Bessel distributions (BD), spec ifically designed for nonstationary signals, have been optimized for s pectral width estimation accuracy and compared to the STFT under diffe rent signal to noise ratios using simulated Doppler signals of known t ime-frequency characteristics. The optimum parameter values for each m ethod were determined as a Hanning window duration of 10 ms for the SF TF, 40 ms for the WVD and CWD and 20 ms for the BD and dimensionless t ime-frequency smoothing constant values of five in the CWD and two in the BD. Thresholding was used to reduce the effect of cross terms and side lobes in the WVD and BD. With no added noise the WVD gave the low est estimation error followed by the CWD. At signal-to-noise ratios (S NR's) of 10 dB and 20 dB the CWD and BD had similar errors and were ma rkedly better than the other estimators. Overall the CWD gave the best performance.