Comparison of time-frequency distributions for Doppler spectral estimation

The time-frequency distribution (TFD) of Doppler blood flow signals is usually obtained using the spectrogram, which requires signal stationarity and is known to produce large estimation variance. The authors examine three alternative, nonstationary spectral estimators: the Choi-Williams distribution (CWD), the Bessel distribution (ED) and the novel, adaptive-Q distribution (AQD) for their applicability to Doppler ultrasound. A synthetic Doppler signal, simulating the nonaxial and pulsatile flow of the common carotid artery, was used as the test signal. The theoretical distribution was compared to each technique, with the cross-correlation (ρ) and the root-mean-square-error (RMSE) providing a quantitative assessment. The AQD had the lowest RMSE and the highest ρ of all the TFDs, while the CWD and the ED were very similar and better than the spectrogram (but both were prone to low-level noise). In conclusion, the AQD performed better than the traditional spectrogram and the other TFDs, but it was more computationally demanding