Time-scale analysis of quadrature Doppler ultrasound signals

Most Doppler ultrasound systems employ quadrature demodulation techniques at the detection stage. The information concerning flow direction, encoded in the phase relationship between in-phase and quadrature-phase channels, is not obvious at this stage. The complex fast Fourier transform can be used to obtain directional information in the frequency domain, as well as time-frequency analysis of Doppler signals. However, it has an inherent time-frequency resolution limitation. The wavelet transform allows the time-frequency resolution compromise to be optimized. Mapping directional information in the scale domain is also desirable. A method is described, based on the utilization of complex wavelets and negative scales. It eliminates the intermediate processing stages for obtaining directional Doppler signals for time-scale analysis.