Wavelet 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. Complex FFT 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 scale domain is also desirable. A method based on the utilisation of complex wavelets and negative scales is described. It eliminates the intermediate processing stages for obtaining directional Doppler signals for time-scale analysis.