IEE PROCEEDINGS-SCIENCE MEASUREMENT AND TECHNOLOGY, cilt.148, ss.15-22, 2001
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 optimised. Mapping directional information in the scale domain is also desirable. A method is described, based on the utilisation of complex wavelets and negative scales. It eliminates the intermediate processing stages for obtaining directional Doppler signals for time-scale analysis.