Spectral velocity profiles for detailed ultrasound flow analysis

The operation of a novel ultrasound multigate instrument capable of computing in real-time the fast Fourier transform (FFT) of Doppler signals detected from 64 equally spaced range cells is presented. The new system provides up to 50 velocity profiles per second, which are displayed in such a manner that information about the full spectral content of Doppler signals at all the investigated depths is continuously monitored over a PRF-wide frequency range which can be set arbitrarily between -PRF and +PRF. Experimental results are presented, which demonstrate that the true velocity profile can be accurately detected through the computation of "local" maximum velocities obtained by properly correcting the maximum frequency of each spectrum. There is also a discussion on how the results of multigate analysis are influenced by the sample volume length, a parameter which can be usually set by modifying the duration of the transmitted burst. In particular, it is shown that, in regions close to the vessel walls, the shear rate can be measured with a spatial resolution related to the spacing between subsequent range cells and not to the sample volume length.