Theory of Ultrasound Doppler-Spectra Velocimetry for Arbitrary Beam and Flow Configurations

Conventional ultrasound Doppler velocimetry is based on the frequency shifts produced when the beam axis and the flow direction produce acute angles. Recently it was shown, theoretically as well as experimentally, that by using a pulsed Doppler system with the beam transversely oriented with respect to the flow, the flow velocity can be derived from the limits of the relevant Doppler spectrum. The theory discussed previously was limited to transverse motion, constant flow velocity and uniformly insonified apertures, as well as monochromatic excitation. Presently these results are generalized to take into account arbitrary directions of incidence, effects of velocity gradients, arbitrary apertures and arbitrary source pulses. For uniform apertures and transverse flow, it has been previously shown that the Doppler output spectrum is symmetrical about zero frequency, with its width depending in a straightforward manner on the Doppler effect due to the velocity and t he geometry of t.he problem. For a beam direction oblique to the particle's direction, this spectrum is shifted, so that instead of zero frequency, the reference is the classical Doppler frequency corresponding to the velocity component parallel to the beam. Previously only a constant velocity flow field was considered. It is shown here that for velocity gradients and transverse flows the spectrum remains symmetrica1,with the edges corresponding to the maximal velocity, however, the profile becomes peaked at the center. For oblique incidence an asymmet,rical spectrum is obtained and its edges are related to the maximal and minimal velocities within the measurement volume. For the simple case of a long strip transducer discussed previously, it was shown that the Doppler system output spectrum is essentially obtained by convolving the transmitter and receiver aperture functions. The general discussion given here, even for the single particle, is more complicated. Nevertheless, using the reciprocity theorem it is shown that, the output spectrum is obtained by convolving the particle excitation spectrum due to monochromatic excitation, with the receiver input spectrum due t o a moving monochromatic source, all this shifted to the classical Doppler frequency mentioned above. It is shown that when the excitation, rather than being monochromatic, possesses an arbitrary (narrow band) s pectrum, this spectrum, replicated in a prescribed manner, has to be convolved with the spectra derived above. The spectrum produce by an ensemble of particles is more complicated. The strategy used here is to derive semiquantitative graphical interpretations for various configurations, and to further substantiate t he results by analytically treating simplified models.