| Popis: |
Ultrasound Doppler velocimetry for medical diagnostics is based on displays of spectra derived from time signals due to acoustical waves backscattered by random collections of moving particles. Those spectra are notoriously very noisy, and accurate velocity measurements are usually difficult if not outright impossible. Models describing the stochastic signals have been suggested in the literature, e.g., by Mo and Cobbold (1986). Such models account for the interference produced by the random phased time signals. However, it is very well known that the often encountered non-steady, nonuniform, sometimes turbulent flows are conducive to randomly fluctuating spectra as well, causing also spectrum broadening. While the presence of a superposition of randomly phased time signals, originating from randomly located scatterers, adequately explains the fluctuations in the spectrum, giving it its typical “jagged” or “ridged” appearance, broadening is mainly due to Doppler frequency shifting, introduced by the motional modes mentioned above, whose analysis requires additional considerations. These broadening phenomena are manifested whenever additional spurious Doppler type frequency shifts occur, even without the presence of a fully developed turbulent flow regime. We therefore refer to them generically as microturbulences. While the statistical nature of the fluctuating phase signals and the associated spectra can be satisfactorily modeled, there is only a cursory understanding as to how these typically ridged spectra are related to the characteristics of the microturbulent flow in question. This problem might be very important for medical diagnostics, because the resulting displays might look similar in cases of turbulence due to stenoses, and cases of regurgitation or retrograding flows. |
