EXPERIMENTAL EVALUATION OF INTRINSIC AND NONSTATIONARY ULTRASONIC DOPPLER SPECTRAL BROADENING IN STEADY AND PULSATILE FLOW LOOP MODELS

Intrinsic and nonstationary Doppler spectral broadening, and the skewn ess of the spectral representation, were evaluated experimentally usin g porcine red cell suspensions as ultrasonic scatterers. Intrinsic bro adening, by definition, refers to the broadening produced by the range of angles sustained by each scatterer viewed by the finite dimension of the transducer. Nonstationary broadening refers, on the other hand, to the broadening associated with the acceleration and deceleration o f the scatterers within the Doppler sample volume. Theoretically, the relative Doppler bandwidth, defined as the intrinsic bandwidth divided by the mean Doppler frequency shift, should be velocity independent. In the present study, the relative Doppler bandwidth invariance theore m was experimentally verified with an in vitro steady laminar blood fl ow model. We showed that the relative bandwidth was both independent o f the flow velocity and blood hematocrit. Using a pulsatile laminar fl ow model, we demonstrated that the relative Doppler bandwidth invarian ce theorem did not hold during flow acceleration and deceleration. In addition, a positive skewness of the Doppler spectra was observed duri ng acceleration while a negative skewness was measured during the dece leration of blood. The effect of the window duration used in the Fouri er spectral computation, on nonstationary broadening, was also charact erized. For a window of 2.5 ms, broadening due to spectral leakage dom inated over nonstationary broadening. The limitation of the spectrum a nalyzer was less important for windows of 5 and 10 ms. Experiments wer e also performed in pulsatile turbulent flow to verify the behavior of the relative Doppler bandwidth and spectral skewness. In this flow re gime, both parameters significantly varied within the flow cycle, with a pattern of variation different from that observed in pulsatile lami nar flow. Generally, good matching was found between experimental and theoretical results. Significant basic information on the backscatteri ng of ultrasound from blood in both steady and pulsatile flow is prese nted in this study.