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 skewness of the spectral representation, were evaluated experimentally using porcine red cell suspensions as ultrasonic scatterers. Intrinsic broadening, 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 of 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 theorem was experimentally verified with an in vitro steady laminar blood flow model. We showed that the relative bandwidth was both independent of the flow velocity and blood hematocrit. Using a pulsatile laminar flow model, we demonstrated that the relative Doppler bandwidth invariance theorem did not hold during flow acceleration and deceleration. In addition, a positive skewness of the Doppler spectra was observed during acceleration while a negative skewness was measured during the deceleration of blood. The effect of the window duration used in the Fourier spectral computation, on nonstationary broadening, was also characterized. For a window of 2.5 ms, broadening due to spectral leakage dominated over nonstationary broadening. The limitation of the spectrum analyzer was less important for windows of 5 and 10 ms. Experiments were also performed in pulsatile turbulent flow to verify the behavior of the relative Doppler bandwidth and spectral skewness. In this flow regime, both parameters significantly varied within the flow cycle, with a pattern of variation different from that observed in pulsatile laminar flow. Generally, good matching was found between experimental and theoretical results. Significant basic information on the backscattering of ultrasound from blood in both steady and pulsatile flow is presented in this study.