The effect of hemodynamics, vessel wall compliance and hematocrit on ultrasonic Doppler power: An in vitro study

Previous in vitro studies in rigid tubes under pulsatile flow conditions ha ve reported a lack of a cyclic variation in blood echogenicity that contrad icts ill vivo results. To investigate whether or not these variations can b e attributed to the compliance of the vessel wall, a series of in vitro exp eriments with compliant tubes, under pulsatile flow conditions, was perform ed. Two important factors that may affect the Doppler power were investigat ed: 1, the dependence on hematocrit and 2, the effect of the vessel wall el asticity. In the present study, it is shown that, at the low beat rates, th e peak of the mean Doppler power within the Bow cycle depends on the vessel wall compliance. When the vessel becomes more compliant, the peak is shift ed from the early to the late systole, Additionally, there is a correlation between the power peak and hematocrit that is more evident in compliant ve ssels. At a higher pulsation rate of 37 beats/min, a different variation is observed, A drop in the power occurs near peak systole in compliant tube e xperiments and is more pronounced as the vessel becomes more constricted. T he observed power drop agrees with previously reported in vivo results, but is not seen in rigid tube experiments. The results of this study suggest t hat proper interpretation of cyclic variations in Doppler power requires a knowledge of hemodynamic parameters, such as the modulus of elasticity of t he vessel wall, propagation velocity or, possibly, the phase angle of input impedance. (C) 1999 World Federation for Ultrasound in Medicine & Biology.