An ultrasonic method for noninvasive estimation of vascular impedance to an absolute level: in vitro validation of the relationship between impedance and phase velocity

Vascular impedance is a quantity that characterizes the properties of the vascular bed. Assuming Newtonian fluid, laminar flow and linear system, impedance is related to phase velocity. The agreement between theoretical relationship and measured data is evaluated in an in vitro study. The setup consist of a computer-controlled flow generator, a variable fluid impedance and a measuring section. Phase velocity is calculated from multiple recorded vessel pulsations detected by ultrasonic pulse-echo technique. The reference impedance is calculated from simultaneously measured high-fidelity pressure and flow data. The impedance is analyzed in the range of 1-40 Hz with 1-Hz resolution. The evaluation of the noninvasive ultrasonic technique is focused on the resonance frequencies and the characteristic impedance. No significant difference (p>0.05) is found in the resonance frequencies nor in the characteristic impedance. However, the impedance below 5 Hz is underestimated and the extension of the measuring section causes spatial averaging effects with reduced frequency resolution.