In vitro evaluation of volumetric flow from Doppler power-weighted and amplitude-weighted mean velocities

Ultrasound theory suggests that the volume of flow is directly related to the power and amplitude of the backscattered Doppler signals. To evaluate the accuracy of volume flow calculated with power-weighted and amplitude-weighted mean velocities (PWMV and AWMV), volume flows were measured in a pulsatile flow-tank system equipped with a 1.25 cm diameter simulated femoral artery. Analyses were performed throughout a range of physiologic flows, mean driving pressures, and pulse rates. At each hemodynamic setting, volume flow in the simulated artery was measured with an electromagnetic flow probe and with pulsed Doppler echocardiography by use of 7.0 and 3.5 MHz transducers. In addition, to determine the effects of vessel size and parabolic flow on the accuracy of the Doppler volumes, volume flow was evaluated in several differently sized vessels at sampling distances of 20 times the vessel diameter downstream from the orifice. On the ultrasound system, PWMV was calculated as the sum of the individual velocities multiplied by their respective power fractions (the fraction of the total instantaneous power represented by the individual signal power). The instantaneous PWMV was plotted continuously in time and superimposed on the spectral recording. Similarly, AWMV was calculated with amplitudes measured as the square root of the signal power. The PWMV and AWMV were integrated over the flow period and multiplied by the known cross-sectional area of flow to obtain the Doppler volume. In all analyses performed, volumetric flows calculated with Doppler echocardiography with PWMV and AWMV correlated extremely well with those measured with the electromagnetic flow probe. Thus, over a wide range of physiologic conditions, transducers frequencies, and vessel sizes, volume flow can be accurately calculated from PWMV and AWMV Doppler data. This technique provides an accurate, automatic method for on-line determination of volumetric flow.