Detection of moving flow separation in pulsatile flow and the degree of stenosis by power of Doppler shift signals.

Power ratios were derived from the principle of ultrasonic Doppler velocimetry to determine a ratio of the volume of vortices to the total vessel volume under the ultrasonic beam. This ratio also equals the ratio of the cross-sectional area of vortices to the vessel lumen. In vitro pulsatile flow experiments were performed in a Reynolds number range of 1,230-4,320 with axisymmetric constrictions with area reductions of 55%, 65%, 70%, and 85% to model carotid stenosis. Flow separation downstream from the model stenoses was detected, and the power ratio fluctuated when vortices with the forward- and reverse-flow velocity components passed by the measurement position. The power ratio estimated the degree of stenosis within 10% of error. Ensemble average of the power ratio was computed to obtain the statistically averaged separated flow region. The moving flow reattachment point was revealed downstream from the 85% stenosis at a Reynolds number of 900.