Changes in ultrasonic Doppler backscattered power downstream of concentric and eccentric stenoses under pulsatile flow.

The main objective of the present work was to investigate, under pulsatile flow, the patterns of variation of the Doppler power backscattered by blood and Sephadex particles upstream and downstream of concentric and eccentric stenoses ranging from 47% to 91% area reduction. Doppler measurements were performed at 5 diameters upstream and 5, 10, 15 and 20 diameters downstream of the constriction. For the concentric 75% and 85%, and the eccentric 79% and 91% area reduction stenoses, a progressive increase of the power backscattered by red cell suspensions at 40% hematocrit was measured downstream of the narrowing. The maximal power usually occurred around 10 diameters after the stenosis and dropped further downstream. In addition to the increase in the power, a cyclic variation of the backscattered intensity was observed within the flow cycle. For the concentric 52% and eccentric 47% area reduction stenoses, no variation of the Doppler power was measured during flow acceleration and deceleration for all recording sites. A coefficient of correlation of 0.82 was measured between the percentage of area reduction and the ratio of the Doppler mean power at 10 diameters downstream to that at 5 diameters upstream of the stenoses. Using Sephadex particles at low concentration, no increase of the Doppler power was found downstream of the 85% and 91% area reduction stenoses. The possible link between the intensity of turbulence and the power backscattered by blood is discussed along with the influence of the correlation between the scattering particles, under turbulent flow.