Cyclic variation of the power of ultrasonic Doppler signals backscattered by polystyrene microspheres and porcine erythrocyte suspensions.

Factors affecting the power of the ultrasonic Doppler signal within the flow cycle have been evaluated experimentally using a pulsatile flow loop model. Polystyrene microspheres and porcine red cells suspended in saline solution for hematocrits between 2 and 40% were used as scattering fluid in the flow model. Experiments were performed at mean flow velocities of 11, 64, and 76 cm/s. In laminar flow experiments performed at a mean velocity of 11 cm/s, no variation of the Doppler power was found for both polystyrene microspheres and red cell suspensions (40% hematocrit). When turbulence was induced in the flow model, the power increased during systole, a maximum was observed early after peak systole, and a decrease was obtained in diastole during deceleration of flow. At higher mean flow velocities (64 and 76 cm/s), a significant cyclic variation of the Doppler power was also measured for all values of hematocrits (between 2 and 40%). The power of the signal scattered by microspheres and red cell suspensions at 4% hematocrit dropped in systole, reached a minimum at peak systole, and then increased during early diastole. For red cells suspended in saline at 40% hematocrit, a slightly different pattern of variation was obtained. The cyclic variations observed at high flow velocities and in the presence of turbulence are believed to be associated with cyclic changes in the correlation among particles. In the present study, the effect of red cell aggregation on the cyclic variation has not been addressed.