Effect of the insonification angle on the Doppler backscattered power under red blood cell aggregation conditions

It has been reported that power color Doppler ultrasound has important advantages over conventional color Doppler flow imaging. Some of these advantages are the aliasing free capability, the increased sensitivity to flow, and the angular independence. This last characteristic of power Doppler ultrasound was evaluated to verify if it was still valid in some well-defined flow conditions where porcine whole blood, calf red cells suspended in saline solution, and carbon fibers suspended in a water-glycerol mixture were used as scattering particles. Experiments were conducted under steady flow conditions (mean shear rates across the tube between 8.5 and 102 s/sup -1/) for insonification angles between 40 and 80/spl deg/. Different hematocrit values (5, 10, 20, and 40%) were specifically tested for porcine whole blood. Results indicated no angular dependence for the saline suspension of calf red cells while a strong anisotropy was observed for the carbon fibers. In this last situation, the Doppler power in decibels increased linearly with the insonification angle. The maximum found at 800 suggests that the fibers were aligned with the direction of the flow. For porcine whole blood, an angular dependence was observed at some specific shear rate conditions. At 40% hematocrit, the anisotropy was about 5 dB for shear rates between 17 and 51 s/sup -1/, while for a lower (8.5 s/sup -1/) or higher shear rate (102 s/sup -1/), the anisotropy was reduced to approximately 2 dB. In all of these situations, the maximum Doppler power was observed for an insonification angle between 45 and 60/spl deg/, For hematocrit values of 5, 10, and 20%, the anisotropy was respectively on the order of 2, 3, and 4 dB or less, depending on the shear rate conditions. Among the possible mechanisms that may explain the anisotropic effect observed in the present study, the structure of the red cell aggregates is believed to be the determinant factor. A hypothesis concerning the structure of the aggregates under flowing conditions in large diameter tubes is proposed.