Spatial variation of ultrasonic backscatter from flowing whole blood

Ultrasonic backscatter of flowing whole blood was measured using a modified B-mode real-time scanner capable of acquiring both video and radio frequency data. Whole blood with hematocrit values varying from 4% to 45% was circulated in a mock flow loop under steady laminar flow at different mean shear rates, ranging from 52 to 160 s^-1. Results show that backscatter from whole blood varies across the vessel lumen and exhibits a peak located approximately in the center stream of the flow. It is postulated that the observed spatial variation of backscatter is related to the degree of red cell aggregation, which is affected by the spatial variation of the shear rate in the tube. A hypoechoic region or black hole surrounded by two bright rings was observed to occur at high shear rates and high hematocrits. These rates differ from those used by previous investigators. When pulsatile flow was introduced, backscatter dropped significantly and a nearly parabolic echogenic front could be readily observed under certain conditions.