AGGREGATION EFFECTS IN WHOLE-BLOOD - INFLUENCE OF TIME AND SHEAR RATEMEASURED USING ULTRASOUND

Ultrasound B-mode imaging (7 MHz) was used to measure blood echogenici ty and velocity profiles simultaneously as they developed with axial d istance for a steady flow of 28% hematocrit whole blood flowing in a l ong (> 60 D) large diameter (D = 2.54 cm) tube. At selected sites alon g the flow axis, velocity profiles were measured using block matching (cross correlation) between successive digitized images with a known t ime separation; from these shear rate profiles were calculated. The co rresponding echogenicity profiles were also determined by averaging th e digitized images. It was found that over a range of low shear rates, the echogenicity is enhanced in a manner similar to the previously re ported influence on aggregation. Evidence is presented confirming the important role of aggregation in controlling the echogenicity. The tra nsient effects of abrupt flow stoppage were studied and shown to provi de useful insights into aggregation kinetics. Based on the above resul ts, a detailed explanation is provided of the echogenicity variations seen in B-mode ultrasound images of slow-moving blood.