Aggregation effects in whole blood: influence of time and shear rate measured using ultrasound

Ultrasound B-mode imaging (7 MHz) was used to measure blood echogenicity and velocity profiles simultaneously as they developed with axial distance for a steady flow of 28% hematocrit whole blood flowing in a long (> 60 D) large diameter (D = 2.54 cm) tube. At selected sites along the flow axis, velocity profiles were measured using block matching (cross correlation) between successive digitized images with a known time separation; from these shear rate profiles were calculated. The corresponding echogenicity profiles were also determined by averaging the digitized images. It was found that over a range of low shear rates, the echogenicity is enhanced in a manner similar to the previously reported influence on aggregation. Evidence is presented confirming the important role of aggregation in controlling the echogenicity. The transient effects of abrupt flow stoppage were studied and shown to provide useful insights into aggregation kinetics. Based on the above results, a detailed explanation is provided of the echogenicity variations seen in B-mode ultrasound images of slow-moving blood.