Red cell aggregation as a cause of blood-flow echogenicity.

The role of red cell aggregation as a cause of ultrasonic echogenicity in flowing blood was evaluated by in vitro experiments using fresh human blood. Blood was circulated in tubes of varying diameter (12 mm to 6 mm). In all experiments, echogenicity increased as blood approached static conditions. Echogenicity was greater in tubes with a larger diameter over the same range of blood velocity. However, echogenicity in tubes of various diameters was the same when evaluated in terms of shear rate. Thus, shear rate and not velocity is the flow condition that determines echogenicity. Since shear rate determines the degree of red cell aggregation, while other conditions affecting red cell aggregation (hematocrit, erythrocyte membrane conditions, and plasma macromolecules) are held constant, we conclude that these results provide additional evidence that red cell aggregation is a cause of echogenicity in flowing blood. Furthermore, a red cell aggregation mechanism for blood-flow echogenicity would explain the increased prominence of internal echoes in lower shear rate venous blood flow compared with those of higher shear rate arterial blood flow.