Intravascular ultrasound imaging of blood: the effect of hematocrit and flow on backscatter

This study evaluates two key parameters influencing the ultrasonic backscatter from blood--hematocrit and flow rate--at 30 MHz in an in vitro flow system. A range of hematocrits from 0 to 50% was studied at a constant flow rate; various flow rates between stagnation and physiologic levels were studied at a constant hematocrit. The relation between backscatter intensity and hematocrit was a convex function with a maximum between a hematocrit of 10% and 20%. In the flow rate studies, the blood backscatter intensity was a maximum at a flow rate of 0 and rapidly decreased at higher flow rates. These in vitro results suggest that blood backscatter intensity is minimally dependent on hematocrit in the physiologic range. However, a dramatic increase in backscatter intensity occurs with stagnant flow, presumably the result of red blood cell aggregation. Clinically, blood backscatter intensity may provide an index for risk of thrombus formation.