Sheer rate dependence of ultrasound backscattering from blood samples characterized by different levels of erythrocyte aggregation

The objectives were (1) to determine the effect of the erythrocyte aggregat ion level (wide range of aggregation) and shear rate (which also affects ag gregation) on the ultrasound backscattered power, and (2) to evaluate the r eproducibility of the ultrasound method. Experiments were performed under s teady flow (100-1250 ml/min) in a 12.7 mm diameter vertical tube. Doppler u ltrasound at 10 MHz was used to measure simultaneously the velocity and the backscattered power across the tube. For each radial position, the shear r ate was computed from the derivative of the velocity profile. The backscatt ered power decayed exponentially as a function of the shear rate, and for a given shear rate, the power increased monotonically with the level of aggr egation measured by laser reflectometry. Using blood samples simulating hyp o-, normal, and hyperaggregating erythrocytes, the power of the ultrasound signal varied respectively by -7.8, -13.2, and -16.1 dB as a function of th e sheer rate (from 0.4 to 50 s(-1)). The reproducibility of the backscatter ed power was 5.5 dB, which is less than the variations observed as a functi on of the shear rate. In conclusion, ultrasound backscattering is sensitive to the level of erythrocyte aggregation. At a first glance, ultrasound see ms less accurate when compared to laser reflectometry but it is suggested t hat this is because ultrasound backscattering may be sensitive to structura l aggregate changes that are not detected by the laser method. (C) 2000 Bio medical Engineering Society.