Attenuation compensated spectral slopes during the kinetics of rouleaux formation for porcine whole blood in Couette flow at 10-70 MHz

The characterization of red blood cell (RBC) aggregation using ultrasound is a promising technique allowing in vivo and in situ assessment of its level in the human body. It is known that pathological levels of RBC aggregation are correlated with diseases involving hemorheological disorders. Important efforts have thus been targeted towards theoretical and empirical studies on ultrasonic absolute RBC backscatter. In this study, we present ultrasonic absolute backscatter values (BSC), and spectral slope (SS) dependencies as a function of temporal shear rate variations. Porcine anti-coagulated blood at 40% hematocrit was placed in a modified Couette flow system at 37°C. Backscattered RF data were digitized and analyzed, using three wideband focused transducers of central frequencies 10, 35 and 55 MHz. BSC and SS were calculated for each transducer to provide kinetic profiles during aggregation and disaggregation of RBC. The reference medium used for absolute BSC values was a 6% hematocrit suspension of RBC in Ringer solution (no aggregation). Frequency dependent beam variations were taken in account using a diffraction correction factor. Different results on BSC and SS sensitivity for "small aggregates" and "large aggregates" are described as a function of frequency. The size of the aggregates was varied by modifying the flow shear rate. According to these experimental results, it is shown that spectral slopes between 10-70 MHz could be of clinical relevance for in vivo detection of red blood cell aggregation. This index may be of interest to display parametric images of RBC aggregates.