11C-6 New Observations on the Anisotropy of Ultrasound Blood Backscatter as a Function of Frequency and Shear Rate

It is recognized that biological tissues with orientated structures show an angular dependence in echogenicity. Previous works on ultrasound (US) blood backscattering have pointed out that red blood cell (RBC) aggregates (or rouleaux) had privileged orientations in a tubular flow inducing measurable US anisotropic behaviours, but results are scarce. The non Newtonian blood velocity profile in tube flow may however be seen as an obstacle to the understanding of the anisotropic spatial organization of RBC aggregates. In this study, whole blood was circulated in a Couette flow apparatus, which allowed to predictably modulate the RBC aggregate sizes and orientations, and to quantify their respective effects on the angle dependent backscatter coefficient (BSC). Anticoagulated porcine bloods were adjusted to 40% hematocrit and inserted into the Couette flow apparatus, at 37degC. US data were acquired and analyzed using two broadband focused transducers at 20 MHz and 35 MHz. The BSC was measured at ten different insonification angles from 30deg to 150deg with respect to the direction of the blood flow, for shear rates of 2 s^-1 and 10 s^-1. A control experiment was also conducted on porcine RBCs suspended in a saline solution (no aggregation). The results indicate no anisotropic effect for the saline suspension and a significant angular dependence for whole blood. At an incident frequency of 20 MHz, the anisotropy was 4.3 dB for a shear rate of 2 s^-1, while it was 4.6 dB for a shear rate of 10 s"1. Similarly, at an incident frequency of 35 MHz, the anisotropy was 5.1 dB for a shear rate of 2 s^-1 and 6.2 dB for a shear rate of 10 s^-1. Thus, the higher anisotropic variation at 35 MHz may indicate a better US sensitivity to the rouleaux orientation at that frequency. Furthermore, at a shear rate of 2 s^-1, the maximum BSC was measured at an angle of 75deg, whereas at a shear rate of 10 s^-1, it was maxim- um at an angle of 90deg. These last results suggest that the RBC aggregate orientation is shear rate dependent. In conclusion, the Couette flow system that was employed is a good strategy to predictably control the orientation of RBC aggregates as a function of the shear rate.