Anisotropy of ultrasonic backscatter by blood in shear flow: Monte Carlo simulations

The relation between the heterogeneous microstructure of biological tissues and their measurable scattering properties is still poorly understood. In particular, physical explanation of blood hyperechogenicity when submitted to low shear forces appears incomplete. To quantify the contribution of erythrocyte aggregation to this phenomenon, Monte Carlo 2D simulations of the red cell spatial pattern are performed. The backscattering coefficient of blood at 5 and 40 MHz is estimated for two orthogonal insonification angles, as a function of effective adhesive energy V/sub agg/ and anisotropy index a. Isotropic aggregation resulted in an enhanced backscatter at 5 MHz (+7 dB) but had a minor effect at 40 MHz. Addition of spatial anisotropy essentially diminished the backscatter at 5 MHz, independently on the angle, whereas at 40 MHz, the perpendicular backscatter largely exceeded the parallel backscatter (+6 dB). This showed that anisotropy present in the spatial microscopic pattern can be detected in the high frequency scattering regime, while low frequency backscatter is more affected by larger geometrical features as the aggregate size.