Ultrasonic backscatter anisotropy of sheared blood suspensions.

The anisotropy of the ultrasonic backscatter coefficient (BSC) was investigated in sheared suspensions of porcine red blood cells (RBCs). Experiments were conducted in Couette and tubular flow devices using a focused mono-element transducer (15 or 32 MHz) at various insonification angles relative to the flow direction. For suspensions of disaggregated RBCs, the BSC was lowest at 90°, when the ultrasound beam was perpendicular to the flow. This angular dependence is attributed to a shear-induced anisotropic microstructure, particularly a pair-depleted region aligned with the velocity direction. A similar angular trend was observed in suspensions of aggregated RBCs, where the minimum BSC occurred around 90°-95°, likely due to regions depleted in aggregate pairs acting as effective scatterers. Finally, the impact of BSC anisotropy on RBC aggregate size estimation was evaluated using model-based inverse methods. Among tested models, the effective medium theory combined with the polydisperse structure factor model provided the most reliable estimates. These findings highlight the role of shear-induced microstructure in shaping BSC anisotropy and its implications for ultrasound-based quantification of RBC aggregation.