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Ultrasound characterization of red blood cells distribution: a wave scattering simulation study

Ultrasonic backscattered signals from blood contain frequency-dependent information that can be used to obtain quantitative parameters describing the aggregation state of red blood cells (RBCs). However the relation between the parameters describing the aggregation level and the backscatterer coefficient needs to be better clarified. For that purpose, numerical wave simulations were performed to generate backscattered signals that mimic the response of two-dimensional (2D) RBC distributions to an ultrasound excitation. The simulated signals were computed with a time-domain method that has the advantages of requiring no physical approximations (within the framework of linear acoustics) and of limiting the numerical artefacts induced by the discretization of object interfaces. In the simple case of disaggregated RBCs, the relationship between the backscatter amplitude and scatterer concentration was studied. Backscatter coefficients (BSC) in the frequency range 10 to 20 MHz were calculated for weak scattering infinite cylinders (radius 2.8 μm) at concentrations ranging from 6 to 36%. At low concentration, the BSC increased with scatterer concentrations; at higher concentrations, the BSC reached a maximum and then decreased with increasing concentration, as it was noted by previous authors in in vitro blood experiments. In the case of aggregated RBCs, the relationship between the backscatter frequency dependence and level of aggregation at a concentration of 24% was studied for a larger frequency band (10 – 40 MHz). All these results were compared with a weak scattering model based on the analytical computing of the structure factor.