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Ultrasonic backscattered signals from blood contain frequency-dependent information that can be used to obtain quantitative parameters reflecting the aggregation state of red blood cells (RBCs). Recently, the structure factor size estimator (SFSE) was developed to solve for two parameters: the packing factor W and mean gyration radius of aggregates Rg, expressed in number of RBCs (Yu and Cloutier 2007, J. Acoust. Soc. Am. 122 645-656). Herein, the SFSE is assessed by computer simulations where deterministic rheology parameters, such as the hematocrit, number of RBCs per aggregate, and mean gyration radius of aggregates, are known. RBC aggregation was simulated at 40% hematocrit and mean gyration radii of clusters ranged from 1.6 (quasi-disaggregated RBCs) to 6.4. US frequencies were varied between 20 and 42 MHz, and the insoniflcation angle was fixed to 90deg (perpendicular to the flow). For the quasi-disaggregated RBCs, the packing factor W evaluated with the SFSE from the simulated signals matched those expected from theory: W was similar to the Perkus Yevick packing factor. For aggregating RBCs, good correlation was obtained between the gyration radii evaluated with the SFSE model and those expected (r2 = 0.96). To conclude, this study shows the SFSE ability to estimate blood backscattering properties at a physiological hematocrit of 40% and open the way to parametric imaging (using the SFSE) in abnormal blood conditions promoting thrombosis.