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The ultrasound characterization of red blood cell (RBC) aggregation is an attractive tool to determine rheological blood flow disorders in vivo and in situ. The backscattered signals from blood can be spectrally analyzed to deduce the size and spatial distribution of RBC aggregates. One difficulty to apply this analysis in vivo is due to the frequency-dependent attenuation caused by intervening tissue layers that distorts the spectral content of backscattered echoes from blood microstructures. An optimization method was recently proposed to simultaneously estimate tissue attenuation and blood structure properties, and was termed the Structure Factor Size and Attenuation Estimator (SFSAE). With in vitro experiments, the method gave satisfactory estimates with relative errors below 25% for attenuations between 0.115 and 0.411 dB/MHz and kR <2.08 (k being the wave number and R the aggregate radius). The current work presents in vivo results to demonstrate the feasibility of the method. Measurements were performed on an arm's vein of a normal subject using an ultrasound scanner equipped with a 25 MHz center frequency probe. The probe was positioned in longitudinal view to examine three blood flow conditions: normal blood flow, stop flow with a proximal and distal constrictions of the ROI, and finally complex flow in the vinicity of two closed venous valves. For each case, the aggregate diameter D, expressed in number of RBCs, the packing factor W and the total attenuation ?0 were estimated by using the SFSAE, i.e. by comparing the spectrum of the backscattered radio-frequency echoes with newly developed models of RBC aggregate scattering. Quantitative ultrasound parametric images of D, W and ?0 estimates were constructed. For the two structural parameters D and W, statistically significant differences were observed between normal and stopped flow conditions, as well as between blood stagnation and circulation zones in the case of the two close- d venous valves. To conclude, this work shows the SFSAE ability to estimate blood backscattering properties in vivo and in situ, and opens the way to parametric imaging for clinical studies in abnormal blood conditions.