Ultrasound characterization of erythrocyte aggregation (EA) is attractive because it is a non-invasive imaging modality that can be applied in vivo and in situ. An experimental validation of the Structure Factor Size Estimator (SFSE), a non-Rayleigh scattering model adapted for dense suspensions, was performed on 4 erythrocyte preparations with different aggregation tendencies. Erythrocyte preparations were circulated in Couette and tube flows while acoustically imaged over a bandwidth of 9-28 MHz. Two acoustically derived parameters, the packing factor (W) and ensemble averaged aggregate size (D), predictably increased with increasing EA, a finding corroborated by bulk viscosity measurements. In tube flow, a "black hole" reflecting the absence of aggregates was observed in the center stream of some parametric images. The SFSE clearly allowed quantifying the EA spatial distribution with larger aggregates closer to the tube walls as the aggregation tendency was increased. In Couette flow, W and D were uniformly distributed across the shear field. Assuming that the viscosity increase at low shear is mainly determined by EA, viscosity maps were computed in tube flow. Interestingly, erythrocyte suspensions with high aggregabilities resulted in homogeneous viscosity distributions, whereas a "normal" aggregability promoted the formation of concentric rings with varying viscosities.