The echogenicity of whole blood is known to vary during the flow cycle under pulsatile flow both in vitro and in vivo. However, the fundamental underlying mechanisms remain uncertain. The combined effects of flow acceleration and the shear rate were recently used to explain the cyclic variations of signals backscattered from flowing blood. However, testing this hypothesis requires determination of the spatial distributions of red blood cells (RBCs) in flowing blood. Recently, the Nakagami (m) and scaling (Ω) parameters have been used, respectively, to detect the spatial distributions of RBCs and the intensity of backscattering signal from blood under steady flow. For a better understanding of the relationship between the spatial distributions of RBCs and erythrocyte aggregation under pulsatile flow condition, these ultrasound backscattering statistical parameters were used, in this study, to characterize signals backscattered from both whole blood and RBC suspensions at different peak flow velocities (from 10 to 30 cm/s) and hematocrits (20% and 40%). The experiments were carried out by a 35-MHz ultrasound transducer. The m and Ω parameters were calculated for different blood properties and conditions, and the flow velocity in the center of blood flowing through a tube was measured synchronously. In whole blood, the results demonstrated that most RBCs were aggregated progressively toward the center of tube as the flow velocity started to accelerate, and that the increase in the intensity of the backscattered signal envelope to a maximum was attributable to larger rouleaux being formed in the center of tube. This phenomenon became apparent at a lower peak flow velocity with 40% hematocrit. However, there were no cyclic and spatial variations of the backscattering signal over a pulsatile cycle in RBC suspensions.
