The echogenicity of whole blood is known to vary during the flow cycleunder pulsatile flow both in vitro and in vivo. However, the fundamentalunderlying mechanisms remain uncertain. The cyclic variations of high frequencyultrasonic backscattering signals have been measured for explaining the temporalvariations of red blood cells (RBCs) aggregation under pulsatile flow. Thelevel of RBC aggregation was enhanced by reducing the flow velocity. However,the size of rouleau still increased during the flow acceleration phase. Thisobservation is opposite to previous studies which the formation of rouleaushould be broke up by increasing the shear force of flow. In order to furtherexplore this phenomenon, the effect of flow acceleration on RBC aggregation wasstudied by high frequency duplex ultrasound. The experiments were performed onporcine whole blood with different hematocrits from 20 to 40%. The blood wascirculated in a pulsatile Couette flow apparatus under different flowaccelerations. Both ultrasound M-mode images and Doppler flow information fromflowing blood were obtained by the 35 MHz and 30 MHz transducers, respectively.The backscattering signals and Doppler flow velocities were acquiredsynchronously to compare the relationship between flow acceleration and bloodaggregation. The results indicated that the cyclic variations became weaker asthe stroke rate increased and the variation was higher for a hematocrit of 40%than for one of 20%. However, the cyclic variation became stronger with increaseof peak flow velocity. The flow velocity between 10 to 20 cm/s can be treatedas a threshold for rouleaux disaggregation under pulsatile flow during theacceleration phase.