Study of the effects of flow acceleration on blood aggregation by high frequency duplex ultrasound: Under pulsatile flow

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 cyclic variations of high frequency ultrasonic backscattering signals have been measured for explaining the temporal variations of red blood cells (RBCs) aggregation under pulsatile flow. The level of RBC aggregation was enhanced by reducing the flow velocity. However, the size of rouleau still increased during the flow acceleration phase. This observation is opposite to previous studies which the formation of rouleau should be broke up by increasing the shear force of flow. In order to further explore this phenomenon, the effect of flow acceleration on RBC aggregation was studied by high frequency duplex ultrasound. The experiments were performed on porcine whole blood with different hematocrits from 20 to 40%. The blood was circulated in a pulsatile Couette flow apparatus under different flow accelerations. Both ultrasound M-mode images and Doppler flow information from flowing blood were obtained by the 35 MHz and 30 MHz transducers, respectively. The backscattering signals and Doppler flow velocities were acquired synchronously to compare the relationship between flow acceleration and blood aggregation. The results indicated that the cyclic variations became weaker as the 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 increase of peak flow velocity. The flow velocity between 10 to 20 cm/s can be treated as a threshold for rouleaux disaggregation under pulsatile flow during the acceleration phase.