| The goal of the acoustic hemostasis project is to locate a hemorrhage using diagnostic ultrasound and halt the bleeding using high intensity focused ultrasound (HIFU). To enhance the imaging of blood, the use of ultrasound contrast agents (UCA; gas filled microbubbles that increase the echogenicity of fluids), has been proposed as a means to locate internal bleeding (Schmiedl et al., 1999). This study investigates the mechanisms of HIFU-induced bioeffects in blood which may lead to primary, or platelet-related, hemostasis.; Using platelet rich plasma (PRP), we investigated the effect of 1.1-MHz continuous wave (CW) HIFU on platelet activation, aggregation and adhesion to a collagen-coated surface. Flow cytometry, laser aggregometry, and environmental scanning electron microscopy (ESEM) were used to quantify platelet activation and aggregation, and to observe adhesion to a collagen-coated surface.; To investigate the role of cavitation as a mechanism for platelet aggregation, a 5-MHz passive cavitation detector was used during aggregation trials to monitor cavitation activity, which was then quantified to provide a relative measure of the amount of cavitation that occurred in each aggregation trial. The effects of HIFU induced cavitation on platelet aggregation in PRP were investigated by enhancing cavitation activity through use of UCA, and by limiting cavitation activity through use of an overpressure system. Platelet aggregation was measured in PRP with laser aggregometry, and in whole blood using an impedance measurement technique. Doppler ultrasound and high speed camera observations were used to measure bulk streaming velocities, providing an estimate of shear stresses due to bulk flow.; Our results show that in PRP, increased cavitation activity lowers the intensity threshold to produce platelet aggregation and decreased cavitation activity in the overpressure system raises the intensity threshold for platelet aggregation. Resulting platelet aggregation in whole blood is less than in samples of PRP for comparable cavitation doses. Shear stresses due to bulk flow are insufficient to stimulate platelet aggregation.; It was determined that HIFU can activate platelets, stimulate them to aggregate and promote their adherence to a collagen-coated surface. In principle, HIFU can stimulate primary, or platelet-related, hemostasis. This study explores the HIFU exposure mechanisms responsible for inducing platelet activity in vitro. |
