The purpose of this study was to investigate the sequential changes in rat artery blood flow and tissue degeneration after exposure to high-intensity focused ultrasound (HIFU) in vivo. HIFU was applied through the skin to the femoral artery of Sprague-Dawley rats. The peak intensities used were 530, 1080, 2750, and 4300 W/cm2. After exposure, we measured the peak systolic velocity (PSV) in the artery every 1 min until the velocity stabilized. The vessel was resected and examined histologically 7 days after exposure. PSV was not significantly affected by HIFU exposure at 530 W/cm2. PSV increased immediately after HIFU exposure at intensities of 1080 and 2750 W/cm2. PSV after HIFU exposure at 1080 W/cm2 fell to the control level within minutes; however, PSV increased immediately after HIFU exposure at 2750 W/cm2 and then decreased slowly but remained at a higher level than the control for 15 min. On HIFU exposure at 4300 W/cm 2, the target artery was completely occluded. Histological studies 7 days after HIFU exposure demonstrated that exposure at 530 and 1080 W/cm 2 induced vacuolar degeneration in the tunica media of the femoral artery in rats; exposure to HIFU at 2750 and 4300 W/cm2 resulted in strong necrotic degeneration in the tunica media. These histological changes were more marked than those found immediately after HIFU exposure. Organized thrombus formation was observed only for HIFU exposure at 4300 W/cm2. Sequential changes in arterial blood flow after HIFU exposure vary with the intensity, and the histological changes in arterial tissue progress over time. These phenomena should be considered when HIFU is clinically applied to achieve arterial occlusion.
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