A study on influence of HIFU exposure to arterial blood flow in the rat

The purpose of this study was to investigate responses of arterial contractility to high intensity focused ultrasound (HIFU) and histological changes of the artery with various intensities. We constructed a prototype transducer which provides both color flow image and HIFU sonication. HIFU was sonicated to deep femoral arteries in the left thigh of Sprague-Dawley (SD) rats (female, 250-300 g) through the skin, where the blood flow was visualized by color imaging. Peak intensities used were 530, 1080, 2750, and 4300 w/cm ². The duration of HIFU sonication was 5 seconds; subsequently, 5-sec. sonications were perfomed 5 times on the left thigh across the vessel with spacing of 1.0 mm. Blood flow occlusion was accomplished by HIFU exposure of 4300 w/cm² intensity, but the flow was preserved with intensities of 530, 1080 and 2750 w/cm². Peak systolic velocity (PSV)s of blood flow measured by Doppler velocimetry increased in sonicated arteries with HIFU of 1080 and 2750 w/cm² intensity and correlated with HIFU intensity. HIFU exposure of 530 w/cm² intensity did not change blood flow velocity. Histological studies have demonstrated vacuolar degeneration with HIFU exposure of 2750 and 4300 w/cm² intensity. HIFU exposure of 1080 w/cm² increased PSV but did not induce histological changes in the vessel wall. In conclusion, the response of the artery to HIFU exposure grades with intensity is in such a way that vascular contraction without tissue degeneration occurs as a first step; subsequently, histological changes diminish vascular diameter and finally occlude the blood flow. It is suggested that these phenomena are probably caused mainly by thermal effects, but cavitaion might have an influence particularly on vascular contraction.