Objective:To evaluate the effect of reactivated ultrasound contrast agent on prevention of ischemia-reperfusion (IR) injury in rat hindlimb ischemia model.Methods:Microbubbles were compressed into nanodroplets (NDs) and the particle size range was determined.In vitro experiments were carried out to observe the acoustic phase transition of nanodroplets in the ultrasound field and the change of dissolved oxygen (DO) concentration in the surrounding solution. Forty-one male Sprague Dawley rats were divided into 5 groups: acoustic nanodroplet vaporization treatment group (NDs+ US group, n=9), saline+ ultrasound treatment group (Saline+ US group, n=8), nanodroplet without ultrasound treatment group (NDs group, n=8), ischemia/reperfusion injury group (IRI group, n=8) and sham operation group (Sham group, n=8). Ultrasound imaging was performed before operation and 12 hours after reperfusion to evaluate the improvement of hemodynamics of criminal artery under different treatments. Tissue injury were evaluated by analyzing immunohistochemistry staining results. Results:The formed NDs ranged in size from approximately 68.0-295.4 nm and the highest concentration in the 100 nm range. In vitro studies, a decrease in DO was measured during the phase transition.In the animal experiment, after ischemia/reperfusion, NDs+ US, Saline+ US, NDs and IRI groups demonstrated a significantly higher resistance index (RI) and pualsatility index (PI) of the right common iliac artery compared with before operation (NDs+ US group: PI 1.79±0.17 vs 1.57±0.23, P=0.014; RI 0.80±0.02 vs 0.75±0.04, P=0.002. Saline+ US group: PI 2.29±0.16 vs 1.57±0.16, P<0.001; RI 0.90±0.06 vs 0.74±0.03, P<0.001. NDs group: PI 2.17±0.14 vs 1.53±0.15, P<0.001; RI 0.91±0.04 vs 0.75±0.04, P<0.001. IRI group: PI 2.12±0.22 vs 1.58±0.20, P<0.001; RI 0.88±0.04 vs 0.75±0.04, P<0.001). The increases of PI and RI (ΔPI, ΔRI) in NDs+ US group were higher than those in sham group (all P<0.05), but significantly lower than those in saline+ US group, NDS group and IRI group (all P<0.05). Immunohistochemical results indicated the percentages malondialdehyde (MDA) positive cells in Saline+ US, NDs and IRI groups were higher than those in NDs+ US and sham groups (all P<0.05). Conclusions:The acoustic phase transition of nanodroplets in the ultrasound field can reduce ischemia/reperfusion injury and improve hemodynamics abnormality after reperfusion.