Purpose To observe the thrombolytic effect of different ultrasonic frequencies combined with urokinase-containing targeted microbubble contrast agent on rabbit femoral artery,to explore the main influencing factors,and to determine the potential indicators related to recurrent embolism in the microcirculation.Materials and Methods Unilateral femoral arterial thrombosis models were established in the selected 72 New Zealand white rabbits,which were randomly divided into 12 groups,6 rabbits in each group.This study was performed on an experimental combination of three factors and different levels,including different ultrasonic frequencies (1.6,2.2,2.8 MHz),different ultrasonic irradiation time (30 and 60 min),and different urokinase dose (3 and 6 mg).Thrombolysis with urokinase-containing targeted microbubble was performed under low frequency ultrasonic assisted irradiation,the recanalization status of blood vessels was observed,and recurrent embolism in the microcirculation was confirmed by HE staining.Furthermore,rabbit blood samples were collected,with indicators,including 6-keto-prostaglandin F1a (6-keto-PGF1a),Thromboxane B2 (TXB2),P/T ratio (6-keto-PGF1a/TXB2) and P-selectin (SP) being detected.Results All the vessels were recanalized.There was no occurrence of recurrent embolism in the group with ultrasonic frequency of 2.2 MHz,radiation time of 30 min,and urokinase dose of 3 mg.Rabbits' blood vessels were observed to be not completely recanalized in other groups,accompanied with different degree of recurrent embolism in the microcirculation.6-keto-PGF 1 a content of the rabbits in the group without recurrent embolism obviously increased after thrombolysis,and the difference was statistically significant (P＜0.05).While,there was no statistical difference in other indicators (P＞0.05).Conclsion The thrombolysis with ultrasound frequency of 2.2 MHz,irradiation time of 30 min,and urokinase dose of 3 mg could achieve complete recanalization of blood vessels.Under the certain conditions of ultrasonic frequency,irradiation time and urokinase dosage,thrombus can be effectively dissolved.However,there may be risk of recurrent embolism in the microcirculation during thrombolysis process.The increase of 6-keto-PGF 1a content has a certain effect on reducing the formation of recurrent embolism in the microcirculation.