Abstract: Objective To investigate the change of Echinococcus granulosus hydatid cysts in vitro treated with single-frequency and multi-frequency focused ultrasound. Methods Mice were inoculated with echinococcus protoscolices from infected sheep, and sacrificed after one year. Forty cysts （about 2 cm in diameter）were taken from infected mice, and randomly divided into four groups. Cysts in the control group were immediately opened, fixed with 3% glutaraldehyde solution, and frozen. Cysts in groups A, B and C were treated with transducer No. 2（4 W）, transducer No.2（4 W）+No. 3（5 W）, and transducer No.1（4 W）+No. 2（4 W）+No. 3（5 W）, respectively. After irradiated for 1 minute, the cysts were all fixed and refrigerated, then observed by naked eyes, transmission electron microscopy and scanning electron microscopy. Results The wall of hydatid cyst in the experimental groups became thicker and less transparent. Images from the transmission electron microscopy showed more serious damage of cyst wall with the increase in ultrasonic power and irradiation time. The fiber of the laminated layer became thicker and disordered, the nucleus in germinal layer became swollen and broken, the mitochondria were destroyed remarkably and the microvillus got shorter, fractured or even disappeared. In some occasions, only cell debris and broken nucleolus left. Scanning electron microscopy also demonstrated that with the increase in ultrasound power and irradiation time, the damage of the cyst wall became more serious even the normal structure was completely destroyed. Conclusion Multi-frequency focused ultrasound causes significant damage of the Echinococcus granulosus hydatid in vitro, and the damage level is related to the ultrasound power and irradiation time.

Key words: Multi-frequency focused ultrasound, Echinococcus granulosus, Hydatid cyst, Scanning electron microscopy, Transmission electron microscopy