Objective:This study focuses on the optimization on stability and size of MBs with poly-L-lactic acid(poly L-lactic acid,PLLA)and polyvinyl alcohol (polyvinyl alcohol,PVA)as shell materials,which made MBs more suitable to work in vivo at the same time keeping the ability of enhancing the ultrasound imaging. Methods:First,PVA was modified to obtain telechelic PVA with carboxylic groups at the chain ends. The modified PVA was used to fabricate MBs present carboxyl on their surface,whose Zeta potential was -38.87 ± 2.19 mV,thus improving the stability of MBs. Next,The particle size and stability of MBs were regulated through adjustments the three factors,i.e. the concentration of PVA aqueous solution,the water-oil ratio of the first emulsification and the rate of second emulsification during the preparation process. Results:In vitro/vivo US imaging experiment was performed on the optimized MBs,which had a average particle size of 1.38 -滋m,uniform particle size distribution and good stability. In vitro experiments showed that the MBs exhibited significantly ultrasound(US)contrast enhancement. In vivo US imaging of tumors in nude mice showed that,by intratumoral injection,MBs can rapidly improve the ultrasound imaging of tumor;by intravenous injection,MBs can reach the tumor site through the blood circulation,accumulate in the tumor site after a period of time,and enhance ultrasound imaging. Conclusion:This study lays a good foundation for the application of MBs in vivo.