Objective:To study the effects of different duty cycles [modulated by pulse repetition frequency(PRF)] of diagnostic ultrasound on enhancing blood perfusion in hypovascular tumors.Methods:Walker-256 tumor cells were injected subcutaneously into the inner thigh of 15 SD rats, and the rats were randomly divided into 3 groups according to PRF: group A(PRF=50 Hz), group B(PRF=1.0 kHz) and group C(PRF=2.0 kHz), with 5 rats in each group. The tumor-bearing rats in each group were treated with ultrasound combined with microbubbles for 10 minutes. Contrast-enhanced ultrasound was performed before treatment, immediately after treatment, and 4 hours after treatment, respectively, to compare the peak intensity(PI), area under curve(AUC), and tumor perfusion area in each group.Results:Immediately after treatment, PI and AUC values of group A and B increased compared with those before treatment (all P<0.05); 4 hours after treatment, PI and AUC values of group B continued to increase, superior to that immediately after treatment (all P<0.05), while PI and AUC values of group A and C were decreased compared with those immediately after treatment (all P>0.05). Immediately after treatment, the tumor blood perfusion area in group A and B increased significantly compared with that before treatment (all P<0.05); 4 hours after treatment, the tumor blood flow perfusion area of group B continued to increase, which was better than that immediately after treatment ( P<0.05), and the tumor blood perfusion areas of group A and C decreased compared with that immediately after treatment (all P>0.05). Conclusions:Low-intensity diagnostic ultrasound stimulated microbubble cavitation with PRF=1.0 kHz can enhance tumor blood flow significantly which can last for more than 4 hours.