Hyperthermia effects （39-44 ℃） induced by pulsed high-intensity focused ultrasound （HIFU） have been regarded as a promising therapeutic tool for boosting immune responses or enhancing drug delivery into a solid tumor. However, previous studies also reported that the cell death occurs when cells are maintained at 43 ℃ for more than 20 minutes. The aim of this study is to investigate thermal responses inside in vivo rabbit auricular veins exposed to pulsed HIFU （1.17 MHz, 5300 W/cm2, with relatively low-duty ratios （0.2%-4.3%）. The results show that： （1） with constant pulse repetition frequency （PRF） （e.g., 1 Hz）, the thermal responses inside the vessel will increase with the increasing duty ratio; （2） a temperature elevation to 43 ℃ can be identified at the duty ratio of 4.3%; （3） with constant duty ratios, the change of PRF will not significantly affect the temperature measurement in the vessel; （4） as the duty ratios lower than 4.3%, the presence of microbubbles will not significantly enhance the thermal responses in the vessel, but will facilitate HIFU-induced inertial cavitation events.
