## 3 Results and Discussion  
The initial temperature of the experiment and simulation was 30 \({}^{\circ}\)C. Figure 4 shows the measured temperature rise of the bovine liver exposed for 5 s at 10 W, 40 W, and 125 W acoustic power. The maximum acoustic intensity at the focus was 500, 3000, and 10,000 W/cm\({}^{2}\), respectively. Under three kinds of power, the temperature of the bovine liver increased rapidly at the initial stage, which was caused by the cavitation effect and viscous heating of the tissue and glass at 125 W and 40 W. When the power was 10 W (lower than the cavitation threshold), this was due to viscous heating. When the power was 125 W, the temperature elevation of the tissue was rapidly above 60 \({}^{\circ}\)C, which indicated that cavitation occurred at the beginning of sonication. Due to thermal equilibrium, the temperature of the tissue reached its maximum and then decreased slightly until the end of the sonication. This trend was similar to that reported by [PERSON] et al. [25]. The temperature increase rate of the bovine liver at 40 W was slower than that at 125 W and reached the maximum temperature of 77 \({}^{\circ}\)C within 1 s; then, the temperature remained almost unchanged. This trend was similar to that reported by [PERSON] et al. [26]. When the power was 10 W (below the threshold of  
Figure 3: Schematic diagram of the experimental system. acoustic cavitation), the temperature of the tissue increased rapidly at first and then slowly under the effect of viscous heating, and the temperature increased steadily to 43 \({}^{\circ}\)C within 5 s. It can be seen that when the intensity of focused ultrasound exceeded the cavitation threshold, the time of heating to the maximum temperature decreased with the increased power. It was difficult to predict the temperature of the tissue during sonication, which was closely related to some complex factors such as cavitation bubbles, thermal necrosis, etc. These processes such as cavitation, boiling, and their effects on temperature have not been accurately evaluated, and are not included in the classical numerical simulation model of the HIFU. Therefore, if the intensity above the cavitation threshold was applied to the tissue, for example, 125 W, its temperature prediction after several milliseconds might not be reliable. The mean temperature increase rate of the tissue was more than 30 \({}^{\circ}\)C/s in the first 1 s of the sonication at 40 W, and the temperature was difficult to control accurately. When the intensity of focused ultrasound was lower than the cavitation threshold (the power was 10 W and the intensity was within the MIFU range), the temperature increased steadily. At this power, the main factors that make the temperature increase were the thermal effect and viscous heating, which facilitated the prediction and control of the temperature. The relationship between temperature and exposure time of the bovine liver at different power (within the MIFU range) was studied as follows. Figure 5 shows the measured, simulated, and modified predicted temperature elevation curves of the bovine liver with time during 40 s of sonication at 4 W, 6 W, and 8 W (200-500 W/cm\({}^{2}\), MIFU). First, the temperature measured was compared with the temperature calculated by numerical simulation considering VH to verify the correctness of the secondary heat source \(Q_{vis}\). The maximum relative error was 4.1%. The measured temperature elevation after sonication was 12.9 \({}^{\circ}\)C (power = 4 W), 18.4 \({}^{\circ}\)C (power = 6 W), and 30.8 \({}^{\circ}\)C (power = 8 W). The simulated temperature elevation after sonication was 13.6 \({}^{\circ}\)C (power = 4 W), 19.4 \({}^{\circ}\)C (power = 6 W), and 28.3 \({}^{\circ}\)C (power = 8 W). With the increase in power, viscous heating would make the tissue rise to a higher temperature within 5 s. After a certain period of time, the contribution of viscous heating to temperature reached the plateau, and further temperature rise was mainly caused by absorbing acoustic energy.