## 3 Results
### BFRs Measurement in the Tissue  
In the phantom experiment, the acoustic power was 25 W, 50 W, and 75 W, respectively, and we repeated three times for each power. As a result of the irreversible thermal damage to the phantom induced by ultrasound, each sample could only be used once; thus, nine samples were required. The BFRs of the phantom under different power \(\times\) time are shown in Figure 8. The exposure time, energy, widths, and axis length ratios of the BFRs when the lengths of the BFRs were approximately equal were counted. These parameters of the BFRs of three samples in each group were statistically processed as shown in Figure 9.  
In the bovine liver experiment, the acoustic power was 50 W, 100 W, and 125 W, respectively, and we repeated three times for each power. It was better to use the same power level for the phantom and bovine liver experiments. However, different power levels of ultrasound were required to induce BFR in the two tissues. BFR could be induced in phantoms with an acoustic power of 25 W, but it was difficult to induce BFR in bovine liver. In addition, the cavitation thresholds of the two tissues were not equal. When the power was 100 W, large bubbles could be seen in the phantom, then, the shapes of BFRs were irregular, and low-power cumulative HIFU was designed to avoid the cavitation effect, so the phantom experiment could only use a low power level compared with bovine liver. Figure 8: BFRs of phantom after ultrasonication with different combinations of acoustic power and time: (**a**) 25 W \(\times\) 449 s, (**b**) 50 W \(\times\) 58 s, (**c**) 75 W \(\times\) 19 s. It could be seen from Figure 9 that in the phantom experiment, HIFU needed 448 s to induce a BFR with a length of 31.58 mm with an acoustic power of 25 W, and this BFR’s width was 9.17 mm, and the axis length ratio was 3.45, which was far from the axis length ratio of the AFR. The axis length ratio was 5.97 with an acoustic power of 75 W, and the shape of the BFR was closer to that of the AFR, and the exposure energy required was reduced by 87.38%. Figure 9: \(Cont.\)The BFRs of bovine liver under different power \(\times\) time are shown in Figure 10. We counted the exposure time, energy, widths, and axis length ratios of the BFRs when the lengths of the BFRs were approximately equal. These values of the BFRs of three samples in each group were statistically processed as shown in Figure 11.  
In the bovine liver experiment, the shape of the BFR induced by 125 W was closer to the shape of the AFR than that induced by 50 W, and the exposure time and energy required for 125 W were also significantly less than those required for 50 W. The rules reflected by the results of the bovine liver experiment were consistent with the laws of the phantom experiment, when the lengths of BFRs and the length of the AFR were approximately equal, the shape of the BFR induced by 'high power \(\times\) short time' exposure was closer to that of the AFR than the shape of the BFR induced by 'low power \(\times\) long time' exposure, and the exposure energy required was significantly reduced. In this study, the shapes of the BFRs of the phantom and bovine liver during the 1-10 s before the ultrasonication was statistically analyzed are shown in Figures 12 and 13. In the simulation of the phantom (Figure 12), the lengths of BFRs induced by 50 W \(\times\) 10 s and 100 W \(\times\) 4 s were similar, their widths were 2.86 and 2.14 mm, respectively, and the axial length ratios were 7.63 and 10.07. The shape of the BFR induced by 100 w \(\times\) 4 s was closer to that of the AFR. The widths of the BFRs induced by 50 W \(\times\) 7 s, 75 W \(\times\) 5 s, and 100 W \(\times\) 4 s were similar, they were 2.14 mm, their lengths of BFRs were 17.62, 20.05, and 21.58 mm, and the axial length ratios were 8.23, 9.37, and 10.08, respectively.