# Enhancement of High-Intensity Focused Ultrasound Heating by Short-Pulse Generated Cavitation
## 4 Temperature Increase and Coagulation Region by Trigger HIFU Exposure
### Coagulation Region Size and Predictability  
Multi-trigger HIFU sequences using spot scanning trigger pulses can generate a large coagulation volume of a nearly cylindrical shape [32; 33; 34; 35]. Figure 10 shows coagulation volumes and their standard deviations of chicken breast tissues by various HIFU sequences [33]. Each coagulation volume was calculated as a cylindrical column, whose diameter and height were measured in its frontage and  
Figure 9: Temperature increases averaged over two measurements at the geometric focus, and 10 mm backward and 7 mm forward from it [35]. **(a,c)** heating bursts alone without trigger pulses and trigger HIFU sequence with spot scanning focus shown in Figure 1b, respectively; **(b,d)** those with a directly synthesized focus shown in Figure 1c. The total duration time of HIFU exposure was approximately 17.7 s.  
depth sections, respectively. The acoustic source was a 128-element array transducer (Imasonic, Voray sur l'Ognon, France) with both diameter and focal length of 120 mm. The HIFU driving system was the same used for the experiments shown in Figures 5-7, and Figure 9. The applied trigger HIFU sequences were similar to that shown in Figure 1b. The total HIFU exposure was 11 s. For the trigger pulses, a single-frequency ultrasound at 0.8 MHz or dual-frequency ultrasound at 0.8 and 1.6 MHz as shown in Figure 3 with a duration of 50 us and acoustic intensity of 10 kW/cm\({}^{2}\) was used. The interval time of the trigger pulses was 3 us and the resulting subtotal durations was 1.272 ms. The intentional focal plane shift of the trigger pulses was not applied in this case. The same heating bursts were used in all sequences. The driving frequency was 1 MHz and the acoustic intensity was 1.7 kW/cm\({}^{2}\), corresponding to an instantaneous total acoustic power of 51 W. The duration and interval time of the heating bursts were 25 and 2 us, respectively, resulting in the subtotal durations of 135.1 ms. The temperature and dissolved oxygen in water were kept at 36 \({}^{\circ}\)C and 20%-30%, respectively. As shown in Figure 10, the average coagulation volumes by the NP and single-frequency sequences were significantly larger than those by the other sequences. Especially, the volume by the NP sequence was almost two times larger than that by the sequence without trigger pulses. The results show that the trigger pulses in the NP and single-frequency sequences were effective for employing cavitation bubbles in the chicken breast for the ultrasonic heating while the trigger pulses in other sequences were not. This indicates that the trigger pulses in the NP and single-frequency sequences could generate sufficient amount of cavitation bubbles through the formation of cavitation clouds, but the others could not, as shown in Figure 4. A small difference of \(p=0.054\) is seen between the results by the NP and single-frequency sequences with the same acoustic intensity of the trigger pulses estimated by assuming a linear relation between the input electric power and acoustic intensity, although the total acoustic power of the trigger pulses in the NP sequence was less than that in the single-frequency sequence because of a smaller focal region by the second-harmonic superimposition. Cavitation clouds are formed backward from a focal point, in other words, toward a HIFU transducer, through a shock scattering phenomena [34; 46; 53; 54]. In the trigger HIFU heating, the mismatch between the center position of bubble clouds and focal plane of heating bursts may have caused a less predictable coagulation pattern. Figure 11 shows high-speed pulse inversion images of cavitation clouds by transmitting diverging waves in chicken breast tissues during HIFU exposure without and with the focal plane shift of the trigger pulses to adjust the cloud position to the heating burst focal plane [34]. The applied HIFU sequence was the same used for the experiments shown in  
Figure 10: Average coagulation volumes of chicken breast tissues by various HIFU sequences similar to that shown in Figure 1b [33].