# Enhancement of High-Intensity Focused Ultrasound Heating by Short-Pulse Generated Cavitation
## 4 Temperature Increase and Coagulation Region by Trigger HIFU Exposure  
### Enhancement of Temperature Increase by Trigger Pulses  
The effect of bubble-enhanced ultrasonic heating has been quantitatively assessed by measuring temperature increase with a thermocouple or magnetic resonance (MR) thermometry in many studies [11; 13; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 35]. Figure 8 shows the temperature increases with and without the exposure of trigger pulses in a PAA gel including 15% BSA placed in a water tank [31]. The temperature was measured at a lateral distance of 2.7-mm above the HIFU focus with a sheath thermocouple of 0.15 mm in diameter (K-type, Chino, Tokyo, Japan). The certain distance away from the HIFU focus was to reduce the effect of viscous heating. The trigger pulse sequence was similar to that shown in Figure 1a. The driving frequency was 1.2 MHz. The instantaneous total acoustic powers of the trigger pulses and heating bursts were 900 and 13 W, respectively. The duration times of trigger pulses and heating bursts were 0.1 and 499.9 ms, respectively. There were no interval time for ultrasound imaging and the total  
Figure 7: High-speed images of generated and sustained cavitation bubbles in a PAA gel including 15% BSA by a sequence of spot scanning trigger pulses followed by spot scanning heating bursts which is similar to that shown in Figure 1b [34]. The instantaneous total acoustic powers of the trigger pulses and heating bursts were 1900 and 65 W, respectively. The intentional focal plane shift of the trigger pulses was not applied in this case. The same camera and light source were used as in Figure 5, except for the optical magnification. duration time of HIFU exposure was 5 s. The result without trigger pulses was averaged over four measurements to provide the baseline for comparison. The maximum temperature increase and the temperature increase 35 s after the stop of HIFU exposure (\(t\) = 40 s) in the trigger pulse sequence were approximately three times as large as those without trigger pulses. Comparing the heat enhancement by cavitation with the results in other studies, the temperature increase in an agar phantom with a 10.2 s exposure at 1.42 MPa and 1 MHz at 0.5 mm away from the focus was approximately three times as large as that at 1.39 MPa [11], and the maximum temperature increase in an ex vivo porcine liver sample with a combination of 0.5 s burst at 500 W and 30 s exposure at 20 W and 1.2 MHz was 155% of that without a 0.5 s high-intensity burst [22]. The comparable ratio of the temperature increase with to without the trigger pulses for a long time range in this study indicates that the ultrasonic heat generation in the gel should have also increased approximately threefold by the effect of cavitation bubbles. However, as the estimated ultrasonic absorption coefficient of the gel by fitting between the measurement and thermal simulation result was 1.28 Np/m, which is much less than the typical value of soft tissues, the temperature increase ratio in a soft tissue would be less. This ratio may be improved by increasing the intensity or repetition frequency of trigger pulses. Figure 9 shows temperature increases in a PAA gel including 15% BSA placed in a water tank by spot scanning sequences as shown in Figure 1b,c [35]. The temperature increases measured with a sheath thermocouple of 0.3 mm in diameter (K-type, Chino, Tokyo, Japan) were averaged over two measurements at the geometric focus, and 10 mm backward and 7 mm forward from it. The results without and with the trigger pulse exposure are shown in Figure 9a-d, respectively. Spot scanning heating bursts shown in Figure 1b and heating bursts with a directly synthesized focus in Figure 1c were exposed in the case of Figure 9a-d, respectively. The triggered HIFU sequences shown in Figure 1b,c were used for the results shown in Figure 9c,d, respectively.