# 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
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 time of 50 μs and acoustic intensity of 10 kW/cm\({}^{2}\) was used. The focal points of trigger pulses were not shifted 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.  
Figure 7 except for a focal plane shift by 10 mm. The focal plane of the heating bursts was the same as the geometric focal plane corresponding to a depth of 120 mm. The total HIFU exposure time was 16.5 s. After the HIFU exposure, cross-sectional pictures of the coagulated chicken breast tissues were taken to investigate the position of the coagulation region and the sharpness of the coagulation region boundary from their brightness distribution. Figure 12 shows typical pictures of the coagulated tissues. Table 2 shows thicknesses of intermediate brightness regions at the ends of a coagulation region in a chicken breast tissue without and with a focal plane shift of trigger pulses [34]. The thickness was calculated from the region where the brightness changed from 20% to 50% of the maximum. In ultrasound images, a significant difference in the position of the bubble clouds is seen between the focal plane shifts of 0 and 10 mm shown in Figure 11a,b, respectively. The position of the clouds shifted by 5-10 mm backward from the geometric focal plane without a focal plane shift. In contrast, it was approximately on the geometric focal plane with a focal point shift of 10 mm. Although the appropriate distance of the focal plane shift may depend on each situation, the real-time ultrasonic imaging of cavitation bubbles seemed to be useful for adapting the amount of the shift. The brightness distribution of coagulated chicken breast tissues showed that the coagulation region significantly shifted backward from the geometric focal position without a focal plane shift. Furthermore, the coagulation region with the focal shift had more distinctive boundaries along the HIFU propagation direction, especially at the forward end of coagulation regions as shown in Table 2. The results show that an intentional focal plane shift of trigger pulses to match the cavitation clouds position to the focal region of heating bursts will make the trigger HIFU treatment more accurate and predictable. However, the lengths of coagulation regions with and without a focal plane shift were longer than those of cavitation regions in ultrasound images, especially in the backward direction. One of the possibilities is the effect of nonlinearly distorted waveform owing to the high acoustic intensity and focusing effect. The higher harmonic components of the distorted ultrasound increase ultrasonic absorption especially in the region shallower than the focal depth. To investigate the effect of the nonlinear ultrasonic waveform, the results using the sequences shown in Figure 1b,c were compared. As the directly synthesized focus using a sector vortex method has a large focal region, a spatial peak acoustic intensity is much less than that by the spot scanning. It reduces the ultrasonic absorption caused by the nonlinear propagation and focusing. Figure 13 shows typical results of ultrasound images of cavitation bubbles during the HIFU exposure and slices of chicken tissue samples after the HIFU exposure [35]. Heating bursts of spot scanning and direct synthesis were exposed for Figure 13a-h, respectively. Trigger HIFU sequences shown in Figure 1b,c were applied for Figure 13a-c,e-g, respectively. The total HIFU exposure time was 11.8 s. Time-averaged variance images of high-speed pulse inversion data by transmitting diverging waves were used for the ultrasound cavitation images because the acoustic signals from cavitation bubbles change much more in time and space than those from tissue itself.