# 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 variance was calculated from the absolute value of complex signals (IQ data) corresponding to every five consecutive B-mode images and the variance images were then averaged. Since the variance of the background was approximately 30 dB lower than the maximum variance, an area with a variance higher than the maximum variance of \(-\)15 dB was defined as the area of cavitation bubbles. The gross pathology in the longitudinal and lateral cross sections is shown in Figure 13b,c,f,g, respectively. The gross pathology in the longitudinal cross section without trigger pulses are shown in Figure 13d,h. Figure 14 shows the ultrasonically detected cloud size in the axial direction and the coagulation area size with spot scanning and direct synthesis [35]. The cavitation area observed in the variance image matches the coagulation area in the corresponding gross pathology significantly better with direct synthesis than with spot scanning. With spot scanning, the coagulation area extended significantly backward from the focal plane, resulting in the longitudinal size approximately twice as large as the cavitation area. Spot scanning induced tissue coagulation even by heating bursts alone, while direct synthesis barely induced it by heating bursts alone. The results indicate that the ultrasonic absorption of nonlinear higher harmonic components had a significant influence on the coagulation region with spot scanning. In spot scanning, cavitation images could only show a part of coagulation area. On the other hand, real-time ultrasound images of cavitation bubbles could predict the coagulation region with direct synthesis much better than that with spot scanning, although the overall heat generation may be smaller. The energy efficiencies calculated from the ratio of coagulation volume to total acoustic energy were 1.7 and 1.0 mm\({}^{3}\)/J with spot scanning and direct synthesis, respectively. Even with direct synthesis, the efficiency was 1.0 mm\({}^{3}\)/J, which was comparable to those in volumetric HIFU ablation studies [68; 69; 70], for example, the ratios of 240 equivalent minutes (EM) uterine fibroids volume to applied energy of 0.29 and 0.91 mm\({}^{3}\)/J with treatment cell sizes of 8 and 16 mm, respectively [70]. As a result, the spot scanning trigger pulses followed by the spot scanning heating bursts achieved an efficient heating and those followed by the heating bursts with a directly synthesized focus generated a coagulation region with high efficiency and predictability. Figure 14: Ultrasonically detected cloud size and the coagulation area size [35]. **(a)** spot scanning; **(b)** direct synthesis. Figure 13: Typical ultrasound images of cavitation bubbles during HIFU exposure and slices of chicken tissue samples after HIFU exposure [35]. Heating bursts with spot scanning and direct synthesis were exposed for **(a–h)**, respectively. Trigger HIFU sequences shown in Figure 1b,c were used for **(a–c,e–g)**, respectively; **(a,d)** variance images of high-speed pulse inversion data by transmitting diverging waves; **(b,f)** the gross pathology in the longitudinal cross section; **(c,g)** the gross pathology in the lateral cross section; and **(d,h)** the gross pathology in the longitudinal cross section without trigger pulses.