Highly efficient cavitation-enhanced heating with dual-frequency ultrasound exposure in high-intensity focused ultrasound treatment

Abstract

High-intensity focused ultrasound (HIFU) treatment is a noninvasive therapy for malignant as well as benign tumors. In this method, the ultrasound is generated outside the body and focused to the target tissue. Therefore, physical and mental stresses on the patient are minimal. A drawback of HIFU therapy is the long treatment time for a large tumor owing to a small therapeutic volume by a single exposure. Enhancing the heating effect of ultrasound by cavitation bubbles may solve this problem. They are generated by high-intensity ultrasound exposure and can convert the acoustic energy into heat at a high efficiency. In this study, we combined "dual-frequency ultrasound exposure" for the efficient generation and expansion of a cavitation bubble cloud, and "multiple-triggered HIFU" for the efficient enhancement of ultrasonic heating. The result suggests that the newly proposed combined method is more efficient than the conventional triggered HIFU method.

Enhancement of High-Intensity Focused Ultrasound Heating by Short-Pulse Generated Cavitation

“…The larger coagulation region contributed to the efficient use of the ultrasonically generated heat through minimizing the surface-area-to-volume ratio. Similarly, as mentioned in the previous section, multi-trigger HIFU sequences for the lateral enlargement of the treated region have been investigated to effectively coagulate a target tissue [29,[32][33][34][35]. Figure 7 shows the high-speed images of bubble clouds generated by trigger pulses in six focal positions [34].…”

“…Figure 10 shows coagulation volumes and their standard deviations of chicken breast tissues by various HIFU sequences [33]. Each coagulation In the results without the trigger pulses, the maximum temperature increases with spot scanning heating bursts in Figure 9a,c are greater than those with a directly synthesized focus in Figure 9b,d, respectively.…”

“…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][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].…”

“…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].…”

“…Although increased heat depositions were obtained using microbubbles in the previous research, distorted coagulation regions or temperature distribution were observed, such as tad-pole shaped [12,22,24] or pre-focal [13,19,[24][25][26] heating regions. A "trigger 2 of 20 HIFU" sequence that consists of high-intensity short pulses and following moderate-intensity long bursts has been investigated to avoid such distorted coagulation regions while enhancing the ultrasonic heating [27][28][29][30][31][32][33][34][35][36].…”