Magnetic Resonance Imaging for the Exploitation of Bubble-Enhanced Heating by High-Intensity Focused Ultrasound: A Feasibility Study in ex Vivo Liver

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

“…Cavitation-enhanced heating by a trigger HIFU sequence can generate a coagulation region with high efficiency and reproducibility without distortion of coagulation region such as to a tadpole shape [12,22,24]. Good agreement between the ultrasonic cavitation image and coagulation area Coagulation (lateral) Coagulation (axial) Cavitation cloud (axial) Figure 13.…”

“…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.…”

“…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].…”

“…The volumetric oscillation of bubbles increases ultrasonic energy dissipation in the vicinity of the bubbles through three mechanisms such as acoustic radiation, thermal, and viscous damping, which resulted in the enhanced heating effect. The effect of bubble-enhanced ultrasonic heating has been investigated experimentally [11][12][13][14][15][16][17][18][19][20][21][22][23][24] and numerically [25,26]. 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.…”

“…The effect of bubble-enhanced ultrasonic heating has been investigated experimentally [11][12][13][14][15][16][17][18][19][20][21][22][23][24] and numerically [25,26]. 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].…”