Relationship between Thermal Effect and Bubble Behavior in HIFU

Abstract

It is known that ultrasound contrast agents enhance the thermal effect in HIFU. However, it has not been clarified how the contrast agent bubbles relate to the enhancement of the thermal effect in HIFU. In this study of contrast agent solutions, we take photographs with a high‐speed camera and measure the temperature change with a thermocouple. The HIFU frequency is 2.15 MHz and the pressure amplitudes are 2.5, 5.0 and 12.4 MPap‐p. The frame rate of the high‐speed camera is either 2 kfps or 125 kfps. By the high‐speed photography, cavitation bubbles (several μm to a few tens of μm) and large stable bubbles (around several hundreds of μm) grown from cavitation bubbles are observed in the contrast agent solution. The temperature rise of the contrast agent solution is higher than that of both saturated water and degassed water. It is thought that the small cavitation bubbles convert mechanical energy into thermal energy during their oscillation. Otherwise, the large stable bubbles, whose radius was much larger than the resonant radius, scatter the ultrasound. It is believed that the scattered ultrasound indirectly contributes to the thermal effect by oscillating small bubbles or by being absorbed in the medium.