Thermal effect of microbubbles in the focused ultrasound field

High-intensity focused ultrasound (HIFU) has been developed for the treatment of tumor, and additionally the medical applications with microbubbles such as ultrasound imaging have attracted much attention. In the field of the bubble dynamics, it is known that the microbubble is an energy converter of ultrasound mechanical energy to heat when subjected to an acoustic field. The goal is the tissue heating enhanced by microbubbles. In this study, the relationship between the heating effect and the behavior of microbubbles is analyzed. The temperature rise was measured by a thermocouple and a thermal liquid crystal sheet, and the bubble behavior was simultaneously observed with a camera. The ultrasound frequency is 2.2 MHz and the intensity is 100–2000 W/cm2. The temperature rise became larger as the number density of microbubbles (Levovist) around the focal region increased. As the number density became too large, the conversion efficiency from ultrasound energy to heat became smaller because the shielding effect of ultrasound by the existence of too many bubbles became larger. From these results, it is indicated that it is important to choose the optimal conditions of microbubbles and ultrasound in order to control the heat deposition from microbubbles.