Effective Heat therapy Controlling Heat Deposition of Microbubbles in the Ultrasound Field

Abstract

It is required that the tumor site is coagulated less invasively and more efficiently, and HIFU treatment combined with microbubbles is investigated here. Microbubbles have a potential to enhance the heating effect in the ultrasound field. In this study, the relationship between the thermal effect and the behavior of microbubbles is analyzed. The temperature rise at the focal region was measured by a thermocouple, and the bubble behavior was simultaneously observed with a high‐speed camera. The ultrasound frequency was 2.2 MHz and the intensity was 1000 – 5000 W/cm2. This time Levovist®, one of the contrast agents, was used as microbubbles, and the initial void fraction (the ratio of total gas volume to liquid) around the focal region was 0, 10−5, 10−4, 10−3 and 10−2%. The temperature rise became larger as the intensity of ultrasound and the void fraction increased. Comparing the data of the thermocouple with the photographs taken by the camera, the temperature rise became large as the bubbles were violently generated. When the ratio of heat energy to input acoustic energy for first 1 second irradiation is considered, the ratio mainly depends on the initial void fraction, and it becomes larger as the void fraction increases. On the other hand, when the ratio for 30 seconds is considered, it becomes smaller as the intensity of ultrasound increases. It is indicated that this is because resonance size of bubbles decrease during HIFU irradiation, for example, the microbubbles may break up or dissolve in water, or the bubbles grow too larger by the effect of rectified diffusion or coalescence. It is very important to consider the relationship between the thermal effect and the behavior of bubbles in the ultrasound field in order to control the heating effect of microbubbles and realize microbubble‐enhanced HIFU treatment.