Heating Location Control of HIFU Treatment Enhanced with Microbubbles

High‐intensity focused ultrasound (HIFU) treatment using ultrasound contrast microbubbles for enhancing the heating effect has been developed with the aim of realising a less invasive tumor therapy. The focused sound waves result in an increase in temperature and increased thermal absorption, which necroses tumor cells. In addition, microbubbles are used as contrast agents for ultrasound imaging, and, in a previous study, we used microbubbles to enhance the heating effect. However, when microbubbles exist in the ultrasound pathway, they disturb ultrasound propagation and distort the acoustic field. Distortion of the acoustic field leads to defocusing and causes unexpected damage to tissue in the body. The objective of the present study is to propose a method by which to destroy microbubbles in the ultrasound pathway and to focus the thermal energy only at the focal point. The proposed method consists of two steps. The first step is to use repetitive high‐intensity, short‐burst waves (20 waves) to destroy the microbubbles in the pathway. In the second step, low amplitude continuous waves are sent in order to heat the focal point. This method was successful for a gel containing microbubbles with a void fraction on the order of 10−4. The results of the present study indicate that more microbubbles were destroyed as the non‐exposure time and the pulse number were increased during the first step.