Heating Location Control of HIFU Treatment Enhanced with Microbubbles Contrast Agents

High intensity focused ultrasound (HIFU) treatment employing ultrasound contrast microbubbles for enhancing the heating effect has been developed in order to achieve a less invasive tumor therapy. The focused sound waves result in an increase of temperature, and its thermal energy increases to necrotize tumor cells. Furthermore, microbubbles are used as contrast agents of ultrasound imaging, and our previous work utilized microbubbles for enhancing the heating effect. However, when microbubbles exist on the pathway of ultrasound, the bubbles disturb ultrasound propagation and distort the acoustic field. Distortion of acoustic field leads to defocus and damage unexpected tissue in the body. The objective of present work is to propose a method to destroy microbubbles on 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 irradiating repetitive high intensity short burst waves (20 waves) for destroying the microbubbles on the pathway. In the second step, weak continuous waves are sent for heating the focal point. This method was successful for a gel that contains microbubbles with the void fraction of the order of 10

− 4

. The results indicate that the more microbubbles were destroyed with the increased in the non-exposure time and pulse number at the first step. For applying the method to a living tissue, the optimization of pulse number is difficult because the void fraction distribution within the tissue is unclear. To overcome this problem, we suggested an advanced method, which is irradiating high intensity burst waves and weak waves in turn. This method contributes to reduce the required surgical procedure, because the destruction of microbubbles and heating the focal point is possible without optimization of burst wave number.