Development of HIFU treatment in which the heating location is controlled using microbubbles

High-intensity focused ultrasound (HIFU) treatment that employs microbubbles to provide enhanced heating has been investigated in order to develop a less invasive and more rapid tumor therapy. Previous studies by us have demonstrated that ultrasound propagation is disturbed when there are microbubbles in front of the focus. In this study, we develop a method for obtaining enhanced heating by using microbubbles just at the focus, thus avoiding heating on the transducer side. In this method, microbubbles are destroyed in front of the HIFU focus (on the transducer side) by irradiating a very short burst wave of microsecond order, before irradiating the ultrasound waves for heating the focus. The experiment is conducted in a medium of a gel containing microbubbles, and a temperature-sensing liquid crystal sheet is set in the focus to observe the temperature distribution. The ultrasound frequency was 2.2 MHz and the intensity was 5000 W/cm ², and 20 burst wave waves were irradiated at pulse repetition frequency of 1 kHz. The number of wave pulses was varied. The continuous-wave frequency, intensity and irradiation time are 2.2 MHz, 1000 W/cm ² and 60 sec, espectively. As the number of pulses increased, the heating region moves from the transducer side to the focus. This is because microbubbles in front of the focus are destroyed and the ultrasound propagates around the target position effectively. These results suggest that the microbubble distribution and the heating position in the developed HIFU system can be controlled.