[Basic investigation on hyperthermia by low-frequency ultrasonic]

Ultrasonic heating methods for hyperthermia have been developed because of several advantage i.e., its easiness in steering the applicator for the target, and its non-interference with other electronic equipment. Most of them make hot spots by focusing the ultrasonic beam, using acoustic lens or many transmitters. Therefore, these methods are based on analogous idea to the ray focusing, and higher frequency, i.e., from 500 kHz to 5 MHz, is used. However, there are some problems; hot spots are generated before the focus, and it is difficult to heat the region beyond gas and bones owing to the attenuation and scattering of the ultrasonic beam. Then, we propose a new method for heating the depths and local regions of the body. In this method, to heat the depths of the body low-frequency ultrasound is used since it has larger penetration depth and it is less scattered than higher-frequency ultrasound. To heat only tumours, hot spots are generated by synthesizing acoustic fields resulted from several incident waves. The heat generation and temperature distribution was analyzed using the models with properties similar to tissues for different values of parameters such as frequency, position and the number of sources. The results show that hot spots can be generated at the depths of the body. and that desirable temperature distribution can be obtained by selecting optimal parameters and cooling condition. To determine the temperature distribution more accurately it would be required to introduce the more complicated model of heat removal effect by blood flow and the experimental results. These problems are left for the future investigation.