Measurements of the thermal focus of an experimental focused ultrasound thermotherapy system

The thermal focus of an experimental thermotherapy equipment based on high energy focused ultrasound has been investigated. The in-house built equipment has a treatment head of seven separate focused transducers arranged in a semispherical fixture with a common focus at 100 mm from the transducer surfaces. Each transducer has a diameter of 50 mm and operates at 0.5 MHz. The ultrasound field of this seven-transducer arrangement, as well as of a single transducer of the same type as the ones in the arrangement, has been investigated in terms of temperature distribution evolved by absorption in castor oil. The results of the single transducer measurements show an ellipsoid-shaped focus displaced from the geometrical focus towards the transducer, whereas the measurements of the treatment head show a thermal focus which practically coincides with the geometrical one. Differences in location and shape of the thermal focuses depend on differences in the focusing action and the absorption in the media. Pilot investigations on tissue lesioning in vitro on pig muscle samples and in vivo on pig thigh were also carried out. Well discriminated local lesions with ellipsoid shape were obtained in the experiments in vitro and in vivo. Temperatures of up to 80 degrees C were measured in tissue at the focus in vitro. The results show that the equipment is well suited for thermotherapy applications.