APERTURE SIZE TO THERAPEUTIC VOLUME RELATION FOR A MULTIELEMENT ULTRASOUND SYSTEM - DETERMINATION OF APPLICATOR ADEQUACY FOR SUPERFICIAL HYPERTHERMIA

Three-dimensional acoustic and thermal models were developed to simula te superficial hyperthermia treatments using a new multielement planar ultrasonic system. Typical power density and steady-state temperature distributions are presented. A parametric study was performed to dete rmine the relationship between therapeutic volume (volume at and above 42-degrees-C) and aperture size (number of active elements). The para meters investigated were: maximum allowable temperature, skin surface temperature, blood perfusion (thermal diffusion length), acoustic abso rption, and frequency. Results showed that this device produces well d istributed sound beams with lateral dimensions comparable to the apert ure size. These simulated results were in agreement with experimental measurements. The simulated temperature distributions were uniform at each depth across the applicator's aperture. The main heating characte ristics found were: (1) the therapeutic volume was directly proportion al to the aperture size; (2) the lateral dimensions of the therapeutic volume were independent of the parameters studied and remained practi cally constant with depth for several centimeters, with a very rapid i ncrease near the skin surface and a very rapid fall off at depth; and (3) therapeutic penetration was strongly dependent on maximum allowabl e temperature, frequency, and acoustic absorption; and weakly dependen t on blood perfusion and skin surface temperature. These heating chara cteristics are new in commercial systems for superficial hyperthermia. Despite the well-distributed beams, it was found that in order to pro duce adequate hyperthermia with this device the lateral dimensions of tumors must be smaller that the applicator's active aperture and that thermal depth coverage must be monitored during treatments. Guidelines for aperture selection and thermometry strategies are discussed.