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 simulate 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 determine the relationship between therapeutic volume (volume at and above 42 degrees C) and aperture size (number of active elements). The parameters investigated were: maximum allowable temperature, skin surface temperature, blood perfusion (thermal diffusion length), acoustic absorption, and frequency. Results showed that this device produces well distributed sound beams with lateral dimensions comparable to the aperture 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 characteristics found were: (1) the therapeutic volume was directly proportional to the aperture size; (2) the lateral dimensions of the therapeutic volume were independent of the parameters studied and remained practically constant with depth for several centimeters, with a very rapid increase near the skin surface and a very rapid fall off at depth; and (3) therapeutic penetration was strongly dependent on maximum allowable temperature, frequency, and acoustic absorption; and weakly dependent on blood perfusion and skin surface temperature. These heating characteristics are new in commercial systems for superficial hyperthermia. Despite the well-distributed beams, it was found that in order to produce adequate hyperthermia with this device the lateral dimensions of tumors must be smaller than the applicator's active aperture and that thermal depth coverage must be monitored during treatments. Guidelines for aperture selection and thermometry strategies are discussed.