DESIGN OF FOCUSED ULTRASOUND SURGERY TRANSDUCERS

High-intensity focused ultrasound surgery (RUS) has been developed for the extracorporeal treatment of various benign and malignant soft tis sue tumors. The system developed at the Institute of Cancer Research/R oyal Marsden (ICR/RM) National Health Service (NHS) Trust incorporates a 150 mm focal length focused bowl transducer operated at 1.7 MHz, an d is currently undergoing Phase 1 clinical trials for the treatment of benign prostatic hyperplasia and superficial bladder cancer, However, the application of this transducer is limited by its focal length to a maximum depth of 100 mn, and by power absorption in the skin to a mi nimum depth of 40 mm, A computer model of acoustic fields, which assum es uniform excitation of the transducer over its entire surface, has p reviously been published. This has been used both to calculate the int ensity in nonattenuating media, and to estimate the absorbed power per unit volume in homogeneous tissues in order to allow determination of the transducer configurations (frequency, focal length, and diameter) necessary for the treatment of both deep (similar to 150 mm) and shal low (similar to 20 mm) soft tissue tumors, These depths encompass the typical range for human tissues which are likely to be treated. Calcul ations cover the frequency range 0.5-4.5 MHz, focal lengths from 70 to 200 mm, and transducer diameters from 30 to 190 mm, The results show that appropriate transducers can be designed for the noninvasive treat ment of tumors in specific organs.