A theoretical study of cylindrical ultrasound transducers for intracavitary hyperthermia

Purpose: The purpose of this paper was to examine the heating patterns and penetration depth when a cylindrical ultrasound transducer is employed for intracavitary hyperthermia treatments. Methods and Materials: The present study employs a simulation program based on a simplified power deposition model for infinitely long cylindrical ult rasound transducers. The ultrasound power in the tissue is assumed to be ex ponentially attenuated according to the penetration depth of the ultrasound beam, and a uniform attenuation for the entire treatment region is also as sumed. The distribution of specific absorption rate (SAR) ratio (the ratio of SAR for a point within the tissue to that for a specific point on the ca vity surface) is used to determine the heating pattern for a set of given p arameters. The parameters considered are the ultrasound attenuation in the tissue, the cavity size, and the transducer eccentricity. Results: Simulation results show that the ultrasound attenuation in the tis sue, the cavity size, and the transducer eccentricity are the most influent ial parameters for the distribution of SAR ratio. A low frequency transduce r located in a large cavity can produce a much better penetration. The cavi ty size is the major parameter affecting the penetration depth for a small cavity size, such as interstitial hyperthermia. The heating pattern can als o be dramatically changed by the transducer eccentricity and radiating sect or. In addition, for a finite length of cylindrical transducer, lower SAR r atio appears in the regions near the applicator's edges. Conclusion: The distribution of SAR ratio indicates the relationship betwee n the treatable region and the parameters if an appropriate threshold of SA R ratio is taken. The findings of the present study comprehend whether or n ot a tumor is treatable, as well as select the optimal driving frequency, t he appropriate cavity size, and the eccentricity of a cylindrical transduce r for a specific treatment. (C) 2000 Elsevier Science Inc.