Lin W L, Yen J Y, Chen Y Y, Cheng K S, Shieh M J
Center for Biomedical Engineering, National Taiwan University, Taipei.
Med Phys. 1998 Jun;25(6):1041-8. doi: 10.1118/1.598287.
The purpose of this paper is to examine the optimal driving frequency and to configure the ultrasound energy deposition schema for a various size and location of breast tissues when a portion or the entire cylindrical ultrasound transducer is employed for breast hyperthermia treatments. This work employs a computer simulation program based on an ideal ultrasound power deposition from a cylindrical transducer. The ultrasound power within the breast is assumed to be exponentially attenuated according to the penetration depth of the ultrasound beam and a uniform absorption for the entire breast is also assumed. The distribution of the specific absorption rate (SAR) ratio is employed to determine the heating pattern of a set of given parameters. The control parameters considered are the ultrasound frequency in the breast tissue, the active portion of cylindrical transducer, and the shifting distance between the central axes of the breast and the transducer. The effect of the breast size on the SAR ratio is also considered. Simulation results demonstrate that the breast size, the ultrasound frequency in breast tissue, the shifting distance, and the active portion of the cylindrical transducer are the potential parameters for influencing the distribution of the SAR ratio. High frequencies should be used for the superficial heating treatments and the active portion of the transducer can be changed to obtain a region with an appropriate SAR ratio to cover the treatment region. Low frequencies are used for deep heating treatments and the region of the high SAR ratio can be moved by shifting the transducer and its pattern is varied with the transducer's active portion. The distribution of the SAR ratio indicates the domain of treatable tumor size and tumor depth for a given set of parameters (driving frequency, shifting distance and active portion of the transducer, as well as breast diameter). Findings of this study can be used to know whether or not the tumor is treatable as well as to select the optimal driving frequency and the appropriate active portion of the cylindrical transducer for a treatment, and hopefully to design an appropriate cylindrical ultrasound heating system for breast tumors.
