PARENT SESSION
1:30 PM to 3:30 PM
Monday, April 22, 2002
Poster Session 3 Heating Devices and High Temperature Therapy
Room: Nevada 1-2

(MP03-33) A study of parameters of multi-focus lens/transducer combinations for ultrasound thermal therapy.

Wu, Xia^*,1,3, Sherar, Michael^1,2,3, ¹ Department of Medical Biophysics, Toronto, Ontario³ Princess Margaret Hospital/Ontario Cancer Institute, Toronto, Ontario² Department of Radiation Oncology, Toronto, Ontario

ABSTRACT-
High intensity focused ultrasound thermal therapy is a promising technique for the non-invasive destruction of solid tumours. One major challenge is that the treatment time currently required is impractically long due to the small individual thermal lesions that highly focused spherical transducers create. One approach to address this is to employ ultrasound transducers such as phased arrays that can form large thermal lesions in tissues so that total treatment times could be reduced. We have demonstrated that multi-focus acoustic lens/transducer combinations [LTC] can produce large thermal lesions of a similar size and shape compared to those produced by phased arrays. In addition, LTCs are simple to manufacture and inexpensive as compared to phased arrays. The objective of this work was to investigate theoretically the effect of lateral focus spacing and number of foci on the shape and size of thermal lesions produced by LTCs. A 3-D ultrasound thermal model was developed to calculate the thermal dose profiles generated by LTCs. A lens design method was developed to determine the lens surface profile required for an LTC to produce a specified multi-focus pattern. We found that as the LTC focus spacing or number of foci increased, thermal lesion size increased in both lateral and axial directions. The shape of thermal lesions became less regular as the number of foci was increased. The use of a 2000 element LTC with a 10 cm aperture, 8.5 cm focal length and 1 MHz operating frequency resulted in 90% of an 1×1×2 cm³ target receiving a thermal dose greater than a thermal damage threshold of 240 equivalent minutes at 43 degC. When the number of foci increased to 16, the use of such an LTC resulted in 86% of the target tissue receiving greater than the thermal damage threshold. Therefore, in order to increase the size of thermal lesions only in the lateral direction, some parameters of LTCs, such as the focusing strength of the LTC transducer must also be changed.

KEYWORDS: ultrasound, acoustic lens, thermal therapy