Thursday, April 2, 1998
Noninvasive thermotherapy through the application of high intensity focused ultrasound beams offers an attractive alternative to conventional resection surgery. However, the lack of a real-time noninvasive imaging technique capable of visualizing this thermal effect has limited its applicability in the clinic. To overcome this difficulty we developed a technique to estimate the temperature distribution on a two-dimensional plane using diagnostic backscattered ultrasound. The ultrasound echo from a region of tissue undergoing thermal therapy experiences time shifts due to the temperature dependence of speed of sound and thermal expansion in the heated region. A linear relationship between these time shifts and the underlying tissue temperature was derived from first principles. Two-dimensional maps of the temperature distribution are estimated and overlaid to the gray-scale ultrasound images to provide guidance for noninvasive thermotherapy.
Depending on the application, tissue motion and deformation may cause echo shifts up to two orders of magnitude larger than the time shifts due to the thermal effect, potentially affecting the temperature estimator. In order to accommodate for these cases, we developed algorithms that compensate for planar motion and uniform deformation of the tissue prior to estimating the temperature distribution. A number of experiments were designed to characterize the capabilities and limitations of these techniques. A phased array therapeutic ultrasound transducer was used to create localized heating in a tissue mimicking phantom. At the same time, backscattered ultrasound was either collected using a commercial ultrasound scanner or using the same therapeutic transducer operated in pulse-echo mode. While the former approach provided better spatial resolution, the latter eliminated the need for a second transducer. The results demonstrate the potential of this architecture in providing thermal image guidance for noninvasive thermotherapy.
For more extensive detail on this presentation and for a biosketch -
Please refer to the homepage link above.