Thursday, April 2, 1998
4:30-5:30 pm
1001 EECS
Abstract -
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.
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