The objective in all thermal ablative therapy is delivery of a lethal thermal dose to the target tissue without damaging the surrounding tissue. This common goal led to the interest in focal thermal treatments using a variety of energy delivery methods including cryotherapy, focused ultrasound and interstitial laser therapy. These techniques have the ability to deposit large amounts of energy in a limited target volume of tissue but require a system to guide or control the deposition. This proposal explores the use of magnetic resonance imaging (MRI) as a control system for thermal therapy treatments (cryoablation, interstitial laser therapy and focused ultrasound surgery). The relationship between the images and the biologic results is explored by acquiring serial MR images during the thermal treatment and then extracting the treated tissue to determine the surviving fraction of cells. The images are processed to compute the optical flow velocity vector field and produce a pixel-by- pixel image-derived function that is compared to the surviving fraction measured with a colony growth assay. The imaging is initiated before energy deposition, continues during the thermal treatment, and is completed when the tissue returns to the baseline original temperature (but not necessarily to the original state). This dynamic image data set and the derived optical flow velocity vector field form an accurate tissue history on a fine timescale. The tissue history obtained in this manner is used to calculate a quantitative characterization of the irradiation and cooling periods. Tissue groups exhibiting similar characterizations are correlated with the surviving cell fractions sampled from the same region. This novel method of dynamic tissue characterization is based not only on the thermal history of the tissue sample but also on the degree of cellular damage induced by the treatment. The resulting information can be used for the spatio-temporal control of destructive energy deposition during thermal ablative procedures. In addition, this experimental data will be used for the design of safe and efficient protocols for clinical trials in cryotherapy, interstitial laser treatments and focused ultrasound surgery.
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