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R.D. Glickman, S.M. Maswadi, S.J. Dodd, J. Gao; Mapping 3-Dimensional Temperature Gradients using Magnetic Resonance Thermography in an Ocular Phantom during Laser-Induced Hyperthermia . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3631.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Transpupillary Thermotherapy (TTT), utilizing laser irradiation of target tissue through the pupil, is an emerging treatment for neovascular membranes and other ocular diseases. Currently, there is no validated method for predicting temperature rises in the exposed tissue, which complicates the development of a standardized treatment protocol. We evaluated the use of magnetic resonance thermography (MRT) based on proton resonance frequency to measure temperature gradients in an eye phantom exposed to the output of a near infrared diode laser. This method can be employed noninvasively in animal or human eyes to validate a practical treatment model. Methods: Eye phantoms were made by filling a round glass cuvette of ~3 cm diameter with agarose containing an embedded layer of bovine melanosomes to simulate the retinal pigment epithelium (RPE). The emission from a 1 W, 806 nm, diode laser (Power Technology Corp.) was delivered via a 600 micron core optical fiber to the cavity of a Tecmag 2T magnetic resonance imager, and focused into the eye phantom. Temperature gradients induced in the phantom during laser exposure were imaged utilizing the relationship of the linear dependence of the change in the resonance frequency with the change in the temperature (measured by phase-sensitive MRI pulse sequences). Calibration was done by comparison to actual temperature changes measured with a thermistor probe in the phantom during similar laser exposures outside the MR cavity. Results: Temperature increases up to 30 oC were produced by exposing the phantom to a laser irradiance of ~5 W/cm2 (beam radius = 0.2 cm at 1/e2) for 5 min. The highest temperature was measured at the surface of the phantom, directly under the laser beam. Deeper in the phantom, the temperature declined as the heat dissipated into the volume of the phantom. The time course of heating and cooling in the phantom could also be measured by MRT. Conclusions: MRT can be utilized to measure temperature gradients in a volume approximating that of the human eye, with a resolution sufficient to generate useful 3-dimensional temperature maps. The spatial distributions of the temperature gradients were approximated by a solution of the heat diffusion equation, modified by the incorporation of a large scattering parameter, to account for the effect of melanin. Future work will be aimed at refining the numerical model to predict heating of the eye during laser-induced hyperthermia, as well as applying MRT to in vivo measurements.
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