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David Dewey, Alexander Vankov, Georg Schuele; Modeling and experimental testing of iris thermal damage limits. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3385.
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© ARVO (1962-2015); The Authors (2016-present)
Despite that retinal safety limits are well established and useful tool for scanning laser systems, iris thermal damage threshold is not well described and understood. Here we present experimental results and theoretical data for iris damage threshold for laser system.
Experimental evaluation of iris minimal visible lesions was done on fresh (less than 25h post mortem) porcine eyes with removed cornea placed under water immersion. Laser beam with 180um in diameter and variable spot spacing was scanned over the iris surface, creating rectangular lesions of various sizes. These lesions were analyzed under the surgical microscope to determine the minimal visible lesion threshold, see Fig.1.Analytical Point Spread Function solution and Finite Element Mesh (FEM) methods were independently used for temperature calculation. Temperature rise was calculated for 1uJ 70kHz laser beam exercising lawn-mower pattern with variable spot spacing 1-3um and distant boundary conditions at body temperature. The analytical solution was used to verify the FEM method and all results were compared the experimental MVL threshold data.
We found that thermal steady state is achieved in fraction of a second. Maximum temperature is reached in the center of the exposed area. Temperature profile consists of slow temperature change and fast oscillations, arising when beam is passing through the point of interest (Fig.2). At the same laser power, maximal temperature slowly depend on the spot spacing and length of the cut, but inversely proportional to the width.
Experimental results matches theoretical model for minimal visible lesion prediction. Based on these results, one can calculate parameters of laser scanning system for various incision geometries.© 2015 Abbott Medical Optics Inc.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Figure 1: Thermal lesions created on the iris at different laser powers.
Figure 2: Calculated FEM temperature profiles of the points at the center line of the exposed area at different distance to the center.
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