Prediction of temperature and damage in an irradiated human eye during retinal photocoagulation

The goal of the study is improving the process of titrating laser power to the level of a “mild” coagulation when the damage to the retinal pigmented epithelium (RPE) sells is only performed, but coagulation of photoreceptors is avoided. An analytical thermal model of the irradiated eye tissue is proposed based on the solution of the heat equation for the laminar media having discrete optical properties. Both single-pulse and multi-pulse laser modes are under consideration. The model is applied for predicting the over-threshold heating within the lesion depending on its diameter. It is found that the size of the lesion detected after trial irradiation is a key factor that can be used for correcting the trial power to the level of mild coagulation. The corresponding correction function is introduced that is independent on eye media transmittance. Consequently, the proposed semi-empirical approach to the threshold prediction completely excludes the problem of unknown intraocular transparency. The transmittance-independent criterion of a critical lesion size is introduced, at which the axial superheat does not exceed a predetermined safety level regardless of pulse duration. The critical lesion dimensions are calculated for different diameters of irradiated spot at the retina. The analytical model is validated against experimental data available in literature.