A theoretical analysis of the light interaction with coagulated tissue

During laser treatment, coagulation affects the optical properties of the tissue. In particular, the formation of a white lesion increases the scattering coefficient significantly. This change in the optical properties in turn affects the laser light distribution in the tissue. For example, what is the effect of the white lesion formed during photocoagulation of the retina upon reflection and fluence rate? This problem was simulated on a model medium consisting of a thin absorbing black paint layer covered with a 1cm thick layer of fresh egg white. The egg white layer was subdivided into coagulated (white) and uncoagulated (clear) layers. The optical properties of coagulated and uncoagulated egg white were determined. These values were used to model light distribution in the medium for varying thicknesses of the coagulated egg white layer using the one dimensional Adding Doubling method. Our results show that the fluence reaching the paint layer increases until the coagulated layer reaches 100μm, after which it falls off exponentially. It was also found that the total reflected light increases almost linearly at first as the coagulated layer thickens, and then begins to level off to an R∞ at a coagulation thickness of 2mm. Experimental measurements of reflection from a lesion with a CCD camera confirm the computed trends. These results provide a theoretical foundation for control of lesion thickness using reflectance images.