Thermal analysis of blood undergoing laser photocoagulation

The temperature of blood undergoing laser-induced photocoagulation during long-pulse (10 ms) 532 nm irradiation was measured in a time- and spatially-resolved manner using a novel technique. This method is based on the change in reflectivity of a solid-liquid interface given a dynamically changing refractive index in the liquid phase. In our case, the temperature-dependent change in the refractive index of blood was utilized, and the reflectivity at a glass-blood interface was measured. Measurements were compared to predictions from a finite-element model incorporating the effects of time-dependent changes in the absorption coefficients of the blood, and phase changes representing coagulation and the liquid/vapor transition. Previous studies have linked the onset of blood coagulation to a sharp rise in the 532-nm reflectance of the blood. Based on the thermal measurements and the results of an Arrhenius analysis, we postulate that the reflectance rise is a combination of protein denaturation and red blood cell conformal changes.