BASIC OPTOTHERMAL DIFFUSION-THEORY FOR INTERSTITIAL LASER PHOTOCOAGULATION

A theoretical basis for interstitial laser photocoagulation (ILP) prac ticed with point-emitting fiber tips has been established by solving t he bioheat transfer equation, using basic Green's function methods, fo r steady and instantaneous point sources of both optical energy and di rect heat. Three combination optical and thermal parameters have been identified that strongly influence temperature distributions during IL P. These are defined here as optothermal heat capacities and an optoth ermal diffusion length, all of which characterize how a thermal diffus ion temperature profile is flattened and reduced when optical diffusio n is added. Relevance and limitations of this theory for practical ILP are discussed. A useful result is a mathematical verification of prev ious empirical observations that point optical sources heat tissues le ss than point heat sources of the same power. A comparison of normaliz ed theoretical temperature transients with published measurements sugg ests that in normal liver, blood perfusion cooling may exceed thermal conduction by a factor of 5.6+/-1.7.