Theoretical investigations of the processes of selective laser interaction with melanin granules in pigmented tissues for laser applications in medicine

Theoretical investigations and the results of computer modeling of optical, thermophysical, thermochemical, and hydrodynamical processes during selective laser interaction with melanoprotein granules (melanosomes) in heterogeneous pigmented tissues (retinal pigment epithelium) are reviewed in this paper. Physico-mathematical models and system of equations are formulated which describe interaction processes for “short” laser pulses of duration t _p < 10^?6 s and for “long” pulses of duration t _p > 10^?6 s. The results of numerical simulation of the processes give the space-time distributions of temperature and degrees of thermodenaturation of the protein molecules inside and around melanosomes and in the volume of irradiated tissue. Energy absorption, heat transfer, and thermochemical processes occurring during the interaction of laser pulses with pigmented spherical and spheroidal granules in heterogeneous tissues are theoretically investigated. The possibility for selective interaction of short laser pulses with pigmented granules, which results in the formation of denaturation microregions inside and near the pigmented granules (granular thermodenaturation) without origination of a continuous macroscopic thermodenaturation lesion in tissue, is discussed. An analytical model of heating of a single spherical and spheroidal granule by a laser pulse is presented. Simple equations for the time dependences of particle temperature are obtained. Vapor generation under the action of a laser pulse on pigmented spherical granules in a water-containing tissue and the formation and dynamics of a vapor blanket are theoretically investigated. The values of pulse energy which give rise to granular and ophthalmoscopically visible thermodenaturation lesions on the retina and to vapor generation are discussed, as well as laser-induced breakdown on granules in pigmented tissues, on the basis of experimental results and numerical and analytical calculations. The comparison and agreement of the numerical results with the experimental data validate the models and techniques developed. The presented results are of essential interest for laser applications in ophthalmology and can be used to investigate laser interaction with heterogeneous tissues in dermatology and various fields of laser medicine.