Method of irradiation of surface and underlying layers of skin

Abstract

ABSTRACTMonte Carlo method has been used to simulate the effects of light focusing in highly scattering tissues, such asdermis. Spatial distributions of light both for ideal focusing systems and for systems with aberrations have been investigated.Keywords: Monte Carlo simulations, light focusing, aberrations. 1. ThTRODUCTION Investigation of light propagation in tissues with high scattering is of great importance due to its application to biology andmedicin&. In recent years a significant progress was achieved in understanding of the processes of light scattering inbiotissues, which are in most ccases highly-scattering media. Different aspects ofthese processes have been investigated. Forexample, one subject of interest is a prediction of light distributions for a number ofphotothermal and therapeutic problems,which are solving in medicine'. Another actual problems are the increasing of the light penetration depth and control overthe spatial distribution of intensity in medium.When light is propagating throw a highly scattering tissue, its distribution strongly changes in comparison with the case oftransparent or absorbing medium. If the light beam incidents normally to the surface of tissue one can observe a maximumof irradiance on some distance from the surface within the tissue. This is due to multiply scattering which occurs when theaveraged length of photon trajectory is much longer than the averaged photon step size between two scattering events. Thisis so, when scattering is much stronger than absorption and tissue is thick. Typically the depth where light energy isconcentrating consists in a fraction of a millimeter. In the case of convergent light beam it is possible to increase thepenetration depth of light in tissue and to concentrate energy in a desired volume, so focusing of light may be used for thesepurposes.Propagation of light in tissues in the most of practical cases can be described using transport theory'. As the analyticalsolution of the latter is available only for some simply cases, the numerical simulations based on Monte-Carlo methods arewidely used'. The features of collimated beam propagation in biotissues were accurately investigated recently. On the otherhand the features of focusing light distributions are not well investigated. For example one actual problem is the influence offocusing on light distribution on surface oftissue and under the surface.