MODELING LIGHT DISTRIBUTIONS OF HOMOGENEOUS VERSUS DISCRETE ABSORBERSIN LIGHT IRRADIATED TURBID MEDIA

Laser treatment of port wine stains has often been modelled assuming t hat blood is distributed homogeneously over the dermal volume, instead of enclosed within discrete vessels. The purpose of this paper is to analyse the consequences of this assumption. Due to strong light absor ption by blood, fluence rate near the centre of the vessel is much low er than at the periphery. Red blood cells near the centre of the vesse l therefore absorb less light than those at the periphery. Effectively , when distributed homogeneously over the dermis, fewer red blood cell s would produce the same absorption as the actual number of red blood cells distributed in discrete vessels. We quantified this effect by de fining a correction factor for the effective absorbing blood volume of a single vessel. For a dermis with multiple vessels, we used this fac tor to define an effective homogeneous blood concentration. This was u sed in Monte Carlo computations of the fluence rate in a homogeneous s kin model, and compared with fluence rate distributions using discrete blood vessels with equal dermal blood concentration. For realistic va lues of skin parameters the homogeneous model with corrected blood con centration accurately represents fluence rates in the model with discr ete blood vessels. In conclusion, the correction procedure simplifies the calculation of fluence rate distributions in turbid media with dis crete absorbers. This will allow future Monte Carlo computations of, f or example, colour perception and optimization of vascular damage by l aser treatment of port wine stain models with realistic vessel anatomy .