Modelling light distributions of homogeneous versus discrete absorbers in light irradiated turbid media

Abstract

Abstract. Laser treatment of port wine stains has often been modelled assuming that 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 absorption by blood, fluence rate near the centre of the vessel is much lower than at the periphery. Red blood cells near the centre of the vessel therefore absorb less light than those at the periphery. Effectively, when distributed homogeneously over the dermis, fewer red blood cells would produce the same absorption as the actual number of red blood cells distributed in discrete vessels. We quantified this effect by defining a correction factor for the effective absorbing blood volume of a single vessel. For a dermis with multiple vessels, we used this factor to define an effective homogeneous blood concentration. This was used in Monte Carlo computations of the fluence rate in a homogeneous skin model, and compared with fluence rate distributions using discrete blood vessels with equal dermal blood concentration. For realistic values of skin parameters the homogeneous model with corrected blood concentration accurately represents fluence rates in the model with discrete blood vessels. In conclusion, the correction procedure simplifies the calculation of fluence rate distributions in turbid media with discrete absorbers. This will allow future Monte Carlo computations of, for example, colour perception and optimization of vascular damage by laser treatment of port wine stain models with realistic vessel anatomy.

The effects of intense pulsed light (IPL) on blood vessels investigated by mathematical modeling

“…The optimal parameters for selective photothermolysis in laser treatments have been widely published by using mathematical models simulating selective photothermolysis [9][10][11][12][13]. However, no model exists in order to correlate the IPL spectrum to the temperature distribution within blood vessels.…”

Hemoglobin parameters from diffuse reflectance data

“…Correction factors for a bulk tissue absorption coefficient, μ a , have been derived by several groups [11][12][13][14] and are shown to be similar by van Veen et al 15 All of these correction factors depend only on the blood vessel radius and the absorption coefficient of blood, but not the density of blood vessels. Here, we present a simple derivation for a correction factor for μ a due to blood being heterogenously distributed and demonstrate that the correction factor also depends on the density of blood vessels.…”

“…One is that the hemoglobin is in blood vessels of varying sizes. Here, as in past work, [11][12][13][14][15] the assumption was made that all blood vessels in the sampled tissue are the same size. Another aspect of tissue heterogeneity is that epithelial tissues are layered.…”

Quantitative analysis of optical properties of flowing blood using a photon-cell interactive Monte Carlo code: effects of red blood cells’ orientation on light scattering

“…For developing a photon propagation model in blood based on MC simulation, the phase function derived using the Mie, Rayleigh-Gans, or straight-ray theory has been developed and utilized. [13][14][15] The optical simulation using MC codes, [16][17][18][19][20][21][22][23][24][25][26] however, treats red blood cells (RBCs) as points in the space, hence it is difficult to determine cell orientation in the space. The scattering by RBCs depends on the incident photon direction because RBCs have a non-spherical biconcave shape.…”