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Numerical study of the effect of methemoglobin concentration in the blood on the absorption of light by human skin.

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Lasers are widely used in dermatology to remove telangiectasias. Increasing the efficiency of sclerosis of deep-lying and large telangiectasias with laser radiation is possible by changing the optical transmission of the skin when it is heated and converting the hemoglobin of the blood contained in it into methemoglobin. The influence of the concentration of methemoglobin in the blood on the absorption of light in human skin is poorly understood, which determines the relevance of this study in the context of finding ways to improve the efficiency of laser removal of telangiectasias. Seven-layer optical models of human skin without telangiectasia and with it for numerical simulation were developed. The extinction coefficients and the degree of change in the optical transmission of whole blood and skin layers were calculated in the range of wavelength from 400 to 1600 nm for skin model without and with arteriolar and venular telangiectasias at various concentrations of methemoglobin in the blood. Based on the analysis of these data, the wavelengths with the biggest change in the optical transmission of whole blood and skin layers occurred during the transformation of hemoglobin to methemoglobin were selected. At the selected wavelengths, the Monte Carlo method was used in optical modelling to get the distribution of the absorbed optical power in each layer of the skin model without and with telangiectasia at various concentrations of methemoglobin. It has been shown that the spectra of extinction coefficients for arteriolar and venular telangiectasias do not differ significantly. During the transformation of hemoglobin to methemoglobin, the largest decrease in the degree of change in the optical transmission of whole blood occurs at wavelengths of 629 nm and 1105 nm, and the largest increase occurs at wavelengths of 447 nm and 578 nm. The part of absorbed optical power in the layer of superficial vascular plexus without and with telangiectasia at wavelengths of 629 nm and 1105 nm increases, and at wavelengths of 441 nm and 574 nm it decreases with a growth of the methemoglobin concentration from 0 to 100 % in the skin model. At the same time, in the layer of deep vascular plexus the value of part of absorbed optical power increases at wavelengths of 441 nm, 574 nm and 1105 nm, but at a wavelength of 629 nm first increases with a growth of the methemoglobin concentration up to 25 %, and then decreases, but to values exceeding the value of part of absorbed optical power without methemoglobin. The change in optical transmission associated with the replacement of blood hemoglobin with methemoglobin is more pronounced for the superficial vascular plexus layer, which is associated with high blood content in it and a limited contribution of the overlying skin layers to the deformation of the spectrum of light incident on this layer. In skin with telangiectasia, a change in the concentration of methemoglobin changes the proportion of absorbed optical power by a greater amount than in skin without telangiectasia, which can be associated with an increase in the volume concentration of blood in skin layers with telangiectasia and an increase in their thickness. The obtained results can be applied in the development of laser systems and technologies for the treatment of skin diseases, including laser sclerosis of telangiectasias.

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