Quantitative assessment of skin layers absorption and skin reflectance spectra simulation in the visible and near-infrared spectral regions.

TLDR: The simulation of diffuse reflectance spectra of skin is simulated by assuming a wavelength-independent scattering coefficient for the different skin tissues and using the known wavelength dependence of the absorption coefficient of oxy- and deoxyhaemoglobin and water to convert reflected intensity.

Abstract: We have simulated diffuse reflectance spectra of skin by assuming a wavelength-independent scattering coefficient for the different skin tissues and using the known wavelength dependence of the absorption coefficient of oxy- and deoxyhaemoglobin and water. A stochastic Monte Carlo method is used to convert the wavelength-dependent absorption coefficient and wavelength-independent scattering coefficient into reflected intensity. The absorption properties of skin tissues in the visible and near-infrared spectral regions are estimated by taking into account the spatial distribution of blood vessels, water and melanin content within distinct anatomical layers. The geometrical peculiarities of skin histological structure, degree of blood oxygenation and the haematocrit index are also taken into account. We demonstrate that when the model is supplied with reasonable physical and structural parameters of skin, the results of the simulation agree reasonably well with the results of in vivo measurements of skin spectra.

Figures

Figure 6. Skin reflectance spectra simulated: (a) for 40% blood volume fractions changes in papillary dermis and upper blood net dermis and (b) for 40% blood volume fractions changes in deep blood net dermis. Source and detector separations are: 250 µm, 400 µm and 800 µm. The assumed optical properties of the modelling medium are given in table 2 and in figure 5.

Table 1. The values of the parameters defined in equation (8) and used in our simulation.

Figure 3. An example of a quasi-random periodic surface representing the interfaces between the dermal layers.

Figure 7. Skin reflectance spectra. Squares represent the results of MC spectra simulation and dots are the results of the skin reflectance spectrum measured in vivo.

Table 2. The parameters used in the calculation of the absorption coefficients of skin layersa, and other optical properties of the layers used in the simulation.

Figure 1. Schematic representation of the experimental set-up used for the non-invasive measurements of the diffuse reflectance spectra of skin. A 100 W quartz halogen lamp (L.O.T.ORIEL Ltd, Model 77501) is used as an optical/NIR source of light projected onto the skin surface by an optical fibre bundle. Another optical fibre separated from the source fibre by a short distance collects diffusely reflected light and delivers it to the detector. A SPEX 270M spectrograph/monochromator with a 1024 × 256 CCD camera (Wright Instruments Ltd) used for high speed spectral measurements, where one dimension is responsible for wavelength and the other for the intensity of detected scattered light. The detecting signal is processed by a personal computer (PC), where the spectral curve can be fitted with a multi-linear regression algorithm (Matcher 2002).