Our group has recently presented a novel technique for noninvasive assessment of the structure and composition of human skin in vivo. The approach combines two optical techniques, photothermal radiometry and diffuse reflectance spectroscopy in visible part of the spectrum with numerical modeling of light transport in a four-level model of human skin, and enables assessment of the contents of specific chromophores (e.g., melanin, oxy-, and deoxy-hemoglobin), as well as scattering properties and thicknesses of the epidermis and dermis. In this study, we try to quantitatively validate the described approach by analyzing a series of optically homogeneous skin phantoms with varying concentrations of absorbing dye (Congo red) and optical scatterers. Twelve such phantoms were made from bovine collagen gel and their scattering properties were controlled by adding TiO2 nanoparticles with diameters of 200nm and 490nm, aiming at the range of values typical of human dermis. The applied Monte Carlo model of light transport takes into account the actual diameter and thickness of the phantoms and allows simultaneous assessment of the absorption coefficient as well as the amplitude and spectral power of the reduced scattering coefficient. The obtained values are proportional to the respective concentrations of the absorbing dye and scatterers, but don’t match the theoretical predictions (from a Mie simulator).
We investigate age-related changes of the dermal reduced scattering coefficient in human skin using a recently introduced methodology for non-invasive characterization of structure and composition of skin in vivo. The approach combines pulsed photothermal radiometry (PPTR) with diffuse reflectance spectroscopy (DRS) in visible part of the spectrum. The experimental data are fitted simultaneously with the respective predictions of a dedicated numerical model of light and heat transport in healthy skin (i.e., inverse Monte Carlo). For this purpose, we apply a four-layer optical skin model consisting of epidermis, upper dermis, lower dermis, and subcutaneous adipose tissue. The study is based on 24 measurements of test sites on the ventral side of the forearm in 9 women and 9 men with healthy fair skin, between 20 and 65 years old. Linear regression analysis of the assessed dermal reduced scattering coefficient values at 500 nm (ader) indicated no significant variation with the person’s age. Meanwhile, strong correlations of ader with the blood contents in both papillary and reticular dermis were observed. Separating the respective contributions of these three variables using multiple linear regression (MLR) analysis revealed a highly significant influence of person’s age on ader (with Pearson’s correlation coefficient r = –0.55 and p < 0.0001). Specifically, by excluding the direct influence of the dermal blood contents, ader decreases with age by approximately 0.2 mm-1 per decade. In addition, the values obtained for older persons are in good agreement with the results from a large cohort study performed by Jonasson et al. (J. Biomed. Opt. 2018).
In measurements of diffuse reflectance spectra (DRS) using an integrating sphere (IS), a highly reflective "white standard" (WS) is used to compensate for spectral properties of the incident light and detection setup. The DRS of the investigated sample is then obtained as the ratio between the values obtained with the sample and the white standard, calculated at each included wavelength. However, because the substitution of the WS with the sample may alter light fluence inside the IS, measured DRS are prone to a systematic distortion called single-beam substitution error (SBSE). An earlier report from our group1 has demonstrated a method for rigorous elimination of the SBSE based on additional measurements performed at the IS reference port. In addition, a more practical solution was presented, involving analytical correction of the measured DRS based on advance characterization of the specific IS. However, we have recently observed that such analytical correction can sometimes deviate from the rigorous experimental elimination of SBSE, depending on reflectance spectrum of the sample. In present study, we attribute this discrepancy to the spectral variation of diffuse reflectance of the IS inner wall, which was disregarded in the original derivation of the analytical correction. We describe experimental characterization of this spectral dependence, which improves the accuracy of analytical removal of SBSE for any object, regardless of its spectral properties.
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