The absorption coefficient µa
, the scattering coefficient µs
′, and chromophore concentrations of skin are fundamental properties of tissue that can provide essential information for many aesthetic, therapeutic, and diagnostic applications such as monitoring of skin blood oxygenation, melanin concentration, detection of cancer with fluorescence, laser surgery, and photodynamic therapy.[1
] Many researchers have quantified the optical properties of skin tissue and most of them used ex-vivo
skin samples and integrating sphere techniques. For example, Troy and Thennadil determined the optical properties of 22 human skin samples ex-vivo
in the wavelength range from 1000 to 2200nm using double integrating sphere measurements.[5
] Simpson et al.
measured samples of dermis of five subjects (four Caucasians and one Negroid) ex-vivo
and obtained the optical properties in the wavelength range from 625 to 1000nm using integrating sphere measurements.[6
] Bashkatov et al.
have characterized optical properties of 21 skin samples ex-vivo
in the wavelength range from 400 to 2000nm using an integrating sphere approach.[7
] Salomatina et al.
have determined and compared the ex-vivo
optical properties of human skin samples with those of non-melanoma skin cancers in the spectral range from 370 to 1600nm.[8
Although the integrating sphere based techniques can be used to investigate ex-vivo optical properties of epidermis, dermis, and subcutaneous tissue, the need to take biopsies from subjects limits their applicability in the clinic. Furthermore, once tissue has been excised, it is no longer part of the living organism; oxy- and dexoy-hemoglobin concentrations begin to diverge from physiologic quantities and if not carefully handled, the hydration of the tissue begins to change.
Photon diffusion theory derived from the radiative transport equation is usually employed as an forward model to determine optical properties of in-vivo
samples at source-detector separation longer than five mean-free-paths, where mean-free-path is defined as 1/(µa
′). It has been proven to be a not adequate model for source-detector separations longer than five mean-free-paths, because boundary conditions and the assumption of multiple scattering in a turbid medium cannot be satisfied.[9
] However, in order to limit interrogation to superficial tissue volumes, such as skin, source-detector separations shorter than five mean-free-paths are more favorable. To the best of our knowledge, in-vivo
techniques which employ alternative forward models to determine optical properties of skin do exist, but have some important limitations. For example, Zhang et al.
determined optical properties of in-vivo
skin using visible reflectance spectroscopy with a multi-layer skin model and a genetic optimization algorithm.[11
] A multi-layer skin model and a number of fitting parameters, such as layer thickness, chromophores, and scattering properties for each layer, and their corresponding ranges must be chosen carefully in advance to avoid non-uniqueness in the solution space. Zonios and Dimou proposed a model to extract optical properties from diffuse reflectance spectra collected from human skin in-vivo
] Their technique requires that all of the chromophores contributing to the measured signals are known in advance and the reduced scattering coefficient has a linear relation to the wavelength in order to separate absorption and reduced scattering coefficients from measured reflectance. For the case where all constituent chromophores cannot be determined, the absorption spectra cannot be recovered. In addition, for the case where the reduced scattering coefficient does not have a linear dependence on wavelength, the empirical mathematical model will not recover tissue optical properties properly.
Previously, we used the diffusing probe that has a “modified two layer geometry” in conjunction with the Steady-State Frequency Domain Photon Migration (SSFDPM) technique to quantify the in-vivo
skin optical properties of 15 subjects (five Caucasians, five Asians, and five African-Americans) in the wavelength range from 650 to 1000nm.[13
] The facilitating technologic innovation of this probe is the presence of a highly diffusing Spectralon layer at the input side that enables a diffusion model for small source-detector distances (less than three mean-free-paths), and, thus, the absorption and reduced scattering coefficients of the superficial volumes of samples can be separated and quantified.
In this study, the probe design has been adjusted into multiple source-detector pairs so that it can employ a white light source to obtain continuous spectra of absorption and reduced scattering coefficients. The advantages of this multi source-detector separation probe include relative low instrument cost and self-calibration for instrument response (by using the reflectance of one source detection pair as the reference and normalizing the reflectance of other source detector separation pairs to the reference). The normalized reflectance versus source-detector separation is then fit to a diffusion model by a least square minimization algorithm to determine the absorption and reduced scattering spectra. The recovered absorption spectra are fit linearly with known chromophore absorption spectra to extract chromophore concentrations, and the reduced scattering spectra are fit to a scattering power law to obtain the scattering power.[14
The goal of this paper was to use this new diffusing probe to determine the skin optical properties of 18 subjects of different skin phototypes and also extract the chromophore concentrations and the scattering power of skin. It is found that performing the “two-region chromophore fitting” to the absorption spectrum would result in the best fit with minimal residuals. By two-region chromophore fitting, we mean that the skin absorption spectrum is fit to a set of known chromophore absorption spectra at wavelengths between 500 nm and 600 nm and again fit separately between 600 nm and 1000 nm. The rationale for performing the two-region fitting is that the skin has very different optical properties in the visible and the NIR wavelength regions, and thus the sampling volumes at these two regions are quite different. Monte Carlo simulation results were generated to support this assumption. Likewise, the best fittings for reduced scattering coefficients of skin were obtained when the reduced scattering spectra were fit in the region below and above 600nm separately. This suggests that the average scatterer sizes of skin determined in the visible and the NIR regions are much different. Results from the measurements carried out here also indicate that the scattering power is not only dependent to anatomical location but also on skin phototype. Finally, experimental results obtained from measuring 10 subjects with forearm venous occlusion were provided. We recovered significantly different hemoglobin concentration at the region below and above 600nm. Our results agree with those reported by other researchers and support that our proposed probe and fitting method are capable of studying in-vivo superficial tissue at different depths simultaneously.