Shedding light on the variability of optical skin properties: finding a path towards more accurate prediction of light propagation in human cutaneous compartments

Abstract

Finding a path towards a more accurate prediction of light propagation in human skin remains an aspiration of biomedical scientists working on cutaneous applications both for diagnostic and therapeutic reasons. The objective of this study was to investigate variability of the optical properties of human skin compartments reported in literature, to explore the underlying rational of this variability and to propose a dataset of values, to better represent an in vivo case and recommend a solution towards a more accurate prediction of light propagation through cutaneous compartments. To achieve this, we undertook a novel, logical yet simple approach. We first reviewed scientific articles published between 1981 and 2013 that reported on skin optical properties, to reveal the spread in the reported quantitative values. We found variations of up to 100-fold. Then we extracted the most trust-worthy datasets guided by a rule that the spectral properties should reflect the specific biochemical composition of each of the skin layers. This resulted in the narrowing of the spread in the calculated photon densities to 6-fold. We conclude with a recommendation to use the identified most robust datasets when estimating light propagation in human skin using Monte Carlo simulations. Alternatively, otherwise follow our proposed strategy to screen any new datasets to determine their biological relevance.

Keywords: (160.4760) Optical properties; (170.3660) Light propagation in tissues; (170.6930) Tissue; (350.5500) Propagation.

Fig. 1

Absorption and scattering coefficients versus wavelength from bibliographic sources for epidermis (A, D), dermis (B, E) and subcutaneous fat layer (C, F). Solid lines represent data extracted from the experimental measurements, dashed-lines - from the mathematical models.

Fig. 2

Relative photon density versus depth obtained from the Monte Carlo predictions of optical transport in a three-layer human skin model using the selected optical properties datasets, shown in semi-logarithmic scale. The photon density presented was extracted from a rectangular cylinder of sizes 400 μm by 400 μm centered around the propagation axis.

Fig. 3

Beam profile in the dermal layer (photon density) versus radial axis obtained from the Monte Carlo predictions of optical transport in a three-layer human skin model using the selected optical properties datasets, shown in semi-logarithmic scale. The photon density presented is the beam profile measured at 700 μm under the skin surface and averaged over 164 μm in the direction perpendicular to the propagation axis

Fig. 4

Absorption versus scattering coefficients corresponding to datasets 1, 2, 3 and 4 for the epidermis (left) and dermis (right). The symbol indicates the dataset: 1 (○), 2 (+), 3 (x) and 4 (□). The colour of the dots indicates the wavelength, blue 450 nm, green 530 nm, red 655 nm and black 850 nm.

Fig. 5

Diffuse reflectance of human skin computed based on the Monte Carlo predictions of the propagation of light in a three-layer skin model using the selected optical properties datasets. Also shown is an empirical measurement of the diffuse reflectance of human Caucasian skin as measured by the NIST [55]