Relevance and utility of the in-vivo and ex-vivo optical properties of the skin reported in the literature: a review [Invited]

Abstract

Imaging non-invasively into the human body is currently limited by cost (MRI and CT scan), image resolution (ultrasound), exposure to ionising radiation (CT scan and X-ray), and the requirement for exogenous contrast agents (CT scan and PET scan). Optical imaging has the potential to overcome all these issues but is currently limited by imaging depth due to the scattering and absorption properties of human tissue. Skin is the first barrier encountered by light when imaging non-invasively, and therefore a clear understanding of the way that light interacts with skin is required for progress on optical medical imaging to be made. Here we present a thorough review of the optical properties of human skin measured in-vivo and compare these to the previously collated ex-vivo measurements. Both in-vivo and ex-vivo published data show high inter- and intra-publication variability making definitive answers regarding optical properties at given wavelengths challenging. Overall, variability is highest for ex-vivo absorption measurements with differences of up to 77-fold compared with 9.6-fold for the in-vivo absorption case. The impact of this variation on optical penetration depth and transport mean free path is presented and potential causes of these inconsistencies are discussed. We propose a set of experimental controls and reporting requirements for future measurements. We conclude that a robust in-vivo dataset, measured across a broad spectrum of wavelengths, is required for the development of future technologies that significantly increase the depth of optical imaging.

Fig. 1.

Schematic of the three different layers of the skin detailing the chromophores and major constituents of each layer.

Fig. 2.

Effect of ‘g’ on focus; µ_s= 0.2 mm^-1, n = 1, µ_a= 0, f-number of lens = 1.7 Images generated via Monte Carlo simulation with 25 photon paths shown for illustration purposes.

Fig. 3.

Absorption spectra in the visible and near-infrared region (500 to 1600 nm), normalised to their maximum, of oxygenated and deoxygenated haemoglobin, water, collagen, beta-carotene and lipid, the main constituents of the skin (Reprinted with permission from Ref. [18], Fig. 1(b))

Fig. 4.

The absorption of skin chromophores in the wavelength range 300 nm -1500 nm with respect to their skin volume fraction (Reprinted with permission from Ref. [42], Fig. 2.4).

Fig. 5.

Methods used in the literature for determining the optical properties of the skin.

Fig. 6.

Comparison of average absorption and reduced scattering coefficients collected from the published ex-vivo (blue lines) and in-vivo (green lines) data. Scattering coefficients are shown as dashed lines and absorption coefficients as solid lines.

Fig. 7.

The variability of reported data. Ex-vivo absorption and scattering coefficients versus wavelength from published data for epidermis (A, D), dermis (B, E) and subcutaneous tissue (C, F). Note that units for the coefficients are cm^-1. Solid lines represent data extracted from the experimental measurements; dashed lines represent data from the mathematical models. (Reprinted with permission from Ref. [8], Fig. 1 © The Optical Society).

Fig. 8.

Variability in absorption coefficients (a) and reduced scattering coefficients (b) amongst the published in-vivo data. For readability the data is plotted by linear interpolation of the available data points. Note that Kono et al., measured scattering coefficients which were converted to reduced scattering coefficients for the purpose of this graph using the equation µ’_s = µ_s(1-g) and g = 0.84 (the average anisotropy coefficient amongst all the published papers used in this review).

Fig. 9.

Comparison of average TMFP for published ex-vivo (blue) and in-vivo (green) data.

Fig. 10.

Comparison of transmission of light through in-vivo (green) and ex-vivo (blue) skin. Percentage epidermal transmission represented by solid lines, percentage of dermal transmission represented by dashed line. Epidermis = 0.1 mm thick; dermis = 1.83 mm thick.