In vivo determination of skin near-infrared optical properties using diffuse optical spectroscopy

Abstract

We develop a superficial diffusing probe with a 3 mm source-detector separation that can be used in combination with diffuse optical spectroscopic (DOS) methods to noninvasively determine full-spectrum optical properties of superficial in vivo skin in the wavelength range from 650 to 1000 nm. This new probe uses a highly scattering layer to diffuse photons emitted from a collimated light source and relies on a two-layer diffusion model to determine tissue absorption coefficient mu a and reduced scattering coefficient mu's. By employing the probe to measure two-layer phantoms that mimic the optical properties of skin, we demonstrate that the probe has an interrogation depth of 1 to 2 mm. We carry out SSFDPM (steady state frequency-domain photon migration) measurements using this new probe on the volar forearm and palm of 15 subjects, including five subjects of African descent, five Asians, and five Caucasians. The optical properties of in vivo skin determined using the superficial diffusing probe show considerable similarity to published optical properties of carefully prepared ex vivo epidermis+dermis.

Fig. 1

Layer structure of skin.

Fig. 2

Geometry of a superficial diffusing probe.

Fig. 3

Optical properties (a) μ_a and (b) ${\mu }_s^{\prime }$ recovered from two-layer phantoms having top layer thicknesses from 1 to 8 mm. The dot-dotted and dash-dotted lines represent benchmark optical properties of the top layers and the substrate of two-layer phantoms, respectively.

Fig. 4

Optical properties (a) μ_a and (b) ${\mu }_s^{\prime }$ of in vivo volar forearm skin of one subject of African descent, an Asian, and a Caucasian measured with a superficial diffusing probe. Discrete data points are recovered from frequency-domain photon migration (FDPM) measurements and lines represent broadband optical property spectra of the three subjects.

Fig. 5

Broadband optical properties (a) μ_a and (b) ${\mu }_s^{\prime }$ of in vivo volar forearm skin of five subjects of African descent (solid line), five Asians (dashed line), and five Caucasians (dotted line) measured with a superficial diffusing probe. The error bars represent the standard deviation of optical properties in each group.

Fig. 6

Broadband optical properties (a) μ_a and (b) ${\mu }_s^{\prime }$ of in vivo palm skin of five subjects of African descent (solid line), five Asians (dashed line), and five Caucasians (dotted line) measured with a superficial diffusing probe. The error bars represent the standard deviation of optical properties in each group.