In vivo spatial frequency domain spectroscopy of two layer media

Abstract

Monitoring of tissue blood volume and local oxygen saturation can inform the assessment of tissue health, healing, and dysfunction. These quantities can be estimated from the contribution of oxyhemoglobin and deoxyhemoglobin to the absorption spectrum of the dermis. However, estimation of blood related absorption in skin can be confounded by the strong absorption of melanin in the epidermis and epidermal thickness and pigmentation varies with anatomic location, race, gender, and degree of disease progression. Therefore, a method is desired that decouples the effect of melanin absorption in the epidermis from blood absorption in the dermis for a large range of skin types and thicknesses. A previously developed inverse method based on a neural network forward model was applied to simulated spatial frequency domain reflectance of skin for multiple wavelengths in the near infrared. It is demonstrated that the optical thickness of the epidermis and absorption and reduced scattering coefficients of the dermis can be determined independently and with minimal coupling. Then, the same inverse method was applied to reflectance measurements from a tissue simulating phantom and in vivo human skin. Oxygen saturation and total hemoglobin concentrations were estimated from the volar forearms of weakly and strongly pigmented subjects using a standard homogeneous model and the present two layer model.

Fig. 1

Illustration of the two layer geometry, optical properties, and illumination considered.

Fig. 2

Schematic (a) and photograph (b) of spatial frequency domain imager used in this study.

Fig. 3

Estimation of ${\tau }_1^{\prime }$ for tissue simulating digital phantom (a) without noise and (b) with 2% noise. Relative percent error between the true and estimated value are shown as a function of wavelength in the upper left corner of each panel.

Fig. 4

Estimation of ${\mu }_{a,2}$ for tissue simulating digital phantom (a) without noise and (b) with 2% noise. Relative percent error between the true and estimated value are shown as a function of wavelength in the upper left corner of each panel.

Fig. 5

Estimation of ${\mu }_{s,2}^{\prime }$ for tissue simulating digital phantom (a) without noise and with (b) 2% noise. Relative percent error between the true and estimated value are shown as a function of wavelength in the upper left corner of each panel.

Fig. 6

Estimated and true optical product of the top layer thickness and absorption coefficient of the two layer silicone phantom. The standard deviations of the estimates at each wavelength are also shown.

Fig. 7

Estimated and true absorption coefficient ${\mu }_{a,2}$ of the supporting layer of silicone phantom representing dermis in skin. The standard deviations of the estimates at each wavelength are also shown.

Fig. 8

Estimated and true reduced scattering coefficient ${\mu }_{s,2}^{\prime }$ of the supporting layer of silicone phantom representing dermis in skin. The standard deviations of the estimates at each wavelength are also shown.

Fig. 9

Estimates of (a) oxyhemoglobin $f_{\mathrm{oxy}}$ and (b) deoxyhemoglobin $f_{\mathrm{deoxy}}$ concentration for four subjects with increasingly darker skin type predicted by a homogeneous model (with and without melanin compensation) and by the present two layer model.

Fig. 10

Estimates of (a) total hemoglobin $f_{\mathrm{heme}}$ and (b) oxygen saturation ${\mathrm{SO}}_2$ for four subjects with increasingly darker skin type predicted by a homogeneous model (with and without melanin compensation) and by the present two layer model.

Fig. 11

(a) The product of epidermal thickness and absorption coefficient as a function of wavelength estimated by the present model. An epidermal thickness $d_1$ of 70 µm and epidermal scattering coefficient ${\mu }_{s,1}^{\prime }$ of $1{\mathrm{mm}}^{-1}$ were assumed. (b) Melanin index ${\mathrm{MI}}_i$ predicted by a homogeneous model and the present two layer model.

Fig. 12

Estimates of the reduced scattering coefficient for four subjects with increasingly darker skin type predicted by a (a) homogeneous model and (b) the present two layer model.

Fig. 13

Oxygen saturation measured from lightly and strongly pigment subjects during a forearm occlusion experiment predicted by a (a) homogeneous model and (b) the present two layer model.