An Iterative method to quantify epidermal melanin concentration using Spatial Frequency Domain Imaging and a layered...

Spatial Frequency Domain Imaging (SFDI) is a non-invasive diffuse optical imaging technology that quantifies tissue optical properties by projecting spatially modulated light onto a region of interest. By detecting and fitting the diffuse reflectance to a light transport model, tissue absorption (μ_a ) and scattering (μ_s ') coefficients are calculated. However, when measuring skin, bulk μ_a and μ_s ' from a homogeneous light transport model may not correspond with the actual properties of individual skin layers, especially for highly pigmented skin. To obtain physiologically accurate skin optical property values, we propose an iterative method based on a two-layered Monte Carlo model. We initially assume that μ_s ' in darker skin is the same as that in lighter skin, and set the epidermal μ_a as the free parameter when fitting the diffuse reflectance. To test this algorithm, we analyzed data from the forearms of 6 subjects having various levels of pigmentation, at 8 visible-to-near-infrared wavelengths. We compared the measured reflectance to both the homogeneous model and our layered model to quantify fit accuracy. At 471 nm and 526 nm for patients of Fitzpatrick skin types IV-VI (relatively dark skin), the two-layered model provided a 10-20% improvement in fit to the AC reflectance as a function of spatial frequency, compared to the homogeneous model. These improved fits yielded epidermal absorption coefficients that were notably higher than the bulk μ_a from the homogeneous model. Fitting the extracted epidermal μ_a to a melanin extinction spectrum¹ enabled estimation of the melanin concentration in the epidermis.