Development and validation of a melanin correction factor for a smartphone-based NIRS optical imaging device

A smartphone-based NIRS optical imaging device (SPOT – smartphone-based oxygenation tool) was developed to evaluate hemoglobin-based tissue oxygenation maps toward wound healing applications. Diffuse reflectance (690, 810, and 830 nm) on the wound is recorded and used to generate tissue oxygenation maps using modified Beer-Lambert’s lawbased light propagation model. However, the skin surrounding the wound contains melanin, which is a significant absorber in the NIR range. It largely affects the acquired diffuse reflectance and eventually the tissue oxygenation measurements. Hence, the objective of this study was to develop a melanin correction factor and validate via phantom studies. For the melanin correction factor, Monte Carlo-based light propagation simulations on a two-layer skin model (x-4cm, y-4cm, epidermis: z-2mm, dermis: z-30mm), at SPOT's wavelengths (690, 810, 830 nm) were conducted. The epidermal melanin volume fraction was varied to achieve absorption coefficients (690nm: 0.29-100.2 𝑐𝑚⁻¹, 810nm: 0.17-58.8 𝑐𝑚⁻¹, 830nm: 0.16-54.2 𝑐𝑚⁻¹representative of the Fitzpatrick skin types. The melanin correction factor was developed for each wavelength separately. The sensitivity of the correction factor (at each respective wavelength) was evaluated by varying the epidermal thickness (0.015-0.55cm) and varying the dermal optical properties (690nm: 0.12-1.53 𝑐𝑚⁻¹, 810nm: 0.02- 1.22 𝑐𝑚⁻¹, 830nm: 0.32-0.83 𝑐𝑚⁻¹ for each FST. Across the wavelengths, the melanin correction factors for each FST showed a low variation when epidermal thickness (<1.5%) and dermal optical properties (<5%) were varied. Ongoing validation studies include developing and standardizing agar-based two-layered phantoms of different FSTs. This will be followed by imaging studies using SPOT on the phantoms and applying the correction factor to evaluate the accuracy of tissue oxygenation measurements.