Geometric decoupling of epidermal melanin and dermal hemoglobin signals using coaxial and spatially resolved NIRS

Epidermal melanin can confound NIRS measurements of tissue oxygen saturation and reduce accuracy across skin tones.

We evaluated whether a coaxial detector can provide a superficial reference that improves absolute ${\mathrm{StO}}_2$ estimation when combined with spatially resolved measurements.

Monte Carlo simulations of multilayer skin were evaluated using mutual information and XGBoost regression, with entire melanin levels held out during validation. Three detector configurations were compared: coaxial only, surface only, and the combined geometry. A secondary experimental analysis examined absolute and baseline referenced oxygenation prediction using validation that excluded one recording at a time.

The combined coaxial plus surface detector configuration reduced average ${\mathrm{StO}}_2$ RMSE from 11.34 to 5.41 percentage points, corresponding to a 52.3% relative reduction based on the displayed fold results. Mutual information analysis showed that the coaxial detector carried substantial information about melanin while carrying negligible direct information about ${\mathrm{StO}}_2$ . In the experimental analysis, baseline-referenced features improved pooled agreement, but the recording-based validation did not establish generalization to entirely unseen participants.

These findings support future experimental validation of integrated coaxial plus spatially resolved probes for melanin-robust absolute tissue oxygenation monitoring.