Exploring the impact and influence of melanin on frequency-domain near-infrared spectroscopy measurements.

Significance: Near-infrared spectroscopy (NIRS) is a non-invasive optical method that measures changes in hemoglobin concentration and oxygenation. The measured light intensity is susceptible to reduced signal quality due to the presence of melanin.

Aim: We quantify the influence of melanin concentration on NIRS measurements taken with a frequency-domain near-infrared spectroscopy system using 690 and 830 nm.

Approach: Using a forehead NIRS probe, we measured 35 healthy participants and investigated the correlation between melanin concentration indices, which were determined using a colorimeter, and several key metrics from the NIRS signal. These metrics include signal-to-noise ratio (SNR), two measurements of oxygen saturation (arterial oxygen saturation, ${\mathrm{SpO}}_2$ , and tissue oxygen saturation, ${\mathrm{StO}}_2$ ), and optical properties represented by the absorption coefficient ( ${\mu }_a$ ) and the reduced scattering coefficient ( ${\mu }_s^{\text{'}}$ ).

Results: We found a significant negative correlation between the melanin index and the SNR estimated in oxy-hemoglobin signals ( $r_s=-0.489$ , $p=0.006$ ) and ${\mathrm{SpO}}_2$ levels ( $r_s=-0.413$ , $p=0.023$ ). However, no significant changes were observed in the optical properties and ${\mathrm{StO}}_2$ ( $r_s=-0.146$ , $p=0.44$ ).

Conclusions: We found that estimated SNR and ${\mathrm{SpO}}_2$ values show a significant decline and dependence on the melanin index, whereas ${\mathrm{StO}}_2$ and optical properties do not show any correlation with the melanin index.