Absolute quantification of redox state of cytochrome c oxidase and hemoglobin in human tissue using continuous-wave broadband near-infrared spectroscopy.

Using the diffusion approximation, we developed a continuous-wave (CW) broadband near-infrared spectroscopy (bb-NIRS) system to noninvasively quantify the absolute concentrations of redox state of cytochrome c oxidase [CCO] and other major human tissue chromophores. The algorithm leverages characteristic spectral features obtained from the first and second derivative domains of wavelength-dependent extinction coefficients. The validation of the method was performed through computational simulations to evaluate estimation accuracy, followed by in vivo measurements on the forearms of 20 participants and the foreheads of 18 participants. The recovered values showed strong agreement with established physiological parameters within maximum of 10% deviation, prior literature, and concurrent frequency-domain near-infrared spectroscopy (FD-NIRS) measurements. These findings demonstrate the feasibility of a CW-based approach for accurate and noninvasive absolute quantification of human tissue components.