Single-distance and dual-slope frequency-domain near-infrared spectroscopy to assess skeletal muscle hemodynamics

Significance: Non-invasive optical measurements of deep tissue (e.g., muscle) need to take into account confounding contributions from baseline and dynamic optical properties of superficial tissue (adipose tissue).

Aim: Discriminate superficial and deep tissue hemodynamics using data collected with frequency-domain (FD) near-infrared spectroscopy (NIRS) in a dual-slope (DS) configuration.

Approach: Experimental data were collected in vivo on the forearm of three human subjects during a 3-min arterial occlusion or 1-min venous occlusion. Theoretical data were generated using diffusion theory for two-layered media with varying values of the reduced scattering coefficient (${\mu }_s^{\text{'}}$) (range: 0.5 to $1.1{\mathrm{mm}}^{-1}$) and absorption coefficient (${\mu }_a$) (range: $0.005-0.015{\mathrm{mm}}^{-1}$) of the two layers, and top layer thickness (range: 2 to 8 mm). Data were analyzed using diffusion theory for a homogeneous semi-infinite medium.

Results: Experimental data in vivo were consistent with simulated data for a two-layered medium with a larger ${\mu }_s^{\text{'}}$ in the top layer, comparable absorption changes in the top and bottom layers during venous occlusion, and smaller absorption changes in the top vs. bottom layers during arterial occlusion.

Conclusions: The dataset generated by DS FD-NIRS may allow for discrimination of superficial and deep absorption changes in two-layered media, thus lending itself to individual measurements of hemodynamics in adipose and muscle tissue.