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 (µs') (range: 0.5 to 1.1 mm-1) and absorption coefficient (µa) (range: 0.005-0.015 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 µs' 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.

Shortwave infrared diffuse optical wearable probe for quantification of water and lipid content in emulsion phantoms using deep learning.

Significance: The shortwave infrared (SWIR, ∼900 to 2000 nm) holds promise for label-free measurements of water and lipid content in thick tissue, owed to the chromophore-specific absorption features and low scattering in this range. In vivo water and lipid estimations have potential applications including the monitoring of hydration, volume status, edema, body composition, weight loss, and cancer. To the best of our knowledge, there are currently no point-of-care or wearable devices available that exploit the SWIR wavelength range, limiting clinical and at-home translation of this technology. Aim: To design and fabricate a diffuse optical wearable SWIR probe for water and lipid quantification in tissue. Approach: Simulations were first performed to confirm the theoretical advantage of SWIR wavelengths over near infrared (NIR). The probe was then fabricated, consisting of light emitting diodes at three wavelengths (980, 1200, 1300 nm) and four source-detector (S-D) separations (7, 10, 13, 16 mm). In vitro validation was then performed on emulsion phantoms containing varying concentrations of water, lipid, and deuterium oxide (D2O). A deep neural network was developed as the inverse model for quantity estimation. Results: Simulations indicated that SWIR wavelengths could reduce theoretical water and lipid extraction errors from ∼6% to ∼1% when compared to NIR wavelengths. The SWIR probe had good signal-to-noise ratio (>32 dB up to 10 mm S-D) and low drift (<1.1% up to 10 mm S-D). Quantification error in emulsion phantoms was 2.1±1.1% for water and -1.2±1.5% for lipid. Water estimation during a D2O dilution experiment had an error of 3.1±3.7%. Conclusions: This diffuse optical SWIR probe was able to quantify water and lipid contents in vitro with good accuracy, opening the door to human investigations.