SignificancePulsatile blood oxygen saturation (SpO2) via pulse oximetry is a valuable clinical metric for assessing oxygen delivery. Individual anatomical features, including skin tone, may affect current optical pulse oximetry methods.AimWe developed an optical pulse oximetry method based on dual-ratio (DR) measurements to suppress individual anatomical confounds on SpO2.ApproachWe designed a DR-based finger pulse oximeter, hypothesizing that DR would suppress confounds from optical coupling and superficial tissue absorption. This method is tested using Monte Carlo simulations and in vivo experiments.ResultsDifferent melanosome volume fractions in the epidermis, a surrogate for skin tone, cause changes in the recovered SpO2 on the order of 1% in simulation and in vivo. Different heterogeneous pulsatile hemodynamics cause greater changes on the order of 10% in simulations. SpO2 recovered with DR measurements showed less variability than the traditional single-distance (SD) transmission method.ConclusionsFor the models and methods considered here, SpO2 measurements are strongly impacted by heterogeneous pulsatile hemodynamics. This variability may be larger than the skin tone bias, which is a known confound in SpO2 measurements. The partial suppression of variability in the SpO2 recovered by DR suggests the promise of DR for pulse oximetry.
While near-infrared spectroscopy has been shown to be a useful technique for the non-invasive monitoring of cerebral hemodynamics, sensitivity to superficial hemodynamic changes continues to be a challenge in the field. Here, we apply a previously designed hexagonal dual-slope module to human subjects during a visual stimulation protocol. The enrolled subjects have different scalp-to-cortex distances, as measured with ultrasound imaging. This work investigates the cerebral hemodynamic response to visual stimulation as measured non-invasively by optical intensity (I) collected with a single distance (SD) or dual-slope (DS) source-detector arrangement [SDI(25 mm), SDI(35 mm), DSI]. The observed results in relation to scalp-to-cortex distance are then validated through theoretical simulations in two-layered media, and these simulations confirm that as the cortical depth increases the sensitivity to the brain decreases faster for single-distance measurements than dual-slope measurements. This finding supports the value of dual-slope measurements for enhanced sensitivity to the brain.
Measuring scattering samples’ absolute optical properties is valuable in bio-medicine, agriculture, material characterization, and beyond. These measurements may be achieved by analyzing the sample’s frequency-domain diffuse reflectance or transmittance. However, successfully achieving these absolute measurements is complicated by the need for calibration. We present a calibration-free method to conduct these measurements. This method, dubbed dual-ratio, creates a measured data type that cancels most coupling and calibration factors that confound traditional reflectance or transmittance measurements, specifically multiplicative factors associated with source emission, detector efficiency, or optical coupling with the sample. Furthermore, we have applied the dual-ratio method to measure absolute optical properties of a small-volume sample (i.e., the size of a standard cuvette). Applications include tissue hemodynamics and oxygenation assessment (brain, muscle, etc.), water turbidity and chemical analysis, food quality determination, and more. This work builds on our previous work developing the dual-ratio method for a cuvette-sized sample volume. We also expand on other previous work, combining frequency-domain and continuous-wave measurements to achieve absolute broadband absorption spectra. Optical properties recovered by small-volume dual-ratio agree well with semi-infinite medium multi-distance scanning, which we consider the gold standard. Such calibration-free methods may make sample quantitative analysis more accessible and allow for easy quantitative measurements outside the traditional laboratory setting.
We used frequency-domain (FD) near-infrared spectroscopy (NIRS) in a dual-slope (DS) configuration to non-invasively assess skeletal muscle hemodynamics in the human forearm muscle. Our objective is to leverage FD-NIRS data (intensity and phase) collected in DS mode (a combination of single-distance and multi-distance measurements) to differentiate superficial and deep tissue hemodynamics. FD-NIRS signals feature contributions from adipose tissue (AT), muscle tissue (MT), and possibly bone tissue, in cases of relatively thin AT and MT layers. We performed measurements of blood flow (BF) and oxygen consumption (OC) using venous occlusion and arterial occlusion protocols, respectively. Additionally, we performed theoretical simulations based on diffusion theory to guide the interpretation of our experimental results. First, we were able to show that our experimental results are consistent with a top layer (adipose tissue) that is more scattering than a bottom layer (muscle) and that absorption changes are greater in the top layer during venous occlusion whereas they are greater in the bottom layer during arterial occlusion, in agreement with previous results on different human subjects [C. Fernandez et al., J. Biomed. Opt. 28, 125004 (2023)]. Second, we started measurements on subjects featuring a range of thicknesses of adipose and muscle tissue to explore the feasibility of discriminating superficial and deeper hemodynamics using the full information content of the data collected with DS FD-NIRS.
SignificanceNon-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).AimDiscriminate superficial and deep tissue hemodynamics using data collected with frequency-domain (FD) near-infrared spectroscopy (NIRS) in a dual-slope (DS) configuration.ApproachExperimental 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.ResultsExperimental 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.ConclusionsThe 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.
SignificanceThe 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.AimTo design and fabricate a diffuse optical wearable SWIR probe for water and lipid quantification in tissue.ApproachSimulations 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.ResultsSimulations 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 % .ConclusionsThis diffuse optical SWIR probe was able to quantify water and lipid contents in vitro with good accuracy, opening the door to human investigations.
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