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.
KEYWORDS: Signal to noise ratio, In vivo imaging, Fluorescence, Flow cytometry, Tissues, Autofluorescence, Systems modeling, Optical testing, Near infrared, Monte Carlo methods
Diffuse in-vivo Flow Cytometry (DiFC) is a fluorescence sensing method to detect labeled circulating cells
in-vivo non-invasively. The Dual-Ratio (DR) is an optical measurement method that aims to suppress most noise
and enhance SNR in deep tissue regions. We investigate DR applied to Near-InfraRed (NIR) DiFC to improve
circulating cells’ maximum detectable depth and SNR. Results indicate that two critical conditions control
the success of DR DiFC. First, the fraction of noise that DR methods cannot cancel (i.e., non-multiplicative
factors) cannot dominate. Second, DR DiFC has an advantage regarding SNR if the spatial distribution of tissue
autofluorescence contributors is surface-weighted.
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.
KEYWORDS: Signal to noise ratio, Fluorophores, Fluorescence, Tissues, Signal detection, Target detection, Near infrared, Interference (communication), In vivo imaging, Skin
SignificanceDiffuse in vivo flow cytometry (DiFC) is an emerging fluorescence sensing method to non-invasively detect labeled circulating cells in vivo. However, due to signal-to-noise ratio (SNR) constraints largely attributed to background tissue autofluorescence (AF), DiFC’s measurement depth is limited.AimThe dual ratio (DR)/dual slope is an optical measurement method that aims to suppress noise and enhance SNR to deep tissue regions. We aim to investigate the combination of DR and near-infrared (NIR) DiFC to improve circulating cells’ maximum detectable depth and SNR.ApproachPhantom experiments were used to estimate the key parameters in a diffuse fluorescence excitation and emission model. This model and parameters were implemented in Monte Carlo to simulate DR DiFC while varying noise and AF parameters to identify the advantages and limitations of the proposed technique.ResultsTwo key factors must be true to give DR DiFC an advantage over traditional DiFC: first, the fraction of noise that DR methods cannot cancel cannot be above the order of 10% for acceptable SNR. Second, DR DiFC has an advantage, in terms of SNR, if the distribution of tissue AF contributors is surface-weighted.ConclusionsDR cancelable noise may be designed (e.g., through the use of source multiplexing), and indications point to the AF contributors’ distribution being truly surface-weighted in vivo. Successful and worthwhile implementation of DR DiFC depends on these considerations, but results point to DR DiFC having possible advantages over traditional DiFC.
Functional Near-InfraRed Spectroscopy (NIRS) (fNIRS) is a powerful method for non-invasively measuring cerebral hemodynamics on human subjects. Measurement contamination from superficial tissue which do not represent the brain continues to be an issue. We have proposed the Dual-Slope (DS) approach which is less sensitive to superficial tissue compared to typical Single-Distance (SD) methods. This DS method has been applied to Diffuse Optical Imaging (DOI), designing and constructing a large source-detector array. Previous results suggested that DS phase (Φ) has intrinsically higher sensitivity to the brain compared to SD Intensity (I). To further investigate this finding, on a large population of subjects, a modular DS array is designed. Allowing for collection from different cortical locations during various protocols. These source-detector modules are hexagonal and contain 4 intra-module DS sets. Tessellation greatly expands the number of measurement sets through the creation of inter-module DS sets. In one example, we found a tessellation of 7 modules which generated 94 DS sets. The modules will be used to enable large population DS DOI studies. Here we present one example trace during a 3-back protocol. Examination of the DS traces suggest the expected higher DS Φ sensitivity to cerebral hemodynamics. Further, close observation of the results demonstrate the importance of considering both the Oxy-hemoglobin concentration change (ΔO) and Deoxy-hemoglobin concentration change (ΔD) during such protocols. The results indicated that if one observed only ΔO they would have mis-identified brain activation in the short SD I measurement. Other data-types and ΔD dynamics suggested that the short SD I was dominated by superficial blood-volume instead of the blood-flow dynamics associated with brain activation.
Measurements of absolute optical properties of scattering samples is valuable in the field of bio-medicine and beyond. However, achieving these measurements is complicated by the need for calibration and by the large sample volumes typically needed to meet common diffusion theory models. We propose a method for calibration free absolute measurements of the absorption coefficient (µa) and reduced scattering coefficient (µ 0 s ) in a relatively small volume (the size of a standard cuvette; 45 mm × 10 mm × 10 mm). This method utilizes the previously proposed Self-Calibrating (SC) / Dual-Slope (DS) geometry by placing two light sources on one face of the cuvette and two optical detectors on the opposite face. This leads to the proposal of the Dual-Ratio of the complex Transmittance (Te) (DR{Te}), a method with the same advantages as SC / DS but with less geometric requirements and constraints. Here we confirm that measurement of DR{Te} from a cuvette may be converted to absolute optical properties. We then investigate differences between two choices of forward models for optical measurements in a cuvette, either Monte-Carlo or diffusion theory. A discrepancy between the two was found, which leads to an error of 10 % in µa and no error in µ's when Monte-Carlo was used to generate data and diffusion theory used to invert it. This result highlights the importance of which model is chosen for the inverse problem when this method is implemented in practice. Implementations would need to evaluate different models against ground truths to identify the optimal method for the measurement of absolute optical properties in a cuvette.
In this work, a miniaturized heterodyning FD-NIRS instrument was presented. The device uses a dual slope probe, which removes the need for pre-calibration. The low-footprint system consists of only the circuit board, the probe and a Raspberry-Pi. Four lasers (685 nm and 830 nm) and two avalanche photodiodes are used, where the lasers are modulated with 80 MHz and the APD signals are amplified and downconverted by the analog front end of the instrument: a custom designed fully differential ASIC in 130 nm CMOS technology. Solid phantom measurements revealed <9% error and significant stability for long-term measurements.
We propose novel frequency-domain data types, that show hybrid features to those of phase and AC intensity, i.e., better CNR features than phase data while preserving preferential sensitivity to deep tissue regions (like phase data). We show the CNR features of some of the novel data types in the dual-slope source-detector arrangement in the semi-infinite homogeneous medium and in the two-layer geometries. The results show that these novel data types indeed may have some hybrid features of AC and phase and may have a potential application in imaging of tissue.
SignificanceThis work targets the contamination of optical signals by superficial hemodynamics, which is one of the chief hurdles in non-invasive optical measurements of the human brain.AimTo identify optimal source–detector distances for dual-slope (DS) measurements in frequency-domain (FD) near-infrared spectroscopy (NIRS) and demonstrate preferential sensitivity of DS imaging to deeper tissue (brain) versus superficial tissue (scalp).ApproachTheoretical studies (in-silico) based on diffusion theory in two-layered and in homogeneous scattering media. In-vivo demonstrations of DS imaging of the human brain during visual stimulation and during systemic blood pressure oscillations.ResultsThe mean distance (between the two source–detector distances needed for DS) is the key factor for depth sensitivity. In-vivo imaging of the human occipital lobe with FD NIRS and a mean distance of 31 mm indicated: (1) greater hemodynamic response to visual stimulation from FD phase versus intensity, and from DS versus single-distance (SD); (2) hemodynamics from FD phase and DS mainly driven by blood flow, and hemodynamics from SD intensity mainly driven by blood volume.ConclusionsDS imaging with FD NIRS may suppress confounding contributions from superficial hemodynamics without relying on data at short source–detector distances. This capability can have significant implications for non-invasive optical measurements of the human brain.
This report is the second part of a comprehensive two-part series aimed at reviewing an extensive and diverse toolkit of novel methods to explore brain health and function. While the first report focused on neurophotonic tools mostly applicable to animal studies, here, we highlight optical spectroscopy and imaging methods relevant to noninvasive human brain studies. We outline current state-of-the-art technologies and software advances, explore the most recent impact of these technologies on neuroscience and clinical applications, identify the areas where innovation is needed, and provide an outlook for the future directions.
Functional Near-InfraRed Spectroscopy (fNIRS) measures cerebral hemodynamics associated with brain activation. Non-invasive optical measurements of cerebral hemodynamics are often confounded by superficial, extra-cerebral hemodynamics and by instrumental and motion artifacts. These confounds are especially prominent in optical intensity data collected at a single source-detector distance. Alternatively, slope methods and frequency-domain measurements of the phase of photon-density waves have been proposed. Here, we first demonstrate the ability of a special slope method (dual-slope) to efficiently suppress instrumental artifacts. Then, a dual-slope imaging array is utilized to generate and compare single-distance and dual-slope intensity and phase data collected on the visual cortex of a human subject during a contrast reversing visual stimulation protocol. The measured hemodynamic traces associated with visual stimulation exhibit a larger amplitude when they are derived from dual-slope versus single-distance data, and from phase versus intensity data. In particular, the functional hemodynamics obtained from dual-slope phase data feature the largest amplitude. These results indicate the greater sensitivity to brain tissue achieved by dual-slope versus single-distance data, and by phase versus intensity data. The conclusion of this work is that dual-slope intensity (in continuous-wave fNIRS) and dual-slope or single-distance phase (in frequency-domain fNIRS) appear to be most effective for functional brain measurements, with the significant practical advantage offered by the minimal sensitivity of dual-slope measurements to a variety of artifacts.
KEYWORDS: Absorption, Chromophores, Calibration, Diffuse reflectance spectroscopy, Spectroscopy, In vivo imaging, Data modeling, Tissues, Scattering, Near infrared
We present a calibration-free instrument to measure absolute broadband absorption spectra of biological tissue. Initial measurements in skeletal muscle show that the fit to the data improves by introducing a spectrally flat absorption background.
We have recently introduced a Dual-Slope (DS) method implemented with Frequency-Domain (FD) NearInfraRed Spectroscopy (NIRS). Its chief advantages are a preferential sensitivity to deeper tissue and the suppression of instrumental artifacts. Thus-far, the work using the DS method has focused on local measurements of tissue hemodynamics. The next step in the development of DS is the measurement of spatially resolved absorption changes of tissue in vivo. To achieve this, we designed a DS imaging array for applications on human tissue. We utilized this array to measure maps of absolute optical properties in human skeletal muscle, and muscle hemodynamics during venous occlusion. In each case, spatial maps were created. The maps of baseline absolute optical properties showed higher scattering in connective tissue and higher absorption in muscle tissue. The analysis of hemodynamics found a greater blood accumulation during venous occlusion in muscle. Overall, muscle hemodynamics were shown to be spatially variable over a large area, suggesting the importance of imaging (as opposed to single-location) measurements. The preliminary data on human subjects with this new DS imaging array pave the way for applications in functional NIRS (fNIRS) for mapping brain activation.
One of the chief applications of diffuse optical spectroscopy is the measurement of chromophore concentrations in biological tissue, which requires measurements of tissue absorption. To achieve absolute absorption measurements, two chief confounds must be accounted for: instrumental contributions and tissue scattering. To account for instrumental contributions, a preliminary calibration on a phantom of known optical properties is typically done. The need for a calibration is eliminated by self-calibrating or dual-slope techniques using specially designed probe geometries. A technique that is capable of measuring tissue scattering is frequency domain near-infrared spectroscopy. However, it is typically not implemented for a spectrum of wavelengths due to instrumental complexity. Here we present a technique that combines self-calibrating frequency-domain at two wavelengths, to account for tissue scattering, and dual-slope continuous-wave broadband diffuse reflectance spectroscopy to achieve spectral measurements of absolute absorption between 600 nm and 1064 nm without any need for calibration. We apply this technique to two human tissues in vivo to determine concentrations of oxy-hemoglobin, deoxy-hemoglobin, lipids, and water. We found that the quality of the spectral fits may be significantly improved by the inclusion of a wavelength-independent background absorption. This leads to a discussion on the origin of this background absorption, and on the meaning of the chromophore concentrations that are recovered from spectral analysis. Current work is seeking to further understand and possibly correct for this apparent background absorption.
In a study on one patient during hemodialysis, we used near-infrared spectroscopy (NIRS) to measure coherent oscillations of cerebral concentrations of oxyhemoglobin ([HbO2]), deoxyhemoglobin ([Hb]), and total-hemoglobin ([HbT]) induced by systemic oscillations in mean arterial pressure (MAP) at a frequency of 0.07 Hz. During hemodialysis, we observed that the phase of [Hb] versus [HbO2] becomes less negative, whereas the phase of [HbT] versus MAP becomes more negative. By applying a quantitative hemodynamic model, we assign these phase changes to an increase in venous blood transit time and a less effective cerebral autoregulation during the hemodialysis process.
In this work we provide some examples of sensitivity of the dual slope method to localized absorption changes in the layered geometry. Reasonably, this model geometry better represents many types of tissue. The sensitivity is shown in a two- and three-layer geometry for alternating current (AC) and phase data for both point-like and layered-like absorption perturbations. Contrary to the homogeneous medium geometry, where the ratio of deep to superficial tissue sensitivity of phase is always greater than that of AC, this is not always the case in the layered geometry. Therefore, depending on the targeted tissue, subject and protocol, in some cases it might be preferable to use AC dual-slopes, whereas in other cases phase dual-slope may be a better choice.
The use of phase (Φ) data collected in Frequency-Domain Near-InfraRed Spectroscopy (FD-NIRS) has not been widespread in measurements of skeletal muscle and has mainly been applied to measure absolute optical properties. We show that single-distance (SD) Φ has a deeper sensitivity compared to SD intensity (I) and can be more sensitive to oxygen consumption in skeletal muscle underneath superficial adipose tissue. We also show the potential benefit of single-slope (SS) or dual-slope (DS) I or Φ in muscle studies.
KEYWORDS: Matrices, Data modeling, Signal detection, Detection and tracking algorithms, Tissues, Sensors, Motion measurement, Brain, Linear filtering, Filtering (signal processing)
Significance: We demonstrated the potential of using domain adaptation on functional near-infrared spectroscopy (fNIRS) data to classify different levels of n-back tasks that involve working memory.
Aim: Domain shift in fNIRS data is a challenge in the workload level alignment across different experiment sessions and subjects. To address this problem, two domain adaptation approaches—Gromov–Wasserstein (G-W) and fused Gromov–Wasserstein (FG-W) were used.
Approach: Specifically, we used labeled data from one session or one subject to classify trials in another session (within the same subject) or another subject. We applied G-W for session-by-session alignment and FG-W for subject-by-subject alignment to fNIRS data acquired during different n-back task levels. We compared these approaches with three supervised methods: multiclass support vector machine (SVM), convolutional neural network (CNN), and recurrent neural network (RNN).
Results: In a sample of six subjects, G-W resulted in an alignment accuracy of 68 % ± 4 % (weighted mean ± standard error) for session-by-session alignment, FG-W resulted in an alignment accuracy of 55 % ± 2 % for subject-by-subject alignment. In each of these cases, 25% accuracy represents chance. Alignment accuracy results from both G-W and FG-W are significantly greater than those from SVM, CNN, and RNN. We also showed that removal of motion artifacts from the fNIRS data plays an important role in improving alignment performance.
Conclusions: Domain adaptation has potential for session-by-session and subject-by-subject alignment of mental workload by using fNIRS data.
We introduce a novel method to enhance the sensitivity of near-infrared spectroscopy (NIRS) to deep tissue (i.e. brain cortex, skeletal muscle, etc.) in non-invasive diffuse optical measurements. Our method relies on the collection of the phase of photon-density waves, launched by intensity-modulated light in frequency-domain NIRS, from two paired sets of multi-distance data. The two sets of data are combined into a phase dual-slope, which features a stronger sensitivity to deeper vs. superficial tissue. For typical conditions of functional NIRS, the maximum sensitivity of phase dual-slopes is at a depth of ~11 mm, which approaches the depth of cortical tissue.
Near-infrared spectroscopy (NIRS) is a non-invasive optical technique that is sensitive to blood volume, blood flow, and oxygen consumption in biological tissue. In particular, a NIRS-measured quantity that has been previously considered as a surrogate for blood flow measurements is the difference of oxy- and deoxy-hemoglobin concentrations ([HbD] = [HbO2] – [Hb]). We propose a new NIRS method for measurements of cerebral blood flow (CBF), which improves on the [HbD] surrogate by accounting for blood volume contributions and for temporal delays due to the blood transit time in the microvasculature. This new NIRS method relies on concepts of coherent hemodynamics spectroscopy (CHS), and we identify it with the acronym NIRS-CHS. We report a comparison of CBF transient dynamics measured on human subjects with NIRS-CHS, with the [HbD] surrogate and with diffuse correlation spectroscopy (DCS). We found a good agreement between the CBF dynamics measured with NIRS-CHS and with DCS, while the [HbD] dynamics lag because of the delayed effect of CBF on [HbD] due to the capillary and venous blood transit times. The NIRS-CHS method also affords absolute measurements of baseline CBF, for which we found a value of 69 ± 6 ml/100g/min (mean ± standard error) in a group of six healthy volunteers. Further studies to characterize and validate CBF measurements with NIRS-CHS are currently ongoing, with an emphasis on the assessment of accuracy, precision, and reproducibility.
We have compared different methods for analyzing dynamic changes of oxy- and deoxyhemoglobin concentrations oscillations, measured by near infrared spectroscopy (NIRS), during cyclic pneumatic thigh cuff occlusion and release at the frequency of 0.1 Hz. This protocol is usually adopted in coherence hemodynamics spectroscopy (CHS) to induce controlled arterial blood pressure perturbations which drive hemodynamic changes in the brain. It is a general problem of NIRS to differentiate hemodynamic signals originated in the brain from those in the extracerebral tissue layer. The purpose of this study is to gain some understanding about the spatial origin of the oscillating optical signals according to these five different methods of data analysis during the thigh cuff occlusion and release protocol. The results obtained on six human subjects show that similar qualitative behavior of oxy- and deoxyhemoglobin dynamic changes are found by using: (1) modified Beer-Lambert law at far source detector separations (d > 25 mm); (2) DC intensity slope method at d > 25 mm; (3) multi-distance method at d >25 mm; (4) Two-layer modified Beer-Lambert law (using d > 25 mm) when we consider dynamic changes in the second (deeper) layer. At short source-detector separations (d < 15 mm), the hemoglobin concentration changes obtained with the modified Beer-Lambert law are consistent with those obtained for the first (superficial) layer with the two-layer modified Beer-Lambert law. For more quantitative assessment of cerebral dynamic changes, we argue that DC slope or two-layer modified Beer-Lambert law should provide better estimates. We support this claim by comparing the sensitivity to layered absorption perturbations obtained by using the modified BeerLambert law and the DC slope methods.
Oscillations in the tissue concentrations of deoxyHemoglobin ([Hb]) and OxyHemoglobin ([HbO]) can be measured in the human brain using Near InfraRed Spectroscopy (NIRS). These oscillations may be driven by temporal dynamics of Arterial Blood Pressure (ABP). Coherent Hemodynamics Spectroscopy (CHS) is a technique that measures oscillations of [Hb] and [HbO] that are coherent with ABP. These oscillations, at a frequency of 0.1 Hz in this work, can then be interpreted with CHS to get physiologically relevant parameters to monitor cerebral AutoRegulation (AR) and microvascular integrity. Systemic oscillations in ABP can be induced with cyclic inflation and deflation of pneumatic thigh cuffs or by paced breathing. ABP oscillations may also occur spontaneously during resting conditions. Here, these three types of ABP oscillations (induced with thigh cuffs, induced with paced breathing, and spontaneously occurring) are considered, and the phase between coherent [Hb] and [HbO] oscillations is interpreted in terms of AR. In two healthy human subjects, it was found that paced breathing may be subjective, either improving or impairing AR depending on the individual paced breathing amplitude. Cuff cyclic inflations and spontaneous hemodynamics resulted in no significant difference in the relative phase of cerebral [Hb] and [HbO] oscillations at 0.1 Hz. These initial results suggest that spontaneous hemodynamics may be used for CHS in place of induced ABP oscillations, with the advantage of not relying on subject’s actions (like paced breathing) or special equipment (like pneumatic thigh cuffs).
We used coherent hemodynamics spectroscopy (CHS) and near-infrared spectroscopy (NIRS) for dynamic measurements of absolute cerebral blood flow (CBF) in one healthy subject over the prefrontal cortex. Temporal transients in mean arterial pressure (MAP) and CBF were induced by rapid deflation of pneumatic thigh cuffs following a sustained 2-minute occlusion at a super-systolic pressure. We studied the sensitivity of relative and absolute measurements of CBF with NIRS-CHS (CBFNIRS-CHS) to the physiological parameters in the CHS model. The temporal dynamics of CBFNIRS-CHS were compared with co-localized NIRS measurements of hemoglobin difference ([HbD] = [HbO2]−[Hb]), and with diffuse correlation spectroscopy (DCS) measurements of relative CBF. We demonstrated that NIRS-CHS provides quantitative measurements of absolute baseline CBF, and corrects [HbD] estimations of CBF dynamics for blood volume contributions and for blood transit times in the microvasculature resulting in a better agreement with CBF dynamics measured by DCS.
We report a near-infrared spectroscopy (NIRS) study of coherent hemodynamic oscillations measured on the human forehead at multiple source–detector distances (1 to 4 cm). The physiological source of the coherent hemodynamics is arterial blood pressure oscillations at a frequency of 0.1 Hz, induced by cyclic inflation (to a pressure of 200 mmHg) and deflation of two thigh cuffs wrapped around the subject’s thighs. To interpret our results, we use a recently developed hemodynamic model and a phasor representation of the oscillations of oxyhemoglobin, deoxyhemoglobin, and total hemoglobin concentrations in the tissue (phasors O, D, and T, respectively). The increase in the phase angle between D and O at larger source–detector separations is assigned to greater flow versus volume contributions and to a stronger blood flow autoregulation in deeper tissue (brain cortex) with respect to superficial tissue (scalp and skull). The relatively constant phase lag of T versus arterial blood pressure oscillations at all source–detector distances was assigned to competing effects from stronger autoregulation and smaller arterial-to-venous contributions in deeper tissue with respect to superficial tissue. We demonstrate the application of a hemodynamic model to interpret coherent hemodynamics measured with NIRS and to assess the different nature of shallow (extracerebral) versus deep (cerebral) tissue hemodynamics.
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