23 research outputs found
Method for Measuring Absolute Optical Properties of Turbid Samples in a Standard Cuvette
Many applications seek to measure a sample's absorption coefficient spectrum
to retrieve the chemical makeup. Many real world samples are optically turbid,
causing scattering confounds which many commercial spectrometers cannot
address. Using diffusion theory and considering absorption and reduced
scattering coefficients on the order of 0.01 per mm and 1 per mm, respectively,
we develop a method which utilizes frequency-domain to measure absolute optical
properties of turbid samples in a standard cuvette (45 mm by 10 mm by 10 mm).
Inspired by the self calibrating method, which removes instrumental confounds,
the method uses measurements of the diffuse complex transmittance at two sets
of two different source-detector distances. We find: this works best for highly
scattering samples (reduced scattering coefficient above 1 per mm); higher
relative error in the absorption coefficient compared to the reduced scattering
coefficient; accuracy is tied to knowledge of the samples index of refraction.
Noise simulations with 0.1 percent amplitude and 0.1 deg (1.7 mrad) phase
uncertainty find errors in absorption and reduced scattering coefficients of 4
percent and 1 percent, respectively. We expect that higher error in the
absorption coefficient can be alleviated with highly scattering samples and
that boundary condition confounds may be suppressed by designing a cuvette with
high index of refraction. Further work will investigate implementation and
reproducibility
Optical imaging and spectroscopy for the study of the human brain: status report
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.
Keywords: DCS; NIRS; diffuse optics; functional neuroscience; optical imaging; optical spectroscop
Domain Adaptation for Robust Workload Level Alignment Between Sessions and Subjects using fNIRS
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. In order 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:
multi-class 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
Optical imaging and spectroscopy for the study of the human brain: status report.
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
Multi-Distance Frequency-Domain Optical Measurements of Coherent Cerebral Hemodynamics
We report non-invasive, bilateral optical measurements on the forehead of five healthy human subjects, of 0.1 Hz oscillatory hemodynamics elicited either by cyclic inflation of pneumatic thigh cuffs, or by paced breathing. Optical intensity and the phase of photon-density waves were collected with frequency-domain near-infrared spectroscopy at seven source-detector distances (11–40 mm). Coherent hemodynamic oscillations are represented by phasors of oxyhemoglobin (O) and deoxyhemoglobin (D) concentrations, and by the vector D/O that represents the amplitude ratio and phase difference of D and O. We found that, on an average, the amplitude ratio (|D/O|) and the phase difference (∠(D/O)) obtained with single-distance intensity at 11–40 mm increase from 0.1° and −330° to 0.2° and −200°, respectively. Single-distance phase and the intensity slope featured a weaker dependence on source-detector separation, and yielded |D/O| and ∠(D/O) values of about 0.5 and −200°, respectively, at distances greater than 20 mm. The key findings are: (1) Single-distance phase and intensity slope are sensitive to deeper tissue compared to single-distance intensity; (2) deeper tissue hemodynamic oscillations, which more closely represent the brain, feature D and O phasors that are consistent with a greater relative flow-to-volume contributions in brain tissue compared to extracerebral, superficial tissue
Coherent Spontaneous Hemodynamics in the Human Brain
Goal: This work investigates the presence of cerebral hemodynamics (namely Oxy (O) and Deoxy (D) hemoglobin concentrations) that are coherent with spontaneous oscillations in Arterial Blood Pressure (ABP) in 78 healthy subjects during a driving simulation task. Methods: Spatially resolved O and D were measured on the prefrontal cortex with multi-channel near-infrared spectroscopy (NIRS). Wavelet coherence and phasor analysis were performed between O and ABP, and between D and ABP to evaluate the amplitude ratio, phase difference, and duration of significant coherence. Results: In the low-frequency range, oscillations at 0.1 Hz featured significant coherence for the longest time fraction (∼10%–30%). At this frequency, the amplitude ratio and phase difference showed a greater variance across subjects than over cortical locations, and no significant difference between driving tasks and baseline. Conclusions: Measuring low-frequency cerebral hemodynamics that are coherent with systemic ABP holds promise for non-invasive assessment of cerebral perfusion and autoregulation at the cerebral microvascular level
Dual-Slope Diffuse Reflectance Instrument for Calibration-Free Broadband Spectroscopy
This work presents the design and validation of an instrument for dual-slope broadband diffuse reflectance spectroscopy. This instrument affords calibration-free, continuous-wave measurements of broadband absorbance of optically diffusive media, which may be translated into absolute absorption spectra by adding frequency-domain measurements of scattering at two wavelengths. An experiment on a strongly scattering liquid phantom (milk, water, dyes) confirms the instrument’s ability to correctly identify spectral features and measure absolute absorption. This is done by sequentially adding three dyes, each featuring a distinct spectral absorption, to the milk/water phantom. After each dye addition, the absorption spectrum is measured, and it is found to reproduce the spectral features of the added dye. Additionally, the absorption spectrum is compared to the absorption values measured with a commercial frequency-domain instrument at two wavelengths. The measured absorption of the milk/water phantom quantitatively agrees with the known water absorption spectrum (R2=0.98), and the measured absorption of the milk/water/dyes phantom quantitatively agrees with the absorption measured with the frequency-domain instrument in six of eight cases. Additionally, the measured absorption spectrum correctly recovers the concentration of one dye, black India ink, for which we could accurately determine the extinction spectrum (i.e., the specific absorption per unit concentration). The instrumental methods presented in this work can find applications in quantitative spectroscopy of optically diffusive media, and particularly in near-infrared spectroscopy of biological tissue