Significance: Developing algorithms for estimating blood oxygenation from snapshot multispectral imaging (MSI) data is challenging due to the complexity of sensor characteristics and photon transport modeling in tissue. We circumvent this using a method where artificial neural networks (ANNs) are trained on in vivo MSI data with target values from a point-measuring reference method.

Aim: To develop and evaluate a methodology where a snapshot filter mosaic camera is utilized for imaging skin hemoglobin oxygen saturation (SO2), using ANNs.

Approach: MSI data were acquired during occlusion provocations. ANNs were trained to estimate SO2 with MSI data as input, targeting data from a validated probe-based reference system. Performance of ANNs with different properties and training data sets was compared.

Results: The method enables spatially resolved estimation of skin tissue SO2. Results are comparable to those acquired using a Monte-Carlo-based approach when relevant training data are used.

Conclusions: Training an ANN on in vivo MSI data covering a wide range of target values acquired during an occlusion protocol enable real-time estimation of SO2 maps. Data from the probe-based reference system can be used as target despite differences in sampling depth and measurement position.

Significance: For optical methods to accurately assess hemoglobin oxygen saturation in vivo, an independently verifiable tissue-like standard is required for validation. For this purpose, we propose three hemoglobin preparations and evaluate methods to characterize them.

Aim: To spectrally characterize three different hemoglobin preparations using multiple spectroscopic methods and to compare their absorption spectra to commonly used reference spectra.

Approach: Absorption spectra of three hemoglobin preparations in solution were characterized using spectroscopic collimated transmission: whole blood, lysed blood, and ferrous-stabilized hemoglobin. Tissue-mimicking phantoms composed of Intralipid, and the hemoglobin solutions were characterized using spatial frequency-domain spectroscopy (SFDS) and enhanced perfusion and oxygen saturation (EPOS) techniques while using yeast to deplete oxygen.

Results: All hemoglobin preparations exhibited similar absorption spectra when accounting for methemoglobin and scattering in their oxyhemoglobin and deoxyhemoglobin forms, respectively. However, systematic differences were observed in the fitting depending on the reference spectra used. For the tissue-mimicking phantoms, SFDS measurements at the surface of the phantom were affected by oxygen diffusion at the interface with air, associated with higher values than for the EPOS system.

Conclusions: We show the validity of different blood phantoms and what considerations need to be addressed in each case to utilize them equivalently.

Significance: Hemoglobin oxygen saturation and red blood cell (RBC) tissue fraction are important parameters when assessing microvascular status. Functional information can be attained using temporally resolved measurements performed during stimulus–response protocols. Pointwise assessments can currently be conducted with probe-based systems. However, snapshot multispectral imaging (MSI) can be used for spatial–temporal measurements.

Aim: To validate if hemoglobin oxygen saturation and RBC tissue fraction can be quantified using a snapshot MSI system and an inverse Monte Carlo algorithm.

Approach: Skin tissue measurements from the MSI system were compared to those from a validated probe-based system during arterial and venous occlusion provocation on 24 subjects in the wavelength interval 450 to 650 nm, to evaluate a wide range of hemoglobin oxygen saturation and RBC tissue fraction levels.

Results: Arterial occlusion results show a mean linear regression R2 = 0.958 for hemoglobin oxygen saturation. Comparing relative RBC tissue fraction during venous occlusion results in R2 = 0.925. The MSI system shows larger dynamic changes than the reference system, which might be explained by a deeper sampling including more capacitance vessels.

Conclusions: The snapshot MSI system estimates hemoglobin oxygen saturation and RBC tissue fraction in skin microcirculation showing a high correlation (R2 > 0.9 in most subjects) with those measured by the reference method.

There are many techniques using sensors and wearable devices for detecting and monitoring patients with Parkinson's disease (PD). A recent development is the utilization of human interaction with computer keyboards for analyzing and identifying motor signs in the early stages of the disease. Current designs for classification of time series of computer-key hold durations recorded from healthy control and PD subjects require the time series of length to be considerably long. With an attempt to avoid discomfort to participants in performing long physical tasks for data recording, this paper introduces the use of fuzzy recurrence plots of very short time series as input data for the machine training and classification with long short-term memory (LSTM) neural networks. Being an original approach that is able to both significantly increase the feature dimensions and provides the property of deterministic dynamical systems of very short time series for information processing carried out by an LSTM layer architecture, fuzzy recurrence plots provide promising results and outperform the direct input of the time series for the classification of healthy control and early PD subjects.

Spatial frequency domain imaging (SFDI) utilizes a digital light processing (DLP) projector for illuminating turbid media with sinusoidal patterns. The tissue absorption (μ_a) and reduced scattering coefficient (μ^,_s) are calculated by analyzing the modulation transfer function for at least two spatial frequencies. We evaluated different illumination strategies with a red, green and blue light emitting diodes (LED) in the DLP, while imaging with a filter mosaic camera, XiSpec, with 16 different multi-wavelength sensitive pixels in the 470-630 nm wavelength range. Data were compared to SFDI by a multispectral camera setup (MSI) consisting of four cameras with bandpass filters centered at 475, 560, 580 and 650 nm. A pointwise system for comprehensive microcirculation analysis was used (EPOS) for comparison. A 5-min arterial occlusion and release protocol on the forearm of a Caucasian male with fair skin was analyzed by fitting the absorption spectra of the chromophores HbO₂, Hb and melanin to the estimatedμ_a. The tissue fractions of red blood cells (fRBC), melanin (/mel) and the Hb oxygenation (^S0₂ ) were calculated at baseline, end of occlusion, early after release and late after release. EPOS results showed a decrease in ^S0₂ during the occlusion and hyperemia during release (^S0₂ = 40%, 5%, 80% and 51%). The fRBC showed an increase during occlusion and release phases. The best MSI resemblance to the EPOS was for green LED illumination (^S0₂ = 53%, 9%, 82%, 65%). Several illumination and analysis strategies using the XiSpec gave un-physiological results (e.g. negative ^S0₂ ). XiSpec with green LED illumination gave the expected change in /RBC , while the dynamics in ^S0₂ were less than those for EPOS. These results may be explained by the calculation of modulation using an illumination and detector setup with a broad spectral transmission bandwidth, with considerable variation in μ_a of included chromophores. Approaches for either reducing the effective bandwidth of the XiSpec filters or by including their characteristic in a light transport model for SFDI modulation, are proposed.

A better understanding of bioelectrodes can be acquired with extended testing, which will lead to better methodology and data quality. Today electrodes are evaluated for intraelectrode differences and performance with a traditional gain-phase method, while using the physical axon action potential generator (Paxon) test platform offers extended test possibilities. The direct gain-phase measurements are useful to extract the transfer function of the electrode, as well as some other base parameters. The Paxon test platform is a complementary method that tests electrodes under conditions that are more realistic, mimicking real measurement situations in comparison to the gain-phase method. The Paxon also allows tests to be performed beyond what the gain-phase methods can measure, for example electrode rotation, which would uncover variations in the symmetry of the electrode. When tested, the symmetry properties of the electrode, where the electrodes are rotated in steps of 90 degrees, resulted in a peak to peak variation in detected amplitude of 5.3 +/- 8.9 mV. Therefore, the Paxon appears to be a feasible test platform for characterizing electrodes beyond the gain-phase tests in a semiautomatic manner.

BACKGROUND:

Therapeutic hypothermia (TH) has become standard treatment for severe and moderate hypoxic-ischemic neonatal encephalopathy (HIE). Our group has developed an optically based, noninvasive concept of assessing the capacity for oxygen delivery from the microcirculation to the cells of a tissue under investigation. The hypothesis was that mechanisms of reduced oxygen delivery due to reduced metabolism in cooled asphyxiated neonates could be characterized with this concept.

METHODS:

The skin of 28 asphyxiated newborn infants was studied on days 1 and 3 during TH and on day 4 following rewarming with laser Doppler perfusion measurements (LDPM), computer-assisted video microscopy (CAVM), and diffuse reflectance spectroscopy (DRS). Twenty-five healthy neonates served as a control group.

RESULTS:

The LDPM decreased during cooling (P < 0.01). Functional capillary density was higher both during and following TH compared with control infants (P < 0.01). Capillary flow velocities were reduced during TH (P < 0.05). The heterogeneity of the flow velocities was larger in the HIE infants than in the control infants. Tissue oxygen extraction was higher during TH (P < 0.01).

CONCLUSION:

This study indicates that assessments of skin microvascular density, capillary flow velocity, and oxygen extraction can be used to characterize reduced oxygen delivery to cells during TH

Evaluation of an electrode for intraelectrode differences using both a traditional gain-phase method and the Paxon test platform. The direct gain-phase measurements are useful to extract the transfer function of the electrode, as well as some other base parameters. The Paxon test platform is a complementary method that tests electrodes under conditions that are more realistic than the gel-to-gel connection used in the gain-phase method. Testing stability over time e.g. DC signal drift (worst set 6,31 ± 43,00 nV) over a one hour of measurement duration was carried out. The Paxon also lets tests be performed beyond what the gain-phase methods can measure, for example electrode rotation, which would uncover variations in the symmetry of the electrode. When tested, the symmetry properties of the electrode (test set variations, start to end, over rotations 0,90,180 and 270 degrees) resulted in a peak to peak variation in detected amplitude of 5.3 ±8.9 mV. Intraelectrode variations were detected and quantized with the Paxon test platform.

Skin blood haemoglobin saturation (𝑠_b) can be estimated with hyperspectral imaging using the wavelength (λ) range of 450-700 nm where haemoglobin absorption displays distinct spectral characteristics. Depending on the image size and photon transport algorithm, computations may be demanding. Therefore, this work aims to evaluate subsets with a reduced number of wavelengths for 𝑠_b estimation. White Monte Carlo simulations are performed using a two-layered tissue model with discrete values for epidermal thickness (𝑇_epi) and the reduced scattering coefficient (μ'_s ), mimicking an imaging setup. A detected intensity look-up table is calculated for a range of model parameter values relevant to human skin, adding absorption effects in the post-processing. Skin model parameters, including absorbers, are; μ'_s (λ), 𝑇_epi, haemoglobin saturation (𝑠_b), tissue fraction blood (𝑐_b) and tissue fraction melanin (𝑐_mel). The skin model paired with the look-up table allow spectra to be calculated swiftly. Three inverse models with varying number of free parameters are evaluated: A(𝑠_b, 𝑐_b), B(𝑠_b, 𝑐_b, 𝑐_mel) and C(all parameters free). Fourteen wavelength candidates are selected by analysing the maximal spectral sensitivity to 𝑠_b and minimizing the sensitivity to 𝑐_b. All possible combinations of these candidates with three, four and 14 wavelengths, as well as the full spectral range, are evaluated for estimating 𝑠_b for 1000 randomly generated evaluation spectra. The results show that the simplified models A and B estimated 𝑠_b accurately using four wavelengths (mean error 2.2% for model B). If the number of wavelengths increased, the model complexity needed to be increased to avoid poor estimations.