In Vitro Validation of a New Tissue Oximeter Using Visible Light.

There is a clinical need to measure local tissue oxygen saturation (StO2), oxy-, deoxy- and total haemoglobin concentration ([O2Hb], [HHb], [tHb]) in human tissue. The aim was to validate an oximeter called OxyVLS applying visible light spectroscopy (VLS) to determine these parameters without needing to assume a reduced scattering coefficient (µ's). This problem is solved by appropriate calibrations. Compared to near-infrared spectroscopy (NIRS), OxyVLS determines the oxygenation in a much smaller more superficial volume of tissue, which is useful in many clinical cases. OxyVLS was validated in liquid phantoms with known StO2, [tHb], and µ's and compared to frequency domain NIRS as a reliable reference. OxyVLS showed a high accuracy for all the mentioned parameters and was even able to measure µ's. Thus, OxyVLS was successfully tested in vitro.

Impact of Changes in Systemic Physiology on fNIRS/NIRS Signals: Analysis Based on Oblique Subspace Projections Decomposition.

Measurements of cerebral and muscle oxygenation (StO2) and perfusion ([tHb]) with functional near-infrared spectroscopy (fNIRS) and near infrared spectroscopy (NIRS), respectively, can be influenced by changes in systemic physiology. The aim of our study was to apply the oblique subspace projections signal decomposition (OSPSD) to find the contribution from systemic physiology, i.e. heart rate (HR), electrocardiography (ECG)-derived respiration (EDR) and partial pressure of carbon dioxide (pCO2) to StO2 and [tHb] signals measured on the prefrontal cortex (PFC) and calf muscle. OSPSD was applied to two datasets (n1 = 42, n2 = 79 measurements) from two fNIRS/NIRS speech studies. We found that (i) all StO2 and [tHb] signals contained components related to changes in systemic physiology, (ii) the contribution from systemic physiology varied strongly between subjects, and (iii) changes in systemic physiology generally influenced fNIRS signals on the left and right PFC to a similar degree.

Absolute Values of Optical Properties (µa, µÎ„s, µeff and DPF) of Human Head Tissue: Dependence on Head Region and Individual.

BACKGROUND: Absolute optical properties (i.e., the absorption coefficient, µa, and the reduced scattering coefficient, [Formula: see text]) of head tissue can be measured with frequency-domain near-infrared spectroscopy (FD-NIRS). AIM: We investigated how the absolute optical properties depend on the individual subject and the head region. MATERIALS AND METHODS: The data set used for the analysis comprised 31 single FD-NIRS measurements of 14 healthy subjects (9 men, 5 women, aged 33.4 ± 10.5 years). From an 8-min measurement (resting-state; FD-NIRS device: Imagent, ISS Inc.; bilateral over the prefrontal cortex, PFC, and visual cortex, VC) median values were calculated for µa and [Formula: see text] as well as the effective attenuation coefficient (µeff) and the differential pathlength factor (DPF). The measurement was done for each subject one to three times with at least 24 h between the measurements. RESULTS: (i) A Bayesian ANOVA analysis revealed that head region and subject were the most significant main effects on µa, [Formula: see text] and µeff, as well as DPF, respectively. (ii) At the VC, µa, [Formula: see text] and µeff had higher values compared to the PFC. (iii) The differences in the optical properties between PFC and VC were age-dependent. (iv) All optical properties also were age-dependent. This was strongest for the properties of the PFC compared to the VC. DISCUSSION AND CONCLUSION: Our analysis demonstrates that all optical head tissue properties (µa, [Formula: see text], µeff and DPF) were dependent on the head region, individual subject and age. The optical properties of the head are like a 'fingerprint' for the individual subject. Assuming constant optical properties for the whole head should be carefully reconsidered.

Long-Term Changes in Optical Properties (µa, µ's, µeff and DPF) of Human Head Tissue During Functional Neuroimaging Experiments.

Frequency-domain near-infrared spectroscopy (FD-NIRS) enables to measure absolute optical properties (i.e. the absorption coefficient, µa, and the reduced scattering coefficient, [Formula: see text]) of the brain tissue. The aim of this study was to investigate how the optical properties changed during the course of a functional NIRS experiment. The analyzed dataset comprised of FD-NIRS measurements of 14 healthy subjects (9 males, 5 females, aged: 33.4 ± 10.5 years, range: 24-57 years old). Each measurement lasted 33 min, i.e. 8 min baseline in darkness, 10 min intermittent light stimulation, and 15 min recovery in darkness. Optical tissue properties were obtained bilaterally over the prefrontal cortex (PFC) and visual cortex (VC) with FD-NIRS (Imagent, ISS Inc., USA). Changes in µa and [Formula: see text] were directly measured and two parameters were calculated, i.e. the differential pathlength factor (DPF) and the effective attenuation coefficient (µeff). Differences in the behavior of the optical changes were observed when comparing group-averaged data versus single datasets: no clear overall trend was presented in the group data, whereas a clear long-term trend was visible in almost all of the single measurements. Interestingly, the changes in [Formula: see text] statistically significantly correlated with µa, positively in the PFC and negatively in the VC. Our analysis demonstrates that all optical brain tissue properties (µa, [Formula: see text], µeff and DPF) change during these functional neuroimaging experiments. The change in [Formula: see text] is not random but follows a trend, which depends on the single experiment and measurement location. The change in the scattering properties of the brain tissue during a functional experiment is not negligible. The assumption [Formula: see text] ≈ const during an experiment is valid for group-averaged data but not for data from single experiments.

In Vitro Comparisons of Near-Infrared Spectroscopy Oximeters: Impact of Slow Changes in Scattering of Liquid Phantoms.

Several cerebral oximeters based on near-infrared spectroscopy (NIRS) are commercially available that determine tissue oxygen saturation (StO2). One problem is an inconsistency of StO2 readings between different brands of instruments. Liquid blood phantoms mimicking optical properties of the neonatal head enable quantitative device comparisons. However, occasionally, the reduced scattering coefficient (µs') of these phantoms decreases over time. AIM: To investigate whether this decrease in µs' affects the validity of comparison of these devices. StO2 was measured by several NIRS oximeters simultaneously on a phantom, which exhibited a particularly strong decrease in µs'. We found that a decrease in µs' by ≤16% from baseline led to deviations in StO2 of ≤3%.

In vivo precision assessment of a near-infrared spectroscopy-based tissue oximeter (OxyPrem v1.3) in neonates considering systemic hemodynamic fluctuations.

The aim was to determine the precision of a noninvasive near-infrared spectroscopy (NIRS)-based tissue oximeter (OxyPrem v1.3). Using a linear mixed-effects model, we quantified the variability for cerebral tissue oxygenation (StO2) measurements in 35 preterm neonates to be 2.64%, a value that meets the often-articulated clinicians' demand for a precise tissue oxygenation measurement. We showed that the variability of StO2 values measured was dominated by spontaneous systemic hemodynamic fluctuations during the measurement, meaning that precision of the instrument was actually even better. Based on simultaneous and continuous measurements of peripheral arterial oxygenation and cerebral StO2 with a second sensor, we were able to determine and quantify the physiological instability precisely. We presented different methods and analyses aiming at reducing this systematic physiological error of in vivo precision assessments. Using these methods, we estimated the precision of the OxyPrem tissue oximeter to be ≤ 1.85 % . With our study, we deliver relevant information to establish highly precise cerebral oxygenation measurements with NIRS-based oximetry, facilitating the further development toward a substantially improved diagnosis and treatment of patients with respect to brain oxygenation.

Tissue oximetry by diffusive reflective visible light spectroscopy: Comparison of algorithms and their robustness.

It is essential to measure tissue oxygen saturation (StO2 ) locally and in thin layers of tissue, for example, the bronchial mucosa, skin flaps and small bones. Visible light spectroscopy (VLS) with a shallow penetration depth is suitable method. Although several VLS algorithms have been developed and described, they have not yet been compared to each other. This hinders attempts to compare the clinical results obtained by different algorithms. To address this issue, we compared the algorithms of Harrison, Knoefel, Pittman-Duling, Sato and our OxyVLS oximeter, which applies the algorithm from Wodick and Lübbers, in a liquid phantom with optical properties of human tissue. We generally observed considerable differences between the algorithms, which were StO2 dependent. Exceptions were OxyVLS and Sato, showing a high level of agreement with negligible StO2 dependency. In spite of the considerable deviation between the other algorithms, the difference of StO2 between them in clinically normal StO2 was <10%. We did not observe any dependency of the algorithms on hemoglobin content of the phantom or temperature.

Quantifying the effect of adipose tissue in muscle oximetry by near infrared spectroscopy.

Change of muscle tissue oxygen saturation (StO2), due to exercise, measured by near infrared spectroscopy (NIRS) is known to be lower for subjects with higher adipose tissue thickness. This is most likely not physiological but caused by the superficial fat and adipose tissue. In this paper we assessed, in vitro, the influence of adipose tissue thickness on muscle StO2, measured by NIRS oximeters. We measured StO2 of a liquid phantom by 3 continuous wave (CW) oximeters (Sensmart Model X-100 Universal Oximetry System, INVOS 5100C, and OxyPrem v1.3), as well as a frequency-domain oximeter, OxiplexTS, through superficial layers with 4 different thicknesses. Later, we employed the results to calibrate OxyPrem v1.3 for adipose tissue thickness in-vivo.

Local Measurement of Flap Oxygen Saturation: An Application of Visible Light Spectroscopy.

The aim was to develop and test a new device (OxyVLS) to measure tissue oxygen saturation by visible light spectroscopy independently of the optical pathlength and scattering. Its local applicability provides the possibility of real time application in flap reconstruction surgery. We tested OxyVLS in a liquid phantom with optical properties similar to human tissue. Our results were in good agreement with a conventional near infrared spectroscopy device.

Evaluation of a textile-based near infrared spectroscopy system in calf muscle oxygenation measurements.

We recently introduced a novel textile-based NIRS sensor (TexNIRS). Here, we evaluate TexNIRS in ten subjects (16 legs, age 28.5 ± 2.32 years, adipose tissue thickness (ATT) 4.17 ± 1.71 mm). Three venous occlusions at 50 mmHg were performed on their calf muscle. After 3 min of occlusion, oxy/deoxy hemoglobin concentration ([O2Hb], [HHb]) changes were 3.71 ± 1.89/1.79 ± 1.08 µM; venous oxygen saturation (SvO2) was 75 ± 9.7 %, oxygen consumption (VO2) was 0.02 ± 0.01 mL/100 g/min, hemoglobin flow (HF) was 0.93 ± 0.48 µmol/100 mL/min, and blood flow (BF) was 2.01 ± 1.04 mL/100 mL/min. Our results are in good agreement with the literature, but the TexNIRS enables a much higher level of comfort.

Textile integrated sensors and actuators for near-infrared spectroscopy.

Being the closest layer to our body, textiles provide an ideal platform for integrating sensors and actuators to monitor physiological signals. We used a woven textile to integrate photodiodes and light emitting diodes. LEDs and photodiodes enable near-infrared spectroscopy (NIRS) systems to monitor arterial oxygen saturation and oxygenated and deoxygenated hemoglobin in human tissue. Photodiodes and LEDs are mounted on flexible plastic strips with widths of 4 mm and 2 mm, respectively. The strips are woven during the textile fabrication process in weft direction and interconnected with copper wires with a diameter of 71 µm in warp direction. The sensor textile is applied to measure the pulse waves in the fingertip and the changes in oxygenated and deoxygenated hemoglobin during a venous occlusion at the calf. The system has a signal-to-noise ratio of more than 70 dB and a system drift of 0.37% ± 0.48%. The presented work demonstrates the feasibility of integrating photodiodes and LEDs into woven textiles, a step towards wearable health monitoring devices.