Source-detector Spacing Research Articles

Side-scattered finger-photoplethysmography: experimental investigations toward practical noninvasive measurement of blood glucose.

The aim of this study was to discover a simple/convenient geometrical arrangement of radiation sources and detector to acquire finger-photoplethysmograms (PPGs) with wavelength regions of blood glucose (BGL) absorption, toward practical noninvasive BGL measurement. First, we compared PPGs with three wavelengths: 808 nm (without water absorption), 1160 nm (with weak water absorption), and 1600 nm (with nearly peak BGL absorption and strong water absorption), while the source-detector spacing was successively increased circumferentially around a fingertip. In 10 healthy subjects, we observed clear cardiac-related pulsatile components of PPG signals at 808 and 1160 nm in any incident positions with more than 15 dB of signal-to-noise ratio ( S / N ), but reliable PPG detections at 1600 nm with more than 10 dB of S / N was only possible when the source-detector distance was less than 3 mm around the fingertip circumference. Second, with this arrangement, an experiment was performed using six wavelengths to cover glucose absorption bands (from 1550 to 1749 nm), obtaining pulsatile PPG signals with more or less 15 dB of S / N . Through the present experiments, this orthogonal arrangement of the source and detector to detect forward- and side-scattered radiation through the tissue is appropriate for PPG measurements with wavelength regions where there is potential for BGL measurement.

Influence of light source-detector spacing on shape of probability density functions of scattering angles in laser Doppler flowmetry.

Analytical models, describing laser Doppler flowmetry and its derived applications, are based on fundamental assumptions of photon scattering angles. It is shown by means of Monte Carlo simulations that, even in the case these assumptions are correct, the presence of a specific source-detector configuration may bias the shape of the probability density functions describing scattering angle behavior. It is found that these biased shapes are generated by selective filtering of photons induced by a particular source-detector configuration. In some specific cases, this phenomenon might invalidate laser Doppler analytical models.

Rapid forward modeling of multidetector logging-while-drilling Sigma measurements

Multidetector logging-while-drilling (LWD) Sigma was introduced to the oil industry to measure neutron absorption cross section (Sigma) and radial length of invasion in shallow-invaded formations. Sigma quantifies the ability of a material to absorb thermal neutrons and is calculated from the late time portion of the time decay of thermal neutrons or gamma rays generated from thermal-neutron absorption. The assessment of invasion is made possible with the combination of a thermal-neutron detector and two gamma-ray detectors with different source-detector spacings. However, the interpretation of LWD Sigma logs is often affected by several environmental and/or geometric effects that can mask the formation response. A fast numerical simulation method embedded with inversion-based techniques can be used to estimate intrinsic formation Sigma from borehole measurements affected by shoulder-bed, invasion, and/or environmental effects. We developed a fast and accurate method to numerically simulate LWD multidetector thermal-neutron and gamma-ray time decays in realistic borehole environments. The method relies on Monte Carlo precalculated libraries of particle time decays and detector-specific flux sensitivity functions, while accounting for detector-specific borehole and diffusion effects. Simulations are benchmarked against test-pit measurements and Monte Carlo N-particle (MCNP) transport code calculations. Results indicate that multidetector time decays acquired under complex geometrical conditions can be numerically simulated in approximately 1e-5 the time required using MCNP, with average difference within two capture units. The simulation of time decays, rather than Sigma itself enables a direct relationship between actual rock Sigma and multidetector diffusion-affected time decays, thereby removing intermediate correction steps often used to convert apparent into intrinsic formation Sigma.

Cytochrome c oxidase response to changes in cerebral oxygen delivery in the adult brain shows higher brain-specificity than haemoglobin

The redox state of cerebral mitochondrial cytochrome c oxidase monitored with near-infrared spectroscopy (Δ[oxCCO]) is a signal with strong potential as a non-invasive, bedside biomarker of cerebral metabolic status. We hypothesised that the higher mitochondrial density of brain compared to skin and skull would lead to evidence of brain-specificity of the Δ[oxCCO] signal when measured with a multi-distance near-infrared spectroscopy (NIRS) system. Measurements of Δ[oxCCO] as well as of concentration changes in oxygenated (Δ[HbO2]) and deoxygenated haemoglobin (Δ[HHb]) were taken at multiple source-detector distances during systemic hypoxia and hypocapnia (decrease in cerebral oxygen delivery), and hyperoxia and hypercapnia (increase in cerebral oxygen delivery) from 15 adult healthy volunteers. Increasing source-detector spacing is associated with increasing light penetration depth and thus higher sensitivity to cerebral changes. An increase in Δ[oxCCO] was observed during the challenges that increased cerebral oxygen delivery and the opposite was observed when cerebral oxygen delivery decreased. A consistent pattern of statistically significant increasing amplitude of the Δ[oxCCO] response with increasing light penetration depth was observed in all four challenges, a behaviour that was distinctly different from that of the haemoglobin chromophores, which did not show this statistically significant depth gradient. This depth-dependence of the Δ[oxCCO] signal corroborates the notion of higher concentrations of CCO being present in cerebral tissue compared to extracranial components and highlights the value of NIRS-derived Δ[oxCCO] as a brain-specific signal of cerebral metabolism, superior in this aspect to haemoglobin.

OPTIMIZATION FOR BRAIN ACTIVITY MONITORING WITH NEAR INFRARED LIGHT IN A FOUR-LAYERED MODEL OF THE HUMAN HEAD

We describe a four-layered model for near infrared light propagation in a human head based on the Monte Carlo method. With the use of three-dimensional voxel-based media discretization, photon migration in the brain is analyzed by both the time-of-∞ight measurement and the spatial sensitivity proflle. In the measurement of brain activity, the selection of light wavelength and the distance between the source and the detector have a great in∞uence on the detected signal. In this study, we compare the detected signals from the detectors with difierent source-detector spacing at wavelengths of 690nm, 800nm and 1300nm, and flnd that in our model, the wavelength of 1300nm is more appropriate for the measurement of brain activity because the signals at 1300nm get better detection sensitivity and spatial resolution. Source-detector spacing is also optimized.

Investigation of spectral interferences on the accuracy of broadband CW-NIRS tissue SO_2 determination

An accurate SO2 prediction method for using broadband continuous-wave diffuse reflectance near infrared (NIR) spectroscopy is proposed. The method fitted the NIR spectra to a Taylor expansion attenuation model, and used the simulated annealing method to initialize the nonlinear least squares fit. This paper investigated the effect of potential spectral interferences that are likely to be encountered in clinical use, on SO2 prediction accuracy. The factors include the concentration of hemoglobin in blood, the volume of blood and volume of water in the tissue under the sensor, reduced scattering coefficient, µs', of the muscle, fat thickness and the source-detector spacing. The SO2 prediction method was evaluated on simulated muscle spectra as well as on dual-dye phantoms which simulate the absorbance of oxygenated and deoxygenated hemoglobin.

Light scattering regimes along the optical axis in turbid media

We inject an angularly collimated laser beam into a scattering medium of a nondairy creamer-water solution and examine the distribution of the scattered light along the optical axis as a function of the source-detector spacing. The experimental and simulated data obtained from a Monte Carlo simulation suggest four regimes characterizing the transition from unscattered to diffusive light. We compare the data also with theoretical predictions based on a first-order scattering theory for regions close to the source, and with diffusionlike theories for larger source-detector spacings. We demonstrate the impact of the measurement process and the effect of the unavoidable absorption of photons by the detection fiber on the light distribution inside the medium. We show that the range of validity of these theories can depend on the experimental parameters such as the diameter and acceptance angle of the detection fiber.

Light source-detector spacing of near-infrared-based tissue oximeters and the influence of skin blood flow

Near-infrared (NIR) spectroscopy is a noninvasive optical technique that is increasingly used to assess muscle oxygenation during exercise with the assumption that the contribution of skin blood flow to the NIR signal is minor or nonexistent. We tested this assumption in humans by monitoring forearm tissue oxygenation during selective cutaneous vasodilation induced by locally applied heat ( n = 6) or indirect whole body heating (i.e., heating subject but not area surrounding NIR probes; n = 8). Neither perturbation has been shown to cause a measurable change in muscle blood flow or metabolism. Local heating (∼41°C) caused large increases in the NIR-derived tissue oxygenation signal [before heating = 0.82 ± 0.89 optical density (OD), after heating = 18.21 ± 2.44 OD; P < 0.001]. Similarly, whole body heating (increase internal temperature 0.9°C) also caused large increases in the tissue oxygenation signal (before heating = −0.31 ± 1.47 OD, after heating = 12.48 ± 1.82 OD; P < 0.001). These increases in the tissue oxygenation signal were closely correlated with increases in skin blood flow during both local heating (mean r = 0.95 ± 0.02) and whole body heating (mean r = 0.89 ± 0.04). These data suggest that the contribution of skin blood flow to NIR measurements of tissue oxygenation can be significant, potentially confounding interpretation of the NIR-derived signal during conditions where both skin and muscle blood flows are elevated concomitantly (e.g., high-intensity and/or prolonged exercise).

Velocity half-sphere model for multiple light scattering in turbid media

We extend the traditional diffusion theory by distinguishing between the energy radiance in the forward and backward directions at each point in space. This approach leads to a new effective source for the diffusion equation that is nonzero for an anisotropic light source. It differs significantly from the diffusion theory for short source-detector spacings. We derive an analytical solution for the two lowest-order velocity moments of the radiance.

Near-Infrared Light Propagation in an Adult Head Model with Refractive Index Mismatch

We investigate near-infrared light (NIR) propagation in a model of an adult head using an extensive Monte Carlo (MC) simulation. The adult head model is a four-layered slab which consists of a surface layer, a cerebrospinal fluid layer, a gray-matter layer, and a white-matter layer. We study the effects of a refractive index mismatch on the model, calculating the intensity of detected light, mean flight time, and partial mean flight time of each layer for various refractive indices of the cerebrospinal fluid layer as functions of source-detector spacing. The Monte Carlo simulation shows that the refractive index mismatch presents very rich results including rapidly decaying intensity of detected light and a peak and cross-over in the partial mean flight time with source-detector spacing. We also investigate spatial sensitivity profiles at various source-detector spacings, discussing the index mismatch effect on the model.

Generalized diffusion solution for light scattering from anisotropic sources

The traditional diffusion theory, often used for isotropic sources, becomes inaccurate at short source-detector spacings and cannot be applied to media with large absorption or with small scattering strengths. We show that for any type of source anisotropy, a Green's-function-based procedure can remove these limitations. The accuracy of the new approach is examined through a comparison with the numerical solution to the radiative transfer equation.

Neutron Log Measurement of Moisture in Unsaturated Basalt: Progress and Problems

The research described here examines whether neutron logs can be used in unsaturated basalt to quantitatively estimate moisture content, and, in tandem with other wireline logs, to determine the relative contribution to neutron log response by minerals containing chemically bound H. Our results show that it should be possible to quantitatively correlate neutron log response to the amount of both bound and unbound H regardless of saturated or unsaturated conditions. Such a correlation is possible only if bulk density, saturated porosity and neutron log response are already known in saturated conditions. It is not yet known, however, what the exact form of this correlation should be. To evaluate the candidate correlation equations, we compare measured permeability in unsaturated basalts with the calculated H content derived from these correlations. As the result of this evaluation, we identify intervals of clay-free impermeable, unsaturated basalt with apparently high H content. The neutron log response in these intervals may be caused by localized variations of the neutron slowing-down length, Ls, due to low concentration of H in the formation and borehole environment. When Ls is greater than the source-detector spacing of a neutron logging tool, the neutron log response can invert, resulting in potentially faulty interpretation of neutron logs. These measurement uncertainties in unsaturated basalts might be overcome by using a neutron logging tool with both a higher-flux neutron source and increased source-detector spacing, or a neutron tool with several detectors placed at different spacing from the source.

Influence of perfusion depth on laser Doppler flow measurements with large source-detector spacing.

Laser Doppler flowmetry with a large source-detector spacing has been applied to measure blood perfusion in the deeper regions of tissue. The influence of the depth of perfusion on the Doppler spectrum for the large source-detector spacing is likely to be different from that for the conventional laser Doppler instruments with small source-detector spacing. In this study, the light propagation in a tissue model with a blood perfusion layer is predicted by the Monte Carlo simulation to discuss the influence of the depth of perfusion, blood volume, and source-detector spacing on the spectrum of the Doppler signal detected with large source-detector spacing. The influence of the depth of perfusion on the Doppler spectrum for the large source-detector spacing is different from that for the conventional laser Doppler instruments with small source-detector spacing, although the influence of source-detector spacing and blood volume on the Doppler spectrum for large source-detector spacing is almost the same as that for the conventional laser Doppler instruments. The influence of the depth of the perfusion on the Doppler spectrum depends on the path length that the detected light travels at different depths.

Near-infrared light propagation in an adult head model. II. Effect of superficial tissue thickness on the sensitivity of the near-infrared spectroscopy signal.

It is important for near-infrared spectroscopy (NIRS) and imaging to estimate the sensitivity of the detected signal to the change in hemoglobin that results from brain activation and the volume of tissue interrogated for a specific source-detector fiber spacing. In this study light propagation in adult head models is predicted by Monte Carlo simulation to investigate the effect of the superficial tissue thickness on the partial optical path length in the brain and on the spatial sensitivity profile. In the case of source-detector spacing of 30 mm, the partial optical path length depends mainly on the depth of the inner skull surface whereas the spatial sensitivity profile is significantly affected by the thickness of the cerebrospinal fluid layer. The mean optical path length that can be measured by time-resolved experiments increases when the skull thickness increases whereas the partial mean optical path length in the brain decreases when the skull thickness increases. These results indicate that it is not appropriate to use the mean optical path length as an alternative to the partial optical path length to compensate the NIRS signal for the difference in sensitivity caused by variation of the superficial tissue thickness.

Correction to "Scale-based diffusive image filtering preserving boundary sharpness and fine structures"

mm , and for the clear layer, and mm , mm , and for highly scattering layers. The dotted line in the figure shows the backward scattering pulse profile of the homogeneous slab without the clear layer at the 20 mm away from laser illumination points. The profile changes by adding the clear layer especially in the trailing edge of the pulse retaining the peak amplitude. When absorption coefficient in the third layer was increased from mm to mm , the decay of the reflectance is also increased as shown in the figure. This means that the absorption changes inside the clear layer are estimated by the time profile changes of the reflectance. The sensitivity for the absorption changes becomes maximum when source detector separation is 20 mm. The mean path lengths are 200, 660, 825, and 870 ps for the source detector spacing 10, 20, 30, and 37.5 mm, respectively. The mean path length saturate for the spacing larger than 25 mm, which are coincident with results reported by Monte Carlo and FEM simulation [9]. V. C ONCLUSION FDTD method has been successfully formulated for analysis of pulse responses in biological tissues. Boundary conditions have been defined using fluence rate at the scattering and no scattering material interfaces. The conditions to give stabilities for numerical solutions have been become clear in terms of scattering coefficients and mean cosine of scattering angles. Using the formulation, the reflectance of three-layered slabs containing a clear layer has been calculated. As a result, it has been become clear that absorption coefficient changes of the highly scattering media beyond the clear layer are estimated from the time profiles of the reflectance. The analysis is easily extended to calculate pulse responses in tissues with 3-D inhomogeneous optical parameters, which is effective for image reconstruction in practical diffused optical tomography.

Modelling the sampling volume for skin blood oxygenation measurements.

The absolute quantified measurement of haemoglobin skin blood saturation from collected reflectance spectra of the skin is complicated by the fact that the blood content of tissues can vary both in the spatial distribution and in the amount. These measurements require an understanding of which vascular bed is primarily responsible for the detected signal. Knowing the spatial detector depth sensitivity makes it possible to find the best range of different probe geometries for the measurements of signal from the required zones and group of vessels inside the skin. To facilitate this, a Monte Carlo simulation has been developed to estimate the sampling volume offered by fibre-optic probes with a small source-detector spacing (in the current report 250 microm, 400 microm and 800 microm). The optical properties of the modelled medium are taken to be the optical properties of the Caucasian type of skin tissue in the visible range of the spectrum. It is shown that, for a small source-detector separation (800 microm and smaller), rough boundaries between layers of different refractive index can play a significant role in skin optics. Wavy layer interfaces produce a deeper and more homogeneous distribution of photons within the skin and tend to suppress the direct channelling of photons from source to detector. The model predicts that a probe spacing of 250 microm samples primarily epidermal layers and papillary dermis, whereas spacings of 400-800 microm sample upper blood net dermis and dermis.

Regional transcranial oximetry with near infrared spectroscopy (NIRS) in comparison with measuring oxygen saturation in the jugular bulb in infants and children for monitoring cerebral oxygenation

Using a dual channel near infrared (NIR) in vivo optical spectroscopy (INVOS) system (INVOS 3100A, Somanetics Corp. Troy, MI, USA) we investigated the relationship between jugular venous oxygen saturation (SjvO2) and regional cerebral oxygen saturation (rSO2) in 30 infants and children (mean age 4.5 years) with congenital heart disease undergoing cardiac catheterisation. The NIRS-SomaSensor (emitter and dual receiver probe) was applied at a standardised right fronto-temporal location (over the right frontal cortex) on the infant's head and covered with an adhesive flexible bandage. Using NIR light (730 and 810 nm) and two source-detector spacings (3 and 4 cm from the transmitter), percentage values of rSO2 were calculated from detected haemoglobin saturations. Simultaneously, jugular venous oxygen saturation (SjvO2) monitoring was performed via a catheter placed in the right internal jugular vein with its tip positioned in the jugular bulb, as verified by fluoroscopy. To compare the reliability of NIRS measurement characteristics, jugular venous blood was analysed for SjvO2 as a reference measure of global cerebral oxygenation, by co-oximetry (OSM3-Hemoximeter, Radiometer Copenhagen, Denmark). Other measured variables included pulse oximetry, arterial blood pressure, and venous and arterial oxygen saturations. Over a jugular venous oxygen saturation range of 31-83%, a significant positive linear correlation was found between rSO2 (NIRS measurement) and SjvO2 (jugular bulb oximetry) (r = 0.93, p < 0.001). No significant correlation was observed between rSO2 values and arterial blood saturation or pulse oximetry. The quantitative correlation between rSO2 (haemoglobin oxygenation in a small hemi-elliptical area of the brain) and reference SjvO2 measurement (method for monitoring global cerebral oxygenation) suggests that NIRS measurement with subtraction algorithm should identify predominantly intracranial saturation in the pediatric age group, and will tend to reflect global oxygenation under physiological conditions. Transcranial oximetry using dual receiving channel NIRS offers a noninvasive, real-time, reliable and practicable means of monitoring cerebral haemoglobin oxygenation changes infants and children with cyanotic and noncyanotic congenital heart disease.

Theoretical and experimental investigation of near-infrared light propagation in a model of the adult head

Near-infrared light propagation in various models of the adult head is analyzed by both time-of-flight measurements and mathematical prediction. The models consist of three- or four-layered slabs, the latter incorporating a clear cerebrospinal fluid (CSF) layer. The most sophisticated model also incorporates slots that imitate sulci on the brain surface. For each model, the experimentally measured mean optical path length as a function of source-detector spacing agrees well with predictions from either a Monte Carlo model or a finite-element method based on diffusion theory or a hybrid radiosity-diffusion theory. Light propagation in the adult head is shown to be highly affected by the presence of the clear CSF layer, and both the optical path length and the spatial sensitivity profile of the models with a CSF layer are quite different from those without the CSF layer. However, the geometry of the sulci and the boundary between the gray and the white matter have little effect on the detected light distribution.

Investigations into the origin of the spurious 17 keV neutrino signal observed in35S beta decay

An exhaustive study has been made of the β spectrum of35S, recorded with a Si(Li) detector. The object was to identify the origin of a distortion in the35S β spectrum some 17 keV below the end point, reported over three years ago and interpreted then as evidence for a 17 keV neutrino. Measurements with different source-detector spacings and with varied collimation have shown that there is a long range curvature in the Kurie plot which is a sensitive function of configuration, but the principal origin of the distortion is energy loss in the35S sources. The35S sources, prepared by chemical adsorption of Ba35SO4 on a gold substrate, are clumped and locally thick. Electrons near the end point lose ∼0.3 keV in the source material and if this is taken into account the spectra are well fitted without any admixture of 17 keV neutrino. The source thickness has been investigated with a proton microprobe and determined from both source tilting and the yield of bariumK X-rays; these studies are discussed in detail. The uncertainties in and justification for the form of the electron response function employed are also thoroughly discussed. If there is no systematic error common to the majority of 14 independent sets of35S data, the admixture of 17 keV neutrino is <10−3 (95% CL). A simple search for a kink at 150 keV in the combined data from all 14 runs yielded a limit of 1.8×10−3 (95% CL). The end point of the35S β spectrum is found to be 167.60±0.05 keV.

Optical pathlength measurements on adult head, calf and forearm and the head of the newborn infant using phase resolved optical spectroscopy

The authors have used an intensity modulated optical spectrometer, which measures the phase shift across tissue experienced by intensity modulated near-infrared light, to determine the absolute optical pathlength through tissue. The instrument is portable and takes only 5 s to record pathlength at four wavelengths (690 nm, 744 nm, 807 nm and 832 nm). The absolute pathlength divided by the known spacing between the light source and detector on the skin is the differential pathlength factor (DPF) which previous studies have shown is approximately constant for spacings greater than 2.5 cm. DPF results are presented for measurements on 100 adults and 35 newborn infants to determine the statistical variation on the DPF. All measurements were made at a frequency of 200 MHz with source-detector spacings of >4 cm. Results at 807 nm show a DPF of 4.16(+or-18.8%) for adult arm, 5.51(+or-18%) for adult leg, 6.26(+or-14.1%) for adult head and 4.99(+or-9%) for the head of a newborn infant. A wavelength dependence was obtained for DPF on all tissues and a difference in DPF between male and female was observed for both the adult arm and leg. The results can be used to improve the quantitation of chromophore concentration changes in adults and newborn infants.