Source-detector Distance Research Articles

Non-invasive optical monitoring of human lungs: Monte Carlo modeling of photon migration in Visible Chinese Human and an experimental test on a human.

The human lung was quantified and visualized by photon transport in this paper. A Monte Carlo (MC) simulation of voxelized media was used with the visible Chinese human (VCH). This study theoretically explored the feasibility of non-invasive optical detection of pulmonary hemodynamics, and investigated the optimal location of the light source in the lung photon migration and optimized the source-detector distance. The light fluence intensity showed that the photon penetration depth was 6-8.4 mm in the human lung. The optimal distance from the light source to the detector was 2.7-2.9 cm, but the optimal distance of the superior lobe of right lung was 3.3-3.5 cm. We then conducted experiments on diffuse light reflectance using NIRS on 14 volunteers. These measurements agree well with the simulation results. All the results demonstrated the great potential of non-invasive monitoring of pulmonary hemodynamics and contribute to the study of human lungs in the biomedical optics community.

Angular insensitivity of a gamma spectrometer for in-field applications

In-field measurements have particular challenges as compared with those conducted under laboratory conditions. Besides unknown source shielding, the source-detector distance varies and the detector orientation relative to the incident radiation is not necessarily constant. The incoming flux facing a detector is a parallel beam at long source-detector distances (>1 m). The counting efficiency depends on the tilting angle relative to the beam facing the detector.In principle, a cylindrical detector with a height-diameter ratio of π/4 (H/D = 0.785) exhibits the lowest angular dependency (41% at low energies for a tilting angle of 45° as compared with the orientation of the detector end cap relative to the beam). However, Monte Carlo simulations of a germanium detector showed that this variability can be greatly improved by slightly increasing H/D (0.84) and introducing a copper cladding around the detector (1.1 mm). The counting efficiency of such a detector is almost independent of the direction of photons arriving to the detector. The maximum deviation of 10% takes place at 200 keV.

Detectability of low-oxygenated regions in human muscle tissue using near-infrared spectroscopy and phantom models.

The present work aims to describe the detectability limits of hypoxic regions in human muscle under moderate thicknesses of adipose tissue to serve as a groundwork for the development of a wearable device to prevent pressure injuries. The optimal source-detector distances, detection limits, and the spatial resolution of hypoxic volumes in the human muscle are calculated using finite element method-based computer simulations conducted on 3-layer tissue models. Silicone phantoms matching the simulation geometries were manufactured, and their measurement results were compared to the simulations. The simulations showed good agreement with the performed experiments. Our results show detectability of hypoxic volumes under adipose tissue thicknesses of up to 1.5 cm. The maximum tissue depth, at which hypoxic volumes could be detected was 2.8 cm. The smallest detectable hypoxic volume in our study was 1.2 cm3. We thus show the detectability of hypoxic volumes in sizes consistent with those of early-stage pressure injury formation and, consequently, the feasibility of a device to prevent pressure injuries.

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 mm-1 and 1 mm-1, respectively, we develop a method which utilizes frequency-domain to measure absolute optical properties of turbid samples in a standard cuvette (45 mm × 10 mm × 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 mm-1); higher relative error in the absorption coefficient compared to the reduced scattering coefficient; accuracy is tied to knowledge of the sample's index of refraction. Noise simulations with 0.1 % amplitude and 0.1° = 1.7 mrad phase uncertainty find errors in absorption and reduced scattering coefficients of 4 % and 1 %, 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.

Correction of the Contribution of Scattered Photon Radiation to the Ionization Chamber ReadingsDuring X-Ray Radiation Quality Assessment

Reduction of the systematic error when determining the characteristics of the reference X-ray radiation fields is an essential task according to the ISO 4037-1:2019 standard. This task is especially important in dosimetry laboratories when establishing the qualities of reference photon fields. The aim of the study was to develop a method that allows taking into account the contribution of radiation scattered on the filter when determining the half-value layer of the photon field generated by the X-ray unit. Another goal was to reduce the computational cost of determining this contribution.One of the major contributors to the systematic error in measuring the half-value layer is the radiation scattered on the filter material. The standard recommends that this error should be taken into account in the measurement. But it does not provide any methodology that would do this.The study investigated the possibility of reducing the contribution of scattered radiation to the ionization chamber readings when assessing the radiation quality of the X-ray unit by the means of half-value layer. The study utilized the (N, H, L) quality series as reference fields according to ISO 4037-1:2019.Contribution of the scattered radiation to the half-value layer was compensated with the correction coefficients; they were calculated with the FLUKA Monte Carlo software according to the zero-aperture approximation method. Unlike other similar methods, the proposed approach employs kinetic energy released to matter (kerma), to air in this case, as the main value, which, when utilized instead of deposited energy, reduces the program’s runtime several fold.Correctness of the results obtained in this work was verified by comparing the calculated values of the half-value layer with the tabulated ones provided in the ISO 4037-1:2019 standard. The deviation of calculated values from those specified in the standard does not exceed 2 %.Calculation results showed that the error contributed by scattered radiation to the magnitude of the halfvalue layer in direct measurements does not exceed 5 %. The use of the air kerma allowed us to significantly reduce the time for calculating the correction coefficients by the factor of 6–16 times with respect to other methods, depending on the radiation quality series. This made it possible to calculate correction factors for the source-detector distance equal to 2.5 meters.

A closer look at the efficiency calibration of LaBr3(Ce) and NaI(Tl) scintillation detectors using MCNPX for various types of nuclear investigations

The nuclear spectroscopy method has long been used for advanced studies on nuclear physics. In order to decrease costs and increase the efficiency of nuclear radiation investigations, quick and efficient solutions are required. The purpose of this research was to calculate the whole energy peak efficiency values for a range of gamma-ray energies, from 30.973 keV to 1408 keV, at various source-detector distances using the MCNPX Monte Carlo code, which is extensively used in nuclear medicine, industry, and scientific research. As a result, the modeled detectors' full-energy peak efficiencies were calculated and compared to both experimental data and Monte Carlo simulations. Experiment results and prior studies using Monte Carlo simulations were found to be very consistent with these results. The counting efficiency against source-detector distance is then calculated using the modeled detectors. The data we have show that LaBr3(Ce) has outstanding detection properties. This study’s findings might be used to improve the design of detectors for use in wide range of high-tech gamma spectroscopy and nuclear research applications.

Justification of the Photoplethysmography Sensor Configuration by Monte Carlo Modeling of the Pulse Waveform

Photoplethysmography (PPG) is an optical technique for detection of blood volume changes in the microvascular bed of a biological tissue. Many aspects of the PPG signal formation are still unclear. In particular, it is not known how the shape of a registered PPG signal depends on the geometry of tissue illumination. The aim of this study is to model the PPG waveform using the Monte Carlo (MC) method. For this, we developed a three-layer optical model of the skin in a reflectance geometry and verified it experimentally for different wavelengths (660, 810, and 940 nm) and source-detector distances (from 4 to 10 mm). The MC simulation results showed that the PPG waveform depends on the source-detector distance. The most pronounced diastolic wave is observed at the distance of 6 mm for the wavelength of 810 nm. The results obtained can be used for the development of reflectance PPG sensors.

Two-layer analytical model for estimation of layer thickness and flow using Diffuse Correlation Spectroscopy.

Diffuse correlation spectroscopy (DCS) has been widely explored for its ability to measure cerebral blood flow (CBF), however, mostly under the assumption that the human head is homogenous. In addition to CBF, knowledge of extracerebral layers, such as skull thickness, can be informative and crucial for patient with brain complications such as traumatic brain injuries. To bridge the gap, this study explored the feasibility of simultaneously extracting skull thickness and flow in the cortex layer using DCS. We validated a two-layer analytical model that assumed the skull as top layer with a finite thickness and the brain cortex as bottom layer with semi-infinite geometry. The model fitted for thickness of the top layer and flow of the bottom layer, while assumed other parameters as constant. The accuracy of the two-layer model was tested against the conventional single-layer model using measurements from custom made two-layer phantoms mimicking skull and brain. We found that the fitted top layer thickness at each source detector (SD) distance is correlated with the expected thickness. For the fitted bottom layer flow, the two-layer model fits relatively consistent flow across all top layer thicknesses. In comparison, the conventional one-layer model increasingly underestimates the bottom layer flow as top layer thickness increases. The overall accuracy of estimating first layer thickness and flow depends on the SD distance in relationship to first layer thickness. Lastly, we quantified the influence of uncertainties in the optical properties of each layer. We found that uncertainties in the optical properties only mildly influence the fitted thickness and flow. In this work we demonstrate the feasibility of simultaneously extracting of layer thickness and flow using a two-layer DCS model. Findings from this work may introduce a robust and cost-effective approach towards simultaneous bedside assessment of skull thickness and cerebral blood flow.

Corrections of HPGe detector efficiency curve due to true coincidence summing by program EFFTRAN and by Monte Carlo simulations

Standard reference sources, used for efficiency curve calibration of detector, often contain radionuclides with complex decay schemes (such as 60Co, 88Y, 152Eu …), introducing a potential problem in gamma-ray spectrometry, due to the appearance of coincidence summing of detected photons, in particular at a low source-detector distance. In this paper, a set of Monte Carlo simulations of an identical experimental setup were performed in order to obtain the efficiency curve of coaxial p-type HPGe detector for energy region (0–2) MeV, with the effect of true coincidence summing and without it. Obtained efficiency curves are compared with the experimental curve after applied EFFTRAN corrections. Fairly well agreement (between simulated and experimental curves with EFFTRAN corrections (with a relative deviation of 10%) proved the reliability of EFFTRAN corrections, as well as the possibility of Monte Carlo simulations for efficiency curve determination.

Determination of Appropriate Source-to-Detector Distance Dependence of Gamma Spectrometry System to Achieve Higher Resolution

Operation of gamma spectrometry requires intensive monitoring of gamma ray fluxes. When a radioactive intense source is placed close to a crystal detector, it becomes saturated as a result of detector dead time. On the contrary, when placed far away, there is loss of count statistics. For this reason, optimal position for placing the source has to be investigated. With the improvement in the instrumentation of radiation detection applications over the years, high count rate measurement accuracy is more crucial than ever. This is due to periodic measurement and new correction models for dead time. The aim of this study is to examine the performance of two gamma spectroscopic systems. The source detector distance dependence on dead time, peak-to-compton ratio, overall amplifier gain, FWHM, and voltage variation were investigated using American standard procedures (ANSI/IEE -325). Measurements were performed at five (5) different distances of detector cap for four (4) point sources (Co-60, Eu-152, Cs-137 and Ba-133). Source-to-detector distance at 25 cm was improved to avoid summing coincidence and dead time correction. The results obtained at 25 cm showed that the dead time was found less than 1% as compared to 4 cm. This will describe the stability of dead time. It also indicated that for both detectors the rise in biased voltage will yield a good resolution at 1332.5 KeV. This study is significant as it provides information to ensure that detectors are kept at optimal distance to achieve good dead time.

Assessment of firmness and soluble solids content of peaches by spatially resolved spectroscopy with a spectral difference technique

Spatially resolved spectroscopy (SRS) provides a new approach to the measurement of optical scattering and absorption properties and quality assessment of horticultural products. In this research, spatially resolved (SR) spectra over 550 – 1,650 nm were acquired for 600 peaches using a recently developed SRS system covering the light source-detector (S-D) distances from 1.5 to 36 mm with 30 fibers of three sizes (i.e., 50 µm, 105 µm and 200 µm). A new method of calculating spectral differences, between the first S-D distance and the remaining 14 S-D distances was proposed to enhance firmness and soluble solids content (SSC) predictions. Partial least squares (PLS) regression models based on SR and difference reflectance (DR) spectra were developed and compared for firmness and SSC prediction. Results showed that when using SR spectra, prediction results for firmness and SSC varied greatly with the S-D distance; SSC prediction results became worse with the increasing S-D distance, while larger S-D distances of 12–32 mm resulted in better firmness predictions. DR spectra gave consistently better results for both firmness and SSC predictions than SR spectra, with the best correlation coefficients of 0.853 and 0.839 for firmness and SSC prediction, respectively. Overall, SRS coupled with the spectral difference technique can enhance the prediction of firmness and SSC for peach fruit.

Effect of the geometrical parameters of an HPGe detector on efficiency calculations using Monte Carlo methods

This paper describes a procedure for constructing a Monte Carlo model of an HPGe detector used to measure activity. The purpose of this parametric study is to obtain a single model that effectively reproduces the detection efficiency for both small (< 1 cm) and large (> 40 cm) source–detector distances. We therefore investigated the effect of the HPGe detector’s geometrical parameters on the Monte Carlo efficiency calculations. The results highlight detector parameters with the greatest effect on the calculations within a wide range of gamma energy levels (60–1900 keV) and source–detector distances (1 to 50 cm): the air gap, the dead layers thicknesses and the crystal height (see Figure 1). Analysis of these results has made it possible to determine the key parameters, together with the optimal energy and distance ranges, significantly reducing the computational time required. A broad range of calibrated point-like radioactive sources were used to ensure a high level of experimental efficiency when constructing the detector model. A single Monte Carlo model for this detector was thus optimised by minimising the difference between the experimental efficiency and the calculations. The single HPGe model resulting from this optimisation process led to an average deviation between experimental and simulated efficiency of 3% for both small and large source–detector distances.

Development of an energy and efficiency calibration method for stilbene scintillators

A method for calibrating the energy scale and detection efficiency of stilbene scintillators is presented herein. This method can be used to quantitatively analyze the Compton continuum of gamma-ray spectra obtained using such scintillators. First, channel–energy calibration was conducted by fitting a semiempirical equation for the Compton continuum to the acquired energy spectrum and a new method to evaluate the intrinsic detection efficiency, called intrinsic Compton efficiency, of stilbene scintillators was proposed. The validity of this method was verified by changing experimental conditions such as the number of sources being measured simultaneously and the detector–source distance. According to the energy calibration, the standard error for the estimated Compton edge position was ±1.56 keV. The comparison of the intrinsic Compton efficiencies calculated from the single- and two-source spectra showed that the mean absolute difference and the mean absolute percentage difference are 0.031 %p and 0.557%, respectively, demonstrating reasonable accuracy of this method. The feasibility of the method was confirmed for an energy range of 0.5–1.5 MeV, showing that stilbene scintillators can be used to quantitatively analyze gamma rays in mixed-radiation fields.

Assessment by Multi‐Distance Hyperspectral NIRS of Changes in the Oxidation State of Cytochrome C Oxidase (oxCCO) to Carotid Artery Compressions

IntroductionThe occlusion of the common carotid artery (CCA) during carotid endarterectomy (CEA) is associated with a risk of cerebral ischemia due to inadequate collateral blood flow1. By monitoring tissue oxygen saturation (StO2), near‐infrared spectroscopy (NIRS) is frequently employed during CEA2 to determine sufficient oxygen delivery. However, StO2 is only a proxy of oxygen metabolism and an unreliable marker of ischemic injury. In contrast, cytochrome c oxidase (oxCCO) is an enzyme in the mitochondrial electron transport chain and therefore a specific marker of aerobic metabolism3. Thus, the aim of this study was to assess the ability of a multi‐distance hyperspectral (hs) NIRS to monitor changes in StO2 and oxCCO during CCA compressions in healthy participants. We hypothesized that changes in oxCCO concentration would be greater in the brain (i.e., long source‐detector distance, SDD) than in the extracerebral tissue (i.e., short SDD), and this difference would be larger than that of StO2.MethodsWe modified an in‐house built hsNIRS4 to include a second channel for acquiring data at two SDD (1 and 3 cm) to detect potential changes in the extracerebral tissue. Data from 10 young, healthy participants was acquired continuously during one 30‐second unilateral digital CCA compression, performed ipsilateral to the location of the optic probes. In addition, cerebral blood flow index was determined using diffuse correlation spectroscopy and mean arterial blood pressure by finger photoplethysmography.ResultsDuring CCA compression, the change in oxCCO at 3 cm vs. 1 cm (0.4 ± 0.3 µM vs. 0.06 ± 0.1 µM, p = 0.027) was approximately 50% greater than the change in StO2 at 3 cm vs. 1cm (‐4.0 ± 2.2% vs. 1.2 ± 0.7% p = 0.007). The corresponding decrease in cerebral blood flow was 57 ± 14% (p &lt; 0.001 vs. baseline). Mean arterial blood pressure increased 4 ± 1 mmHg (p &lt; 0.001 vs. baseline).DiscussionA larger difference between the two distances was observed for oxCCO than StO2, likely due to the greater concentration of mitochondria in the brain compared to extracerebral tissues. Collecting multi‐distance hsNIRS demonstrated the enhanced sensitivity of oxCCO to the brain. Lastly, this study demonstrates the utility of momentary unilateral CCA compressions as a fast, easy, and strong perturbation for transiently altering cerebral inflow and metabolism.References(1) Aceto, Eur. J. Anestesiol.(2020); (2) Yu, Cochrane Database Syst. Rev.(2014); (3) Bale, J. Biomed. Opt. (2020); (4) Rajaram, Brain Sci (2020).

Evaluation of the robustness of cerebral oximetry to variations in skin pigmentation using a tissue-simulating phantom.

Clinical studies have demonstrated that epidermal pigmentation level can affect cerebral oximetry measurements. To evaluate the robustness of these devices, we have developed a phantom-based test method that includes an epidermis-simulating layer with several melanin concentrations and a 3D-printed cerebrovascular module. Measurements were performed with neonatal, pediatric and adult sensors from two commercial oximeters, where neonatal probes had shorter source-detector separation distances. Referenced blood oxygenation levels ranged from 30 to 90%. Cerebral oximeter outputs exhibited a consistent decrease in saturation level with simulated melanin content; this effect was greatest at low saturation levels, producing a change of up to 15%. Dependence on pigmentation was strongest in a neonatal sensor, possibly due to its high reflectivity. Overall, our findings indicate that a modular channel-array phantom approach can provide a practical tool for assessing the impact of skin pigmentation on cerebral oximeter performance and that modifications to algorithms and/or instrumentation may be needed to mitigate pigmentation bias.

Broadband diffuse optical spectroscopy of two-layered scattering media containing oxyhemoglobin, deoxyhemoglobin, water, and lipids.

We investigated the relationship between chromophore concentrations in two-layered scattering media and the apparent chromophore concentrations measured with broadband optical spectroscopy in conjunction with commonly used homogeneous medium inverse models. We used diffusion theory togenerate optical data from a two-layered distribution of relevant tissue absorbers, namely, oxyhemoglobin, deoxyhemoglobin, water, and lipids, with a top-layer thickness in the range 1–15 mm. The generated data consisted of broadband continuous-wave (CW) diffuse reflectance in the wavelength range 650–1024 nm, and frequency-domain (FD) diffuse reflectance at 690 and 830 nm; two source-detector distances of 25 and 35 mm were used to simulate a dual-slope technique. The data were inverted using diffusion theory for a semi-infinite homogeneous medium to generate reduced scattering coefficients at 690 and 830 nm (from FD data) and effective absorption spectra in the range 650–1024 nm (from CW data). The absorption spectra were then converted into effective total concentration and oxygen saturation of hemoglobin, as well as water and lipid concentrations. For absolute values, it was found that the effective hemoglobin parameters are typically representative of the bottom layer, whereas water and lipid represent some average of the respective concentrations in the two layers. For concentration changes, lipid showed a significant cross-talk with other absorber concentrations, thus indicating that lipid dynamics obtained in these conditions may not be reliable. These systematic simulations of broadband spectroscopy of two-layered media provide guidance on how to interpret effective optical properties measured with similar instrumental setups under the assumption of medium homogeneity.

Feasibility of Dual-Energy Cone-Beam CT of Bone Marrow Edema Using Dual-Layer Flat Panel Detectors.

We investigated the feasibility of detection and quantification of bone marrow edema (BME) using dual-energy (DE) Cone-Beam CT (CBCT) with a dual-layer flat panel detector (FPD) and three-material decomposition. A realistic CBCT system simulator was applied to study the impact of detector quantization, scatter, and spectral calibration errors on the accuracy of fat-water-bone decompositions of dual-layer projections. The CBCT system featured 975 mm source-axis distance, 1,362 mm source-detector distance and a 430 × 430 mm2 dual-layer FPD (top layer: 0.20 mm CsI:Tl, bottom layer: 0.55 mm CsI:Tl; a 1 mm Cu filter between the layers to improve spectral separation). Tube settings were 120 kV (+2 mm Al, +0.2 mm Cu) and 10 mAs per exposure. The digital phantom consisted of a 160 mm water cylinder with inserts containing mixtures of water (volume fraction ranging 0.18 to 0.46) - fat (0.5 to 0.7) - Ca (0.04 to 0.12); decreasing fractions of fat indicated increasing degrees of BME. A two-stage three-material DE decomposition was applied to DE CBCT projections: first, projection-domain decomposition (PDD) into fat-aluminum basis, followed by CBCT reconstruction of intermediate base images, followed by image-domain change of basis into fat, water and bone. Sensitivity to scatter was evaluated by i) adjusting source collimation (12 to 400 mm width) and ii) subtracting various fractions of the true scatter from the projections at 400 mm collimation. The impact of spectral calibration was studied by shifting the effective beam energy (± 2 keV) when creating the PDD lookup table. We further simulated a realistic BME imaging framework, where the scatter was estimated using a fast Monte Carlo (MC) simulation from a preliminary decomposition of the object; the object was a realistic wrist phantom with an 0.85 mL BME stimulus in the radius. The decomposition is sensitive to scatter: approx. <20 mm collimation width or <10% error of scatter correction in a full field-of-view setting is needed to resolve BME. A mismatch in PDD decomposition calibration of ± 1 keV results in ~25% error in fat fraction estimates. In the wrist phantom study with MC scatter corrections, we were able to achieve ~0.79 mL true positive and ~0.06 mL false positive BME detection (compared to 0.85 mL true BME volume). Detection of BME using DE CBCT with dual-layer FPD is feasible, but requires scatter mitigation, accurate scatter estimation, and robust spectral calibration.

Photon detector response function methodology using MCNP and shift hybrid radiation transport code for wide-area contamination assay applications

Radiation transport modeling using the Monte Carlo N-Particle (MCNP) radiation transport code and Monte Carlo code, Shift, were employed to model detector responses for a variety of wide-area photon contamination scenarios. In this study, 2′′ × 2′′ and 3′′ × 3′′ cylindrical NaI(Tl) scintillation detector configurations at source detector-distances of 0.5 cm, 1 cm, 2.54 cm, 10 cm, and 30 cm were modeled. Media of soil, concrete, and steel were evaluated for contamination depths ranging from surface to a depth of an infinite thickness in each medium for photon energies ranging from 20 keV to 3 MeV, which correspond to the energies that current detectors can discern. Monoenergetic photon surface contamination detector responses for each of the media, source–detector distances, and detectors were estimated using MCNP v6.2. Shift was harnessed for improved variance reduction of particle transport in highly attenuating media to obtain average cell fluxes in the two MCNP NaI(Tl) scintillation detector configurations. Average cell flux values in Shift were coupled with detector responses from MCNP to convert average cell flux in a void to energy distribution of pulses in the NaI(Tl) scintillation detector crystal of interest. An optimized detector response function methodology was developed by coupling the MCNP radiation transport method with the Consistent Adjoint Driven Importance Sampling (CADIS) hybrid radiation transport method built into Shift to significantly decrease the runtime of thousands of MCNP pulse height simulations. The methodology may be utilized to quickly and accurately facilitate the assessment of a broad range of wide-area environmental contamination assay and decommissioning cleanup applications.

Performance of a 6 × 6 mm2 SiPM Module for Time-Domain Diffuse Optics

In this work we present a detector module based on large-area silicon photomultiplier, whose design has been optimized for time-domain diffuse optics measurements. In this application, to the best of our knowledge, it represents the largest area detector ever demonstrated, with expected breakthrough performance in terms of scattered light harvesting efficiency. It has been characterized using widely adopted protocols for performance assessment of instruments. We demonstrate the superior light harvesting capability and improved depth sensitivity, which can reach 4 cm. Moreover, the retrieval of optical properties has been demonstrated to be linear and with sufficient accuracy in the range corresponding to the human tissues. As a first step toward clinical application, we first performed <i>in-vivo</i> measurement at very large source-detector distances to show the suitability of the realized detector to tackle new challenges like lung or heart optical monitoring as well as imaging of deep cerebral function.

Model equations of light scattering properties and a characteristic time of light propagation for polydisperse colloidal suspensions at different volume fractions.

We developed model equations of light scattering properties and a characteristic time of light propagation for polydisperse colloidal suspensions at different volume fractions. By the model equations, we examined numerical results using the first-order (dependent) scattering theory (FST) and radiative transfer theory in 600-980 nm wavelength. The model equations efficiently treat the interference of electric fields scattered from colloidal particles by a single effective coefficient, providing fast computation. Meanwhile, the FST provides accurate but complicated treatment. We found the interference effects on the scattering properties and characteristic time depend linearly on wavelength. Dimensionless analysis showed a simple mechanism of the interference effects, independently of wavelength and source-detector distance.