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1.  Evaluation of a fiberoptic-based system for measurement of optical properties in highly attenuating turbid media 
Background
Accurate measurements of the optical properties of biological tissue in the ultraviolet A and short visible wavelengths are needed to achieve a quantitative understanding of novel optical diagnostic devices. Currently, there is minimal information on optical property measurement approaches that are appropriate for in vivo measurements in highly absorbing and scattering tissues. We describe a novel fiberoptic-based reflectance system for measurement of optical properties in highly attenuating turbid media and provide an extensive in vitro evaluation of its accuracy. The influence of collecting reflectance at the illumination fiber on estimation accuracy is also investigated.
Methods
A neural network algorithm and reflectance distributions from Monte Carlo simulations were used to generate predictive models based on the two geometries. Absolute measurements of diffuse reflectance were enabled through calibration of the reflectance system. Spatially-resolved reflectance distributions were measured in tissue phantoms at 405 nm for absorption coefficients (μa) from 1 to 25 cm-1 and reduced scattering coefficients (μ′s MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacuaH8oqBgaqbamaaBaaaleaacqqGZbWCaeqaaaaa@3007@) from 5 to 25 cm-1. These data and predictive models were used to estimate the optical properties of tissue-simulating phantoms.
Results
By comparing predicted and known optical properties, the average errors for μa and μ′s MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacuaH8oqBgaqbamaaBaaaleaacqqGZbWCaeqaaaaa@3007@ were found to be 3.0% and 4.6%, respectively, for a linear probe approach. When bifurcated probe data was included and samples with μa values less than 5 cm-1 were excluded, predictive errors for μa and μ′s MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacuaH8oqBgaqbamaaBaaaleaacqqGZbWCaeqaaaaa@3007@ were further reduced to 1.8% and 3.5%.
Conclusion
Improvements in system design have led to significant reductions in optical property estimation error. While the incorporation of a bifurcated illumination fiber shows promise for improving the accuracy of μ′s MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacH8akY=wiFfYdH8Gipec8Eeeu0xXdbba9frFj0=OqFfea0dXdd9vqai=hGuQ8kuc9pgc9s8qqaq=dirpe0xb9q8qiLsFr0=vr0=vr0dc8meaabaqaciaacaGaaeqabaqabeGadaaakeaacuaH8oqBgaqbamaaBaaaleaacqqGZbWCaeqaaaaa@3007@ estimates, further study of this approach is needed to elucidate the source of discrepancies between measurements and simulation results at low μa values.
doi:10.1186/1475-925X-5-49
PMCID: PMC1570472  PMID: 16928274
2.  Using an oblique incident laser beam to measure the optical properties of stomach mucosa/submucosa tissue 
BMC Gastroenterology  2009;9:64.
Background
The purpose of the study is to determine the optical properties and their differences for normal human stomach mucosa/submucosa tissue in the cardiac orifice in vitro at 635, 730, 808, 890 and 980 nm wavelengths of laser.
Methods
The measurements were performed using a CCD detector, and the optical properties were assessed from the measurements using the spatially resolved reflectance, and nonlinear fitting of diffusion equation.
Results
The results of measurement showed that the absorption coefficients, the reduced scattering coefficients, the optical penetration depths, the diffusion coefficients, the diffuse reflectance and the shifts of diffuse reflectance of tissue samples at five different wavelengths vary with a change of wavelength. The maximum absorption coefficient for tissue samples is 0.265 mm-1 at 980 nm, and the minimum absorption coefficient is 0.0332 mm-1 at 730 nm, and the maximum difference in the absorption coefficients is 698% between 730 and 980 nm, and the minimum difference is 1.61% between 635 and 808 nm. The maximum reduced scattering coefficient for tissue samples is 1.19 mm-1 at 635 nm, and the minimum reduced scattering coefficient is 0.521 mm-1 at 980 nm, and the maximum difference in the reduced scattering coefficients is 128% between 635 and 980 nm, and the minimum difference is 1.15% between 890 and 980 nm. The maximum optical penetration depth for tissue samples is 3.57 mm at 808 nm, and the minimum optical penetration depth is 1.43 mm at 980 nm. The maximum diffusion constant for tissue samples is 0.608 mm at 890 nm, and the minimum diffusion constant is 0.278 mm at 635 nm. The maximum diffuse reflectance is 3.57 mm-1 at 808 nm, and the minimum diffuse reflectance is 1.43 mm-1 at 980 nm. The maximum shift Δx of diffuse reflectance is 1.11 mm-1 at 890 nm, and the minimum shift Δx of diffuse reflectance is 0.507 mm-1 at 635 nm.
Conclusion
The absorption coefficients, the reduced scattering coefficients, the optical penetration depths, the diffusion coefficients, the diffuse reflectance and the shifts of diffuse reflectance of tissue samples at 635, 730, 808, 890 and 980 nm wavelengths vary with a change of wavelength. There were significant differences in the optical properties for tissue samples at five different wavelengths (P < 0.01).
doi:10.1186/1471-230X-9-64
PMCID: PMC2743700  PMID: 19715589
3.  Quantitative photoacoustic measurement of tissue optical absorption spectrum aided by an optical contrast agent 
Optics express  2009;17(6):4879-4889.
In photoacoustic imaging, the intensity of photoacoustic signal induced by optical absorption in biological tissue is proportional to light energy deposition, which is the product of the absorption coefficient and the local light fluence. Because tissue optical properties are highly dependent on the wavelength, the spectrum of the local light fluence at a target tissue beneath the sample surface is different than the spectrum of the incident light fluence. Therefore, quantifying the tissue optical absorption spectrum by using a photoacoustic technique is not feasible without the knowledge of the local light fluence. In this work, a highly accurate photoacoustic measurement of the subsurface tissue optical absorption spectrum has been realized for the first time by introducing an extrinsic optical contrast agent with known optical properties. From the photoacoustic measurements with and without the contrast agent, a quantified measurement of the chromophore absorption spectrum can be achieved in a strongly scattering medium. Experiments on micro-flow vessels containing fresh canine blood buried in phantoms and chicken breast tissues were carried out in a wavelength range from 680 to 950 nm. Spectroscopic photoacoustic measurements of both oxygenated and deoxygenated blood specimens presented an improved match with the reference when employing this technique.
PMCID: PMC2689517  PMID: 19293919
4.  Determination of reference values for optical properties of liquid phantoms based on Intralipid and India ink 
Biomedical Optics Express  2014;5(7):2037-2053.
A multi-center study has been set up to accurately characterize the optical properties of diffusive liquid phantoms based on Intralipid and India ink at near-infrared (NIR) wavelengths. Nine research laboratories from six countries adopting different measurement techniques, instrumental set-ups, and data analysis methods determined at their best the optical properties and relative uncertainties of diffusive dilutions prepared with common samples of the two compounds. By exploiting a suitable statistical model, comprehensive reference values at three NIR wavelengths for the intrinsic absorption coefficient of India ink and the intrinsic reduced scattering coefficient of Intralipid-20% were determined with an uncertainty of about 2% or better, depending on the wavelength considered, and 1%, respectively. Even if in this study we focused on particular batches of India ink and Intralipid, the reference values determined here represent a solid and useful starting point for preparing diffusive liquid phantoms with accurately defined optical properties. Furthermore, due to the ready availability, low cost, long-term stability and batch-to-batch reproducibility of these compounds, they provide a unique fundamental tool for the calibration and performance assessment of diffuse optical spectroscopy instrumentation intended to be used in laboratory or clinical environment. Finally, the collaborative work presented here demonstrates that the accuracy level attained in this work for optical properties of diffusive phantoms is reliable.
doi:10.1364/BOE.5.002037
PMCID: PMC4102347  PMID: 25071947
(170.5280) Photon migration; (170.7050) Turbid media; (170.3890) Medical optics instrumentation
5.  Noninvasive diffuse optical monitoring of head and neck tumor blood flow and oxygenation during radiation delivery 
Biomedical Optics Express  2012;3(2):259-272.
This study explored using a novel diffuse correlation spectroscopy (DCS) flow-oximeter to noninvasively monitor blood flow and oxygenation changes in head and neck tumors during radiation delivery. A fiber-optic probe connected to the DCS flow-oximeter was placed on the surface of the radiologically/clinically involved cervical lymph node. The DCS flow-oximeter in the treatment room was remotely operated by a computer in the control room. From the early measurements, abnormal signals were observed when the optical device was placed in close proximity to the radiation beams. Through phantom tests, the artifacts were shown to be caused by scattered x rays and consequentially avoided by moving the optical device away from the x-ray beams. Eleven patients with head and neck tumors were continually measured once a week over a treatment period of seven weeks, although there were some missing data due to the patient related events. Large inter-patient variations in tumor hemodynamic responses were observed during radiation delivery. A significant increase in tumor blood flow was observed at the first week of treatment, which may be a physiologic response to hypoxia created by radiation oxygen consumption. Only small and insignificant changes were found in tumor blood oxygenation, suggesting that oxygen utilizations in tumors during the short period of fractional radiation deliveries were either minimal or balanced by other effects such as blood flow regulation. Further investigations in a large patient population are needed to correlate the individual hemodynamic responses with the clinical outcomes for determining the prognostic value of optical measurements.
doi:10.1364/BOE.3.000259
PMCID: PMC3269843  PMID: 22312579
(170.0170) Medical optics and biotechnology; (170.3660) Light propagation in tissues; (170.3880) Medical and biological imaging; (170.6480) Spectroscopy, speckle
6.  Quantitative, depth-resolved determination of particle motion using multi-exposure, spatial frequency domain laser speckle imaging 
Biomedical Optics Express  2013;4(12):2880-2892.
Laser Speckle Imaging (LSI) is a simple, noninvasive technique for rapid imaging of particle motion in scattering media such as biological tissue. LSI is generally used to derive a qualitative index of relative blood flow due to unknown impact from several variables that affect speckle contrast. These variables may include optical absorption and scattering coefficients, multi-layer dynamics including static, non-ergodic regions, and systematic effects such as laser coherence length. In order to account for these effects and move toward quantitative, depth-resolved LSI, we have developed a method that combines Monte Carlo modeling, multi-exposure speckle imaging (MESI), spatial frequency domain imaging (SFDI), and careful instrument calibration. Monte Carlo models were used to generate total and layer-specific fractional momentum transfer distributions. This information was used to predict speckle contrast as a function of exposure time, spatial frequency, layer thickness, and layer dynamics. To verify with experimental data, controlled phantom experiments with characteristic tissue optical properties were performed using a structured light speckle imaging system. Three main geometries were explored: 1) diffusive dynamic layer beneath a static layer, 2) static layer beneath a diffuse dynamic layer, and 3) directed flow (tube) submerged in a dynamic scattering layer. Data fits were performed using the Monte Carlo model, which accurately reconstructed the type of particle flow (diffusive or directed) in each layer, the layer thickness, and absolute flow speeds to within 15% or better.
doi:10.1364/BOE.4.002880
PMCID: PMC3862160  PMID: 24409388
(110.6150) Speckle imaging; (170.3660) Light propagation in tissues
7.  Portable, Fiber-Based, Diffuse Reflection Spectroscopy (DRS) Systems for Estimating Tissue Optical Properties 
Applied spectroscopy  2011;62(2):206-215.
Steady-state diffuse reflection spectroscopy is a well-studied optical technique that can provide a noninvasive and quantitative method for characterizing the absorption and scattering properties of biological tissues. Here, we compare three fiber-based diffuse reflection spectroscopy systems that were assembled to create a light-weight, portable, and robust optical spectrometer that could be easily translated for repeated and reliable use in mobile settings. The three systems were built using a broadband light source and a compact, commercially available spectrograph. We tested two different light sources and two spectrographs (manufactured by two different vendors). The assembled systems were characterized by their signal-to-noise ratios, the source-intensity drifts, and detector linearity. We quantified the performance of these instruments in extracting optical properties from diffuse reflectance spectra in tissue-mimicking liquid phantoms with well-controlled optical absorption and scattering coefficients. We show that all assembled systems were able to extract the optical absorption and scattering properties with errors less than 10%, while providing greater than ten-fold decrease in footprint and cost (relative to a previously well-characterized and widely used commercial system). Finally, we demonstrate the use of these small systems to measure optical biomarkers in vivo in a small-animal model cancer therapy study. We show that optical measurements from the simple portable system provide estimates of tumor oxygen saturation similar to those detected using the commercial system in murine tumor models of head and neck cancer.
doi:10.1366/10-06052
PMCID: PMC3074566  PMID: 21499501
Optical spectroscopy; Diffuse reflection spectroscopy; DRS; Inverse Monte Carlo; Quantitative physiology; Tissue phantoms; Murine tumors; Spectrometers; Cancer
8.  Experimental Validation of the Effects of Microvasculature Pigment Packaging on In Vivo Diffuse Reflectance Spectroscopy 
Lasers in Surgery and Medicine  2010;42(7):680-688.
Background
Diffuse reflectance spectroscopy (DRS) uses the steady-state diffuse reflectance measured from the tissue surface to determine absorption and scattering properties of sampled tissue. Many inverse models used to determine absorber properties have assumed a homogeneous distribution of blood. However, blood in tissue is confined to blood vessels that occupy a small fraction of the overall volume. This simplified assumption can lead to large errors when measuring optical properties. The objective of this study was to examine the effect of confining absorbers to small volumes, such as the microvasculature, on in vivo DRS.
Study Design
We fabricated multi-layer microfluidic devices to mimic blood vessels with a size similar to skin microvasculature. We studied the effect of varying channel size (diameter = 22 and 44 μm) and absorber concentration (10–80% food color dye in water) on diffuse reflectance measurements. We also examined the in vivo reflectance from normal skin and non-melanoma skin cancer on 14 patients.
Results
Our results demonstrate that both absorption coefficient and vessel diameter affect the diffuse reflectance spectra. An empirically calculated packaging correction factor based on our experiments shows good agreement with previous theoretical derivations of the same factor. In vivo measurements on normal skin and basal cell carcinoma show that incorporating a correction factor greatly improves the fit of the inverse model to the spectra. In addition, there were statistically significant differences in measured mean vessel diameter and blood volume fraction between normal skin and basal cell carcinoma.
Conclusion
We have demonstrated experimentally the effect of pigment packaging in blood vessels over a physiologically relevant range of blood vessel size and absorption. The correction factors implemented to account for the packaging effect could potentially be used as diagnostic parameters for diagnosing skin cancers.
doi:10.1002/lsm.20933
PMCID: PMC3336741  PMID: 20740619
diffuse reflectance; pigment packaging; microfluidics
9.  Extraction of intrinsic fluorescence from single fiber fluorescence measurements on a turbid medium: experimental validation 
Biomedical Optics Express  2014;5(6):1913-1925.
The detailed mechanisms associated with the influence of scattering and absorption properties on the fluorescence intensity sampled by a single optical fiber have recently been elucidated based on Monte Carlo simulated data. Here we develop an experimental single fiber fluorescence (SFF) spectroscopy setup and validate the Monte Carlo data and semi-empirical model equation that describes the SFF signal as a function of scattering. We present a calibration procedure that corrects the SFF signal for all system-related, wavelength dependent transmission efficiencies to yield an absolute value of intrinsic fluorescence. The validity of the Monte Carlo data and semi-empirical model is demonstrated using a set of fluorescent phantoms with varying concentrations of Intralipid to vary the scattering properties, yielding a wide range of reduced scattering coefficients (μ′s = 0–7 mm −1). We also introduce a small modification to the model to account for the case of μ′s = 0 mm −1 and show its relation to the experimental, simulated and theoretically calculated value of SFF intensity in the absence of scattering. Finally, we show that our method is also accurate in the presence of absorbers by performing measurements on phantoms containing red blood cells and correcting for their absorption properties.
doi:10.1364/BOE.5.001913
PMCID: PMC4052919  PMID: 24940549
(060.2310) Fiber optics; (170.6280) Spectroscopy, fluorescence and luminescence
10.  Broadband ultraviolet-visible optical property measurement in layered turbid media 
Biomedical Optics Express  2012;3(6):1226-1240.
The ability to accurately measure layered biological tissue optical properties (OPs) may improve understanding of spectroscopic device performance and facilitate early cancer detection. Towards these goals, we have performed theoretical and experimental evaluations of an approach for broadband measurement of absorption and reduced scattering coefficients at ultraviolet-visible wavelengths. Our technique is based on neural network (NN) inverse models trained with diffuse reflectance data from condensed Monte Carlo simulations. Experimental measurements were performed from 350 to 600 nm with a fiber-optic-based reflectance spectroscopy system. Two-layer phantoms incorporating OPs relevant to normal and dysplastic mucosal tissue and superficial layer thicknesses of 0.22 and 0.44 mm were used to assess prediction accuracy. Results showed mean OP estimation errors of 19% from the theoretical analysis and 27% from experiments. Two-step NN modeling and nonlinear spectral fitting approaches helped improve prediction accuracy. While limitations and challenges remain, the results of this study indicate that our technique can provide moderately accurate estimates of OPs in layered turbid media.
doi:10.1364/BOE.3.001226
PMCID: PMC3370964  PMID: 22741070
(170.3660) Light propagation in tissues; (170.3890) Medical optics instrumentation; (170.6510) Spectroscopy, tissue diagnostics; (170.6935) Tissue characterization; (170.7050) Turbid media
11.  Determination of optical properties of normal and adenomatous human colon tissues in vitro using integrating sphere techniques 
AIM: The purpose of the present study is to compare the optical properties of normal human colon mucosa/submucosa and muscle layer/chorion, and adenomatous human colon mucosa/submucosa and muscle layer/chorion in vitro at 476.5, 488, 496.5, 514.5 and 532 nm. We believe these differences in optical properties should help differential diagnosis of human colon tissues by using optical methods.
METHODS: In vitro optical properties were investigated for four kinds of tissues: normal human colon mucosa/submucosa and muscle layer/chorion, and adenomatous human colon mucosa/submucosa and muscle layer/chorion. Tissue samples were taken from 13 human colons (13 adenomatous, 13 normal). From the normal human colons a total of 26 tissue samples, with a mean thickness of 0.40 mm, were used (13 from mucosa/submucosa and 13 from muscle layer/chorion), and from the adenomatous human bladders a total of 26 tissue samples, with a mean thickness of 0.40 mm, were used (13 from mucosa/submucosa and 13 from muscle layer/chorion). The measurements were performed using a double-integrating-sphere setup and the optical properties were assessed from these measurements using the adding-doubling method that was considered reliable.
RESULTS: The results of measurement showed that there were significant differences in the absorption coefficients and scattering coefficients between normal and adenomatous human colon mucosa/submucosa at the same wavelength, and there were also significant differences in the two optical parameters between both colon muscle layer/chorion at the same wavelength. And there were large differences in the anisotropy factors between both colon mucosa/submucosa at the same wavelength, there were also large differences in the anisotropy factors between both colon muscle layer/chorion at the same wavelength. There were large differences in the value ranges of the absorption coefficients, scattering coefficients and anisotropy factors between both colon mucosa/submucosa, and there were also large differences in these value ranges between both colon muscle layer/chorion. There are the same orders of magnitude in the absorption coefficients for four kinds of colon tissues. The scattering coefficients of these tissues exceed the absorption coefficients by at least two orders of magnitude.
CONCLUSION: There were large differences in the three optical parameters between normal and adenomatous human colon mucosa/submucosa at the same laser wavelength, and there were also large differences in these parameters between both colon muscle layer/chorion at the same laser wavelength. Large differences in optical parameters indicate that there were large differences in compositions and structures between both colon mucosa/submucosa, and between both colon muscle layer/chorion. Optical parameters for four kinds of colon tissues are wavelength dependent, and these differences would be useful and helpful in clinical applications of laser and tumors photodynamic therapy (PDT).
doi:10.3748/wjg.v11.i16.2413
PMCID: PMC4305627  PMID: 15832410
Optical properties; Laser; Normal and adenomatous human colon tissues; Integrating sphere
12.  A literature review and novel theoretical approach on the optical properties of whole blood 
Lasers in Medical Science  2013;29(2):453-479.
Optical property measurements on blood are influenced by a large variety of factors of both physical and methodological origin. The aim of this review is to list these factors of influence and to provide the reader with optical property spectra (250–2,500 nm) for whole blood that can be used in the practice of biomedical optics (tabulated in the appendix). Hereto, we perform a critical examination and selection of the available optical property spectra of blood in literature, from which we compile average spectra for the absorption coefficient (μa), scattering coefficient (μs) and scattering anisotropy (g). From this, we calculate the reduced scattering coefficient (μs′) and the effective attenuation coefficient (μeff). In the compilation of μa and μs, we incorporate the influences of absorption flattening and dependent scattering (i.e. spatial correlations between positions of red blood cells), respectively. For the influence of dependent scattering on μs, we present a novel, theoretically derived formula that can be used for practical rescaling of μs to other haematocrits. Since the measurement of the scattering properties of blood has been proven to be challenging, we apply an alternative, theoretical approach to calculate spectra for μs and g. Hereto, we combine Kramers–Kronig analysis with analytical scattering theory, extended with Percus–Yevick structure factors that take into account the effect of dependent scattering in whole blood. We argue that our calculated spectra may provide a better estimation for μs and g (and hence μs′ and μeff) than the compiled spectra from literature for wavelengths between 300 and 600 nm.
doi:10.1007/s10103-013-1446-7
PMCID: PMC3953607  PMID: 24122065
Blood; Optical properties; Spectroscopy; Absorption coefficient; Scattering coefficient; Scattering anisotropy
13.  Accurate optical parameter extraction procedure for broadband near-infrared spectroscopy of brain matter 
Journal of Biomedical Optics  2013;18(1):017008.
Abstract.
Modeling behavior of broadband (30 to 1000 MHz) frequency modulated near-infrared (NIR) photons through a phantom is the basis for accurate extraction of optical absorption and scattering parameters of biological turbid media. Photon dynamics in a phantom are predicted using both analytical and numerical simulation and are related to the measured insertion loss (IL) and insertion phase (IP) for a given geometry based on phantom optical parameters. Accuracy of the extracted optical parameters using finite element method (FEM) simulation is compared to baseline analytical calculations from the diffusion equation (DE) for homogenous brain phantoms. NIR spectroscopy is performed using custom-designed, broadband, free-space optical transmitter (Tx) and receiver (Rx) modules that are developed for photon migration at wavelengths of 680, 780, and 820 nm. Differential detection between two optical Rx locations separated by 0.3 cm is employed to eliminate systemic artifacts associated with interfaces of the optical Tx and Rx with the phantoms. Optical parameter extraction is achieved for four solid phantom samples using the least-square-error method in MATLAB (for DE) and COMSOL (for FEM) simulation by fitting data to measured results over broadband and narrowband frequency modulation. Confidence in numerical modeling of the photonic behavior using FEM has been established here by comparing the transmission mode’s experimental results with the predictions made by DE and FEM for known commercial solid brain phantoms.
doi:10.1117/1.JBO.18.1.017008
PMCID: PMC3545521  PMID: 23322361
functional near-infrared; traumatic brain injury; finite element method; diffusion equation; COMSOL; optical transmitter; optical receiver; tri-wavelength; continuous wave; time domain; frequency domain; photon density wave; vertical cavity surface emitting laser; insertion loss; insertion phase
14.  Broadband continuous-wave technique to measure baseline values and changes in the tissue chromophore concentrations 
Biomedical Optics Express  2012;3(11):2761-2770.
We present a broad-band, continuous-wave spectral approach to quantify the baseline optical properties of tissue and changes in the concentration of a chromophore, which can assist to quantify the regional blood flow from dynamic contrast-enhanced near-infrared spectroscopy data. Experiments were conducted on phantoms and piglets. The baseline optical properties of tissue were determined by a multi-parameter wavelength-dependent data fit of a photon diffusion equation solution for a homogeneous medium. These baseline optical properties were used to find the changes in Indocyanine green concentration time course in the tissue. The changes were obtained by fitting the dynamic data at the peak wavelength of the chromophore absorption, which were used later to estimate the cerebral blood flow using a bolus tracking method.
doi:10.1364/BOE.3.002761
PMCID: PMC3493236  PMID: 23162714
(300.6340) Spectroscopy, infrared; (290.5820) Scattering measurements; (290.1990) Diffusion
15.  Measuring and Modeling the Effect of Surface Moisture on the Spectral Reflectance of Coastal Beach Sand 
PLoS ONE  2014;9(11):e112151.
Surface moisture is an important supply limiting factor for aeolian sand transport, which is the primary driver of coastal dune development. As such, it is critical to account for the control of surface moisture on available sand for dune building. Optical remote sensing has the potential to measure surface moisture at a high spatio-temporal resolution. It is based on the principle that wet sand appears darker than dry sand: it is less reflective. The goals of this study are (1) to measure and model reflectance under controlled laboratory conditions as function of wavelength () and surface moisture () over the optical domain of 350–2500 nm, and (2) to explore the implications of our laboratory findings for accurately mapping the distribution of surface moisture under natural conditions. A laboratory spectroscopy experiment was conducted to measure spectral reflectance (1 nm interval) under different surface moisture conditions using beach sand. A non-linear increase of reflectance upon drying was observed over the full range of wavelengths. Two models were developed and tested. The first model is grounded in optics and describes the proportional contribution of scattering and absorption of light by pore water in an unsaturated sand matrix. The second model is grounded in soil physics and links the hydraulic behaviour of pore water in an unsaturated sand matrix to its optical properties. The optical model performed well for volumetric moisture content 24% ( 0.97), but underestimated reflectance for between 24–30% ( 0.92), most notable around the 1940 nm water absorption peak. The soil-physical model performed very well ( 0.99) but is limited to 4% 24%. Results from a field experiment show that a short-wave infrared terrestrial laser scanner ( = 1550 nm) can accurately relate surface moisture to reflectance (standard error 2.6%), demonstrating its potential to derive spatially extensive surface moisture maps of a natural coastal beach.
doi:10.1371/journal.pone.0112151
PMCID: PMC4226492  PMID: 25383709
16.  A Systematic Review of the Effects of Diffuse Optical Imaging in Breast Diseases 
Background
Optical mammography is a new diagnostic method that uses Near-infrared for detection of functional abnormalities and shows tissue activities by measuring absorption and scattering of Near-infrared light. This study aims to evaluate the safety and effectiveness of this technology.
Methods
Cochrane Library (Issue 10, 2012) and Medline (Nov 2012) weresearched using free text and Mesh. Studies that compared optical mammography with other diagnostic methods and used outcomes such as sensitivity, specificity and safety were included.
Results
Twelve studies were included in this review. A multicenter RCT showed that among 875 biopsied lesions, suspicion index led to 97% sensitivity, 14%specificity, 95% negative predictive value and 24% positive predictive value. In terms of oxygenation index, the included studies found that the process should be used with various wavelengths compared to single wavelength technique (690, 750, 788, 856 nm or 683, 912, 975nm). In terms of sensitivity and specificity, Diffuse Optical Tomography Computer Aided Detection is capable of distinguishing healthy tissues from malignant ones with 89% sensitivity and 94% specificity. Also, this technology could show increased blood flow around the tumor tissue compared to the healthy tissue effectively. Included studies did not report any information about the effects of technology on changing the treatment process or the final health outcomes.
Conclusion
Optical mammography is a safe, noninvasive, non-ionized diagnostic technology that can be used as a diagnostic supplement alongside conventional mammography for differentiating benign and malignant tumors. Women with higher breast density should be screened at younger ages and with more persistence than those who have lower densities.
PMCID: PMC4142904  PMID: 25250109
Optical mammography; Diffuse optical imaging; Near-infrared spectroscopy; Breast cancer
17.  Handheld Diffuse Reflectance Spectral Imaging (DRSi) for in-vivo characterization of skin 
Biomedical Optics Express  2014;5(2):573-586.
Diffuse reflectance spectroscopy provides a noninvasive means to measure optical and physiological properties of tissues. To expand on these measurements, we have developed a handheld diffuse reflectance spectral imaging (DRSi) system capable of acquiring wide field hyperspectral images of tissue. The image acquisition time was approximately 50 seconds for a 50x50 pixel image. A transport model was used to fit each spectra for reduced scattering coefficient, hemoglobin concentration and melanin concentration resulting in optical property maps. The system was validated across biologically relevant levels of reduced scattering (5.14% error) and absorption (8.34% error) using tissue simulating phantoms. DRSi optical property maps of a pigmented skin lesion were acquired in vivo. These trends in optical properties were consistent with previous observations using point probe devices.
doi:10.1364/BOE.5.000573
PMCID: PMC3920886  PMID: 24575350
(110.0110) Imaging systems; (110.0113) Imaging through turbid media; (170.6510) Spectroscopy, tissue diagnostics
18.  Effect of dependent scattering on the optical properties of Intralipid tissue phantoms 
Biomedical Optics Express  2011;2(8):2265-2278.
The calibration of optical tissue-simulating phantoms remains an open question in spite of the many techniques proposed for accurate measurements of optical properties. As a consequence, a reference phantom with well known optical properties is still missing. As a first step towards a reference phantom we have recently proposed to use dilutions of Intralipid 20%. In this paper we discuss a matter that is commonly ignored when dilutions are prepared, i.e., the possibility of deviations from the simple linear relationships between the optical properties of the dilution and the Intralipid concentration due to the effects of dependent scattering. The results of an experimental investigation showed that dependent scattering does not affect absorption. As for the reduced scattering coefficient the effect can be described adding a term proportional to the square of the concentration. However, for concentrations of interest for tissue optics deviations from linearity remain within about 2%. The experimental investigation also showed that the microphysical properties of Intralipid are not affected by dilution. These results show the possibility to easily obtain a liquid diffusive phantom whose optical properties are known with error smaller than about 1%. Due to the intrinsic limitations of the different techniques proposed for measuring the optical properties it seems difficult to obtain a similar accuracy for solid phantoms.
doi:10.1364/BOE.2.002265
PMCID: PMC3149524  PMID: 21833363
(160.4760) Optical properties; (170.6510) Spectroscopy, tissue diagnostic; (170.7050) Turbid media
19.  In vivo determination of skin near-infrared optical properties using diffuse optical spectroscopy 
Journal of biomedical optics  2008;13(1):014016.
We develop a superficial diffusing probe with a 3mm source-detector separation that can be used in combination with diffuse optical spectroscopic (DOS) methods to noninvasively determine full-spectrum optical properties of superficial in vivo skin in the wavelength range from 650 to 1000 nm. This new probe uses a highly scattering layer to diffuse photons emitted from a collimated light source and relies on a two-layer diffusion model to determine tissue absorption coefficient μa and reduced scattering coefficient μs′. By employing the probe to measure two-layer phantoms that mimic the optical properties of skin, we demonstrate that the probe has an interrogation depth of 1 to 2mm. We carry out SSFDPM (steady state frequency-domain photon migration) measurements using this new probe on the volar forearm and palm of 15 subjects, including five subjects of African descent, five Asians, and five Caucasians. The optical properties of in vivo skin determined using the superficial diffusing probe show considerable similarity to published optical properties of carefully prepared ex vivo epidermis+dermis.
doi:10.1117/1.2829772
PMCID: PMC2626348  PMID: 18315374
tissue optics; optical properties; epithelial tissue; near-infrared spectroscopy; photon migration
20.  Two dimensional mapping of the perfusion of the retina and optic nerve head. 
The British Journal of Ophthalmology  1995;79(12):1126-1132.
AIM--To present a new non-invasive method of performing a high definition topography of perfused vessels of the retina and the optic nerve head with simultaneous evaluation of blood flow. METHOD--By a combination of a laser Doppler flowmeter with a scanning laser system the perfusion of the retina and the optic nerve head is visualised. The principles of measuring blood flow by laser Doppler flowmetry are based on the optical Doppler effect: laser light scattered by a moving particle is shifted in frequency by an amount delta f. Our data acquisition and evaluation system is a modified laser scanning tomograph. The technical data are retinal area of measurement 2.7 mm x 0.7 mm, 10 degrees field with 256 points x 64 lines, measurement accuracy 10 microns, wavelength 670 nm and 790 nm, light power 100 microW and 200 microW, data acquisition time 2.048 s. Every line is scanned 128 times by a line sampling rate of 4000 Hz. By performing a discrete fast Fourier transformation over 128 intensities of each retinal point the laser Doppler shift is calculated for each retinal point. With these data a two dimensional map with 256 x 64 points of the retinal perfusion is created. The brightness of the pixel is coded by the value of the Doppler shift. Offline capillary blood flow is estimated in arbitrary units according to the theory of laser Doppler flowmetry in every region of interest of the perfusion picture. We estimated the reliability and the validity of the method. Retinal blood flow was measured by scanning laser Doppler flowmetry (SLDF) while varying intraocular pressure by a suction cup of three healthy volunteers. Measurements of retinal blood flow performed in 47 eyes by the presented method (SLDF) were correlated with data gained by a commercially available laser Doppler flowmeter. Perfusion pictures of the superficial retinal layer and of deep prelaminar layers in the optic nerve head are presented. RESULTS--The reliability coefficients r1 of 'flow', 'volume', and 'velocity' were 0.84, 0.85, and 0.84 respectively. We found a significant linear relation between SLDF flow and the ocular perfusion pressure (r = 0.84, p < 0.001). Comparative measurements of the retinal blood flow by SLDF and a commercially available laser Doppler flowmeter showed a linear and significant relation (flow r = 0.6, p < 0.0001, volume r = 0.4, p < 0.01). Capillaries of the retinal superficial vasculature or deep ciliary sourced capillaries of the optic nerve head became visible with a high resolution by the confocal technique dependent on the focus. Offline, the blood flow variables of areas of 100 microns x 100 microns were calculated. CONCLUSION--SLDF enables the visualisation of perfused capillaries and vessels of the retina and the optic nerve head in high resolution by two dimensional mapping of perfusion variables which are encoded by the Doppler signal. This method achieves simultaneously qualitative and quantitative evaluation of capillary blood flow of distinct areas of the capillary meshwork.
Images
PMCID: PMC505355  PMID: 8562550
21.  Diffuse reflectance spectroscopy with a self-calibrating fiber optic probe 
Optics letters  2008;33(16):1783-1785.
Calibration of the diffuse reflectance spectrum for instrument response and time-dependent fluctuation as well as interdevice variations is complicated, time consuming, and potentially inaccurate. We describe a novel fiber optic probe with a real-time self-calibration capability that can be used for tissue optical spectroscopy. The probe was tested in a number of liquid phantoms over a relevant range of tissue optical properties. Absorption and scattering coefficients are extracted with an average absolute error and standard deviation of 6.9% ± 7.2% and 3.5% ± 1.5%, respectively.
PMCID: PMC2802059  PMID: 18709086
22.  Feasibility of Direct Digital Sampling for Diffuse Optical Frequency Domain Spectroscopy in Tissue 
Measurement science & technology  2013;24(4):045501-.
Frequency domain optical spectroscopy in the diffusive regime is currently being investigated for biomedical applications including tumor detection, therapy monitoring, exercise metabolism, and others. Analog homodyne or heterodyne detection of sinusoidally modulated signals have been the predominant method for measuring phase and amplitude of photon density waves that have traversed through tissue. Here we demonstrate the feasibility of utilizing direct digital sampling of modulated signals using a 3.6 Gigasample/second 12 bit Analog to Digital Converter. Digitally synthesized modulated signals between 50MHz and 400MHz were measured on tissue simulating phantoms at six near-infrared wavelengths. An amplitude and phase precision of 1% and 0.6 degrees were achieved during drift tests. Amplitude, phase, scattering and absorption values were compared with a well-characterized network analyzer based diffuse optical device. Measured optical properties measured with both systems were within 3.6% for absorption and 2.8% for scattering over a range of biologically relevant values. Direct digital sampling represents a viable method for frequency domain diffuse optical spectroscopy and has the potential to reduce system complexity, size, and cost.
doi:10.1088/0957-0233/24/4/045501
PMCID: PMC3963501  PMID: 24678143
frequency domain; photon migration; diffuse optics; digital sampling; spectroscopy
23.  In Vivo Optical Signatures of Neuronal Death in a Mouse Model of Alzheimer’s Disease 
Lasers in surgery and medicine  2013;46(1):27-33.
Background
There currently is a need for cost-effective, quantitative techniques to evaluate the gradual progression of Alzheimer’s disease (AD). Measurement techniques based on diffuse optical spectroscopy can detect blood perfusion and brain cellular composition changes, through measuring the absorption (μa) and reduced scattering (μs′) coefficients, respectively, using non-ionizing near-infrared light. Previous work has shown that brain perfusion deficits in an AD mouse model can be detected. The objective of this study was to determine if μs′ is sensitive to the inflammation and neuron death found in AD.
Methods
We used spatial frequency domain imaging (SFDI) to form quantitative maps of μa and μs′ in 3-month old male CaM/Tet-DTA mice harboring transgenes for the doxycyline-regulated neuronal expression of diphtheria toxin. When doxycycline is removed from the diet, CaM/ Tet-DTA mice develop progressive neuronal loss in forebrain neurons. Mice (n =5) were imaged longitudinally immediately prior to and after 23 days of lesion induction, and μa and μs′ (30 wavelengths, 650–970 nm) were compared to properties obtained from Tet-DTA controls (n =5). Neuron death and infiltration of inflammatory cells in brain cortical slices was confirmed with immunohistochemistry.
Results
No significant difference in baseline scattering and absorption were measured between CaM/Tet-DTA mice and controls. After 23 days of lesion induction, brain cortical μs′ was 11–16% higher in the CaM/Tet-DTA mice than in controls (P <0.03). Longitudinal imaging showed no significant difference in μs′ between the first and 23rd day of imaging in controls. Removing doxycycline from the diet was associated with a significant decrease in total hemoglobin concentrations (119 ± 9 μM vs. 91 ± 8 μM) (P <0.05) in controls, but not in CaM/Tet-DTA mice.
Conclusions
Neuronal death and brain inflammation are associated with increased tissue scattering (μs′) and this optical biomarker may be useful in pre-clinical AD therapy evaluation or monitoring of disease progression in AD patients.
doi:10.1002/lsm.22206
PMCID: PMC4001800  PMID: 24284732
diffuse optical spectroscopy; absorption; scattering; spatial frequency domain imaging; neuron death; inflammation
24.  In vivo optical signatures of neuronal death in a mouse model of Alzheimer's disease 
Lasers in Surgery and Medicine  2013;46(1):27-33.
Background
There currently is a need for cost-effective, quantitative techniques to evaluate the gradual progression of Alzheimer's disease (AD). Measurement techniques based on diffuse optical spectroscopy can detect blood perfusion and brain cellular composition changes, through measuring the absorption (µa) and reduced scattering (µs′) coefficients, respectively, using non-ionizing near-infrared light. Previous work has shown that brain perfusion deficits in an AD mouse model can be detected. The objective of this study was to determine if µs′ is sensitive to the inflammation and neuron death found in AD.
Methods
We used spatial frequency domain imaging (SFDI) to form quantitative maps of µa and µs′ in 3-month old male CaM/Tet-DTA mice harboring transgenes for the doxycyline-regulated neuronal expression of diphtheria toxin. When doxycycline is removed from the diet, CaM/Tet-DTA mice develop progressive neuronal loss in forebrain neurons. Mice (n = 5) were imaged longitudinally immediately prior to and after 23 days of lesion induction, and µa and µs′ (30 wavelengths, 650–970 nm) were compared to properties obtained from Tet-DTA controls (n = 5). Neuron death and infiltration of inflammatory cells in brain cortical slices was confirmed with immunohistochemistry.
Results
No significant difference in baseline scattering and absorption were measured between CaM/Tet-DTA mice and controls. After 23 days of lesion induction, brain cortical µs′ was 11–16% higher in the CaM/Tet-DTA mice than in controls (P < 0.03). Longitudinal imaging showed no significant difference in µs′ between the first and 23rd day of imaging in controls. Removing doxycycline from the diet was associated with a significant decrease in total hemoglobin concentrations (119 ± 9 µM vs. 91 ± 8 µM) (P < 0.05) in controls, but not in CaM/Tet-DTA mice.
Conclusions
Neuronal death and brain inflammation are associated with increased tissue scattering (µs′) and this optical biomarker may be useful in pre-clinical AD therapy evaluation or monitoring of disease progression in AD patients.Lasers Surg. Med. 46:27–33, 2014. © 2013 The Authors. Lasers in Surgery and Medicine Published by Wiley Periodicals, Inc.
doi:10.1002/lsm.22206
PMCID: PMC4001800  PMID: 24284732
diffuse optical spectroscopy; absorption; scattering; spatial frequency domain imaging; neuron death; inflammation
25.  Radiative transport in the delta-P1 approximation for semi-infinite turbid media 
Medical physics  2008;35(2):681-693.
We have developed an analytic solution for spatially resolved diffuse reflectance within the δ-P1 approximation to the radiative transport equation for a semi-infinite homogeneous turbid medium. We evaluate the performance of this solution by comparing its predictions with those provided by Monte Carlo simulations and the standard diffusion approximation. We demonstrate that the δ-P1 approximation provides accurate estimates for spatially resolved diffuse reflectance in both low and high scattering media. We also develop a multi-stage nonlinear optimization algorithm in which the radiative transport estimates provided by the δ-P1 approximation are used to recover the optical absorption (μa), reduced scattering ( μs′), and single-scattering asymmetry coefficients (g1) of liquid and solid phantoms from experimental measurements of spatially resolved diffuse reflectance. Specifically, the δ-P1 approximation can be used to recover μa, μs′, and g1 with errors within ±22%, ±18%, and ±17%, respectively, for both intralipid-based and siloxane-based tissue phantoms. These phantoms span the optical property range 4<(μs′/μa)<117. Using these same measurements, application of the standard diffusion approximation resulted in the recovery of μa and μs′ with errors of ±29% and ±25%, respectively. Collectively, these results demonstrate that the δ-P1 approximation provides accurate radiative transport estimates that can be used to determine accurately the optical properties of biological tissues, particularly in spectral regions where tissue may display moderate/low ratios of reduced scattering to absorption ( μs′/μa).
PMCID: PMC3509770  PMID: 18383690

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