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At least 217 records · Page 12Linked to original sources
PubMed2003

Evaluation of optical properties of highly scattering media by moments of distributions of times of flight of photons.

A novel method for the determination of the optical properties of tissue from time-domain measurements is presented. The data analysis is based on the evaluation of the first moment and the second centralized moment, i.e., the mean time of flight and the variance of the measured distribution of times of flight (DTOF) of photons injected by short (picosecond) laser pulses. Analytical expressions are derived for calculation of absorption and of reduced scattering coefficients from these moments by application of diffusion theory for infinite and semi-infinite homogeneous media. The proposed method was tested on experimental data obtained with phantoms, and results for absorption and reduced scattering coefficients obtained by the proposed method are compared with those obtained by fitting of the same data with analytical solutions of the diffusion equation. Furthermore, the accuracy of the moment analysis was investigated for a range of integration limits of the DTOF. The moment analysis may serve as a comparatively fast method for evaluating optical properties with sufficient accuracy and can be used, e.g., for on-line monitoring of optical properties of biological tissue.

Lasers↗
PubMed1999

Smoluchowski dynamics of the vnd/NK-2 homeodomain from Drosophila melanogaster: first-order mode-coupling approximation.

This work is the first in a series devoted to applying mode coupling diffusion theory to the derivation of local dynamics properties of proteins in solution. The first-order mode-coupling approximation, or optimized Rouse-Zimm local dynamics (ORZLD), is applied here to derive the rotational dynamics of the bonds and compare the calculated with the experimental nmr 15N spin-lattice relaxation time behavior of the vnd/NK-2 homeodomain from Drosophila melanogaster. The starting point for the calculations is the experimental three-dimensional structure of the homeodomain determined by multidimensional nmr spectroscopy. The results of the computations are compared with experimentally measured 15N spin-lattice relaxation times T1, at 34.5 and 60.8 MHz, to check the first-order approximation. To estimate the relative importance of internal and overall rotation, both rigid and fluctuating dynamic models are examined, with fluctuations evaluated using molecular dynamics (MD) simulations. The correlation times for the fundamental bond vector time correlation function and for the second-order bond orientational TCF are obtained as a function of the residue number for vnd/NK-2. The stability of the corresponding local dynamics pattern for the fluctuating structure as a function of the length of the MD trajectory is presented. Diffusive dynamics, which is essentially free of model parameters even at first order in the mode-coupling diffusion approach, confirm that local dynamics of proteins can be described in terms of rotational diffusion of a fluctuating quasi-rigid structure. The comparison with the nmr data shows that the first-order mode coupling diffusion approximation accounts for the correct order of magnitude of the results and of important qualitative aspects of the data sensitive to conformational changes. Indications are obtained from this study to efficiently extend the theory to higher order in the mode-coupling expansion. These results demonstrate the promise of the mode-coupling approach, where the local dynamics of proteins is described in terms of rotational diffusion of a fluctuating quasi-rigid structure, to analyze nmr spin-lattice relaxation behavior.

Animals↗
PubMed2005

The diffusion of ecstasy through urban youth networks.

Ecstasy is a drug commonly associated with all-night, or all-weekend electronic dance events known as raves. Upper- and middle-class clubs, gay bars and clubs, and party venues are other common public settings where ecstasy use occurs. During the mid to late 1990s its use was reported in locations as distant as Australia and New Zealand, England and Scotland, and North America. In the United States, use increased dramatically at the end of the millennium, and drug monitoring systems began to report its presence among urban youth. Using social influence, social marketing and diffusion theory, this paper outlines the micro-level processes through which ecstasy traveled from downtown clubs catering to suburban young adults through urban youth networks through distributors and users. The paper is based on participant observation, and in-depth interviews with dealers and users collected during the period of peak diffusion 1999-2001, and survey data collected from 401 poly-drug users between the ages of 16 and 24 and collected at two time points from 1999-2002.

Adolescent↗
PubMed1977

Application of a convective diffusion model to membrane transport.

Studies were carried out on the permeation rate of butambed through a dimethicone membrane. Under conditions of "aqueous diffusion layer control", the permeation rate was accurately described by a mathematical model based on convective diffusion theory. In accordance with model, the rate of permeation from a saturated donor phase was shown to be equal to the rate of dissolution from a pure solid.

Aminobenzoates↗
PubMedDate not supplied

Retracing the steps of Marco Polo: from clinical trials to diffusion of interventions for smokers.

We report the results from a series of four controlled trials which evaluate efficacy, effectiveness, and implementation of interventions for smokers, and then discuss diffusion of our program in feasibility studies both nationally in Australia and internationally. In our first study we reported that when general practitioners delivered moderately brief advice to smokers, they have a 36% abstinence rate at three years. In the second study we found that doctors achieve a 12% abstinence at one year after giving minimal advice to patients, and when using a more involved intervention, quit rates of just less than 20% at one year. In the third and fourth studies of utilization of the smoking cessation program we report that reinforcement contact following a 2-hour training workshop increased doctors' use of a smoking cessation program at 6 months compared to no contact. The challenge of translating research findings into practice for the benefit of doctors in Australia as well as in a low-income country such as China, is described in two feasibility studies. Over the past 11 years we have disseminated the program nationally in Australia, and over 4500 doctors have been trained. Diffusion theory provides a useful model which has guided us in our implementation efforts in Australia and is being used to allow us to introduce the smoking cessation intervention for Chinese doctors to use.

Australia↗
PubMed2004

Memoryless control of boundary concentrations of diffusing particles.

Flux between regions of different concentration occurs in nearly every device involving diffusion, whether an electrochemical cell, a bipolar transistor, or a protein channel in a biological membrane. Diffusion theory has calculated that flux since the time of Fick (1855), and the flux has been known to arise from the stochastic behavior of Brownian trajectories since the time of Einstein (1905), yet the mathematical description of the behavior of trajectories corresponding to different types of boundaries is not complete. We consider the trajectories of noninteracting particles diffusing in a finite region connecting two baths of fixed concentrations. Inside the region, the trajectories of diffusing particles are governed by the Langevin equation. To maintain average concentrations at the boundaries of the region at their values in the baths, a control mechanism is needed to set the boundary dynamics of the trajectories. Different control mechanisms are used in Langevin and Brownian simulations of such systems. We analyze models of controllers and derive equations for the time evolution and spatial distribution of particles inside the domain. Our analysis shows a distinct difference between the time evolution and the steady state concentrations. While the time evolution of the density is governed by an integral operator, the spatial distribution is governed by the familiar Fokker-Planck operator. The boundary conditions for the time dependent density depend on the model of the controller; however, this dependence disappears in the steady state, if the controller is of a renewal type. Renewal-type controllers, however, produce spurious boundary layers that can be catastrophic in simulations of charged particles, because even a tiny net charge can have global effects. The design of a nonrenewal controller that maintains concentrations of noninteracting particles without creating spurious boundary layers at the interface requires the solution of the time-dependent Fokker-Planck equation with absorption of outgoing trajectories and a source of ingoing trajectories on the boundary (the so called albedo problem).

Biological Transport↗
PubMed2006

Investigation of concentrically and eccentrically layered droplets by light scattering.

Two pairs of immiscible liquid compounds are chosen to prepare levitated layered droplets with and without density difference between core and layer phases. The droplets are examined by light scattering along two orthogonal directions. A layered droplet without phase density difference is unambiguously identified as a concentric sphere by matching the observed scattering spectra with those calculated from the Aden-Kerker extension of Mie theory. For layered droplets with phase density difference, only the scattering spectrum from one of the scattering directions can be matched theoretically. These observations suggest that a static layered droplet is predominantly eccentric even though the embedded core is large by volume, as predicted from fluid mechanics. The consistency of the light-scattering characterization with the diffusion theory governing the evaporation of concentrically and eccentrically layered droplets is also established.

Journal Article↗
PubMed2001

Theory and laboratory study of a tall passive chamber for measuring gas fluxes at soil surface.

A tall passive flux chamber with a height significantly greater than its horizontal dimensions is proposed for measuring fluxes of volatile organic compounds (VOCs) at the soil surface. The main feature of this tall chamber is the presence of a vertical concentration gradient of the target gas in the chamber. The emission and transport behavior of the target gas in the soil-chamber system are analyzed using the diffusion theory. A mathematical model is developed to estimate the flux from the soil into the tall chamber, providing the target gas establishes a detectable vertical concentration gradient in the chamber. To obtain the data required for calculating flux, only two gas concentrations (C1 and C2) at two heights (h1 and h2) within the chamber need to be measured at the end of a short chamber placement time (tp). To evaluate the applicability of the tall chamber for measuring flux, several laboratory tests have been conducted, using CH2Cl2 and CH3Br as the target gases. The results indicate that the proposed tall chamber has promising potential as a method for measuring fluxes of VOCs at the soil surface.

Air Pollutants↗
PubMed1988

Kinetics of adsorption of proteins at interfaces: role of protein conformation in diffusional adsorption.

To elucidate the role of protein conformation in the kinetics of adsorption at interfaces, seven structural intermediates of bovine serum albumin were prepared and their adsorption at the air/water interface was studied. Molecular area calculations indicated two distinct molecular processes, the first being the creation of an area, delta A1, for anchoring the molecule during the initial phase of adsorption and the second being the delta A2 cleared during subsequent reorientation and rearrangement of adsorbed molecules at the interface. The delta A1 values for all the albumin intermediates were the same, indicating that the initial work pi delta A1 needed to anchor the molecule at the interface was independent of solution conformation of the protein. Unlike delta A1, delta A2 exhibited a bell-shaped relationship with the extent of refolded state of the intermediates. Calculation of diffusion coefficients indicated that greater the unfolded state of the albumin intermediate, the greater was the diffusion coefficient. It is shown that the simple diffusion theory is inadequate to explain quantitatively the kinetics of protein adsorption. Specific, conformation-dependent, solute-solvent and solute-interface interactions also seem to influence the kinetics of adsorption of proteins.

Adsorption↗
PubMed2006

A probable stellar solution to the cosmological lithium discrepancy.

The measurement of the cosmic microwave background has strongly constrained the cosmological parameters of the Universe. When the measured density of baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis calculations, the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented precision. The predicted primordial lithium abundance is a factor of two to three higher than the value measured in the atmospheres of old stars. With estimated errors of 10 to 25%, this cosmological lithium discrepancy seriously challenges our understanding of stellar physics, Big Bang nucleosynthesis or both. Certain modifications to nucleosynthesis have been proposed, but found experimentally not to be viable. Diffusion theory, however, predicts atmospheric abundances of stars to vary with time, which offers a possible explanation of the discrepancy. Here we report spectroscopic observations of stars in the metal-poor globular cluster NGC 6397 that reveal trends of atmospheric abundance with evolutionary stage for various elements. These element-specific trends are reproduced by stellar-evolution models with diffusion and turbulent mixing. We thus conclude that diffusion is predominantly responsible for the low apparent stellar lithium abundance in the atmospheres of old stars by transporting the lithium deep into the star.

Journal Article↗
PubMed2005

Light diffusion through a turbid parallelepiped.

Solutions of the diffusion approximation to the radiative transport equation are derived for a turbid (rectangular) parallelepiped using the method of image sources and applying extrapolated boundary conditions. The derived solutions are compared with Monte Carlo simulations in the steady-state and time domains. It is found that the diffusion theory is in good agreement with Monte Carlo simulations provided that the light is detected sufficiently far from the incident beam. Applications of the derived solutions, including the determination of the optical properties of the turbid parallelepiped, are discussed.

Animals↗
PubMedDate not supplied

Spectral transmittance and contrast in breast diaphanography.

Diaphanography is an imaging technique using transillumination with visible and near-infrared radiation, and a video camera, to diagnose breast disease, including cancer. Originally based on luminance contrast only, there is now interest in false-color multispectral imaging in selected spectral bands to improve tissue differentiation. Some success has been achieved, but the scientific basis for the results was unknown. This research is concerned with measurements of the diffuse transmittance of breast tissues as a function of wavelength in the 600-1060 nm range (and calculations of contrast with a one-dimensional diffusion theory model). Carcinoma and glandular tissues were found to have similar spectral transmittances with an increase in transmittance between 750 and 900 nm, and an absorption window around 960 nm. Adipose tissue showed a distinct transmittance minimum at 930 nm. In vivo measurement of an intact normal breast showed a minimum at about 825 nm, as yet unexplained. The transmittance data and reflectance data were used to derive the scattering and absorption coefficients. The diffusion length was also determined from radiance versus depth measurements. These coefficients were used for calculations of contrast in a one-dimensional slab model, with and without a layer of cancerous tissue positioned between two slabs of normal tissue.

Biophysical Phenomena↗
PubMed1998

Monte Carlo simulations for EndoBronchial Photodynamic Therapy: the influence of variations in optical and geometrical properties and of realistic and eccentric light sources.

BACKGROUND AND OBJECTIVE: Light dosimetry for endobronchial photodynamic therapy is not very advanced to date. This study investigates the dependency of the fluence rate distribution in the bronchial wall on several parameters. STUDY DESIGN/MATERIALS AND METHODS: A Monte Carlo model is employed for the illumination of a cylindrical cavity by a linear diffuser to compute the fluence rate distribution in the tissue. The influence of optical and geometrical properties (e.g., the absorption coefficient of the bronchial mucosa and the diameter of the treated lumen) have been investigated, as well as the consequences of varying output characteristics of the diffusers. The optical properties used are those of ex vivo pig bronchial mucosa. RESULTS: With on-axis linear diffusers that can be modelled as a row of isotropic point sources, a constant fluence rate buildup factor can be employed for varying diffuser lengths and lumen diameters. Extreme off-axis placement of the diffuser causes a highly variable, considerable increase in the maximum fluence rate as well as a highly asymmetrical fluence rate profile on the circumference of the illuminated lumen. The fluence rate profiles resulting from illumination with realistic diffusers can be evaluated by implementing the measured radiance profiles of these diffusers in the model. The changes in fluence rate caused by variations in the optical properties of the bronchial mucosa could be accounted for by diffusion theory. This relationship can be used to extrapolate the ex vivo results to the clinical situation. CONCLUSION: A set of practical rules of thumb is presented that can help to estimate fluence rate distributions in clinical practice.

Absorption↗
PubMed1975

Uptake of local anaesthetics: movement of ethanol into frog sciatic nerve.

The changes of nerve action potential with time of the excised bullfrog sciaticperoneal nerve were measured at 24 degrees C with the nerve immersed in oxygenated mineral oil containing various concentrations of ethanol. One series of measurements was made with intact nerve and a range of about 20 to 110 millimolar ethanol. Another series was made with nerves in which the epineural sheath was cut open longitudinally and using a range of about 15 to 51 millimolar ethanol. The results were analysed by means of simple diffusion theory, and below applied ethanol concentrations of about 70 millimolar the latent periods and times to total block fitted the theoretical pattern well. Disagreements between theory and measurement of previous workers is explainable by their neglecting to recognize that nerve bundles do not extend to the nerve periphery. It was found that the minimum oil concentration of ethanol capable of producing total block is 15 millimolar. Above 70 millimolar, behaviour was observed consistent with nerve damage. It was found possible, by measuring the ratio latent period: time to total block, to separate the relative contribution of the epineural sheath and epineurium as a diffusion barrier to ethanol from that of the perineurium and perilemma.

Action Potentials↗
PubMed1976

A sociological theory of the diffusion and social setting of opiate addiction.

The theory presented here is intended to apply to socially deprived areas where heroin addiction is endemic, narcotics are available, and adolescents are not naive about the consequences of heroin use. In such settings rapid epidemic changes occur in which within a short period of time large numbers are involved. The transition of heroin use is a conversion-type phemonenon which is facilitated by the basic belief system, the network of social circles and the social and psychological inducements for compliance.

Communication↗
PubMed1988

Effects of Tween 80 and liposomes on the corneal permeability of anti-inflammatory steroids.

The effects of Tween 80 and liposomes on the corneal permeability of dexamethasone (DM) and dexamethasone valerate (DV) were investigated in vitro. A model based on diffusion theory could successfully be applied to analyzing the process by which DM penetrates the cornea from DM preparations. The penetration rate increased according to the concentration of free DM. Although the penetration rate was increased by pretreatment with Tween 80, it was not affected by pretreatment with liposomes. The steroid that penetrated through the cornea from DV preparations had been metabolized to DM. The transfer of DM across the cornea from DV preparations tended to slow down to some extent during the penetration process. Diffusion models were not applicable to this process, in contrast to the corneal penetration of DM from DM preparations. When liposome preparations of DV were applied, the penetration rate of DM across the cornea depended the concentration of free DV in these preparations. When DV aqueous preparations containing different concentrations of Tween 80 were applied, the penetration rate increased as the concentration of the surfactant was increased, even though the concentration of free DV in the suspensions was kept constant. These results suggest that the corneal permeability of anti-inflammatory steroids is not affected by liposomes, but is accelerated by Tween 80.

Animals↗
PubMed2005

A library based fitting method for visual reflectance spectroscopy of human skin.

The diffuse reflectance spectrum of human skin in the visible region (400-800 nm) contains information on the concentrations of chromophores such as melanin and haemoglobin. This information may be extracted by fitting the reflectance spectrum with an optical diffusion based analytical expression applied to a layered skin model. With the use of the analytical expression, it is assumed that light transport is dominated by scattering. For port wine stain (PWS) and highly pigmented human skin, however, this assumption may not be valid resulting in a potentially large error in visual reflectance spectroscopy (VRS). Monte Carlo based techniques can overcome this problem but are currently too computationally intensive to be combined with previously used fitting procedures. The fitting procedure presented herein is based on a library search which enables the use of accurate reflectance spectra based on forward Monte Carlo simulations or diffusion theory. This allows for accurate VRS to characterize chromophore concentrations in PWS and highly pigmented human skin. The method is demonstrated using both simulated and measured reflectance spectra. An additional advantage of the method is that the fitting procedure is very fast.

Algorithms↗
PubMed1978

Potassium contracture of smooth muscle in the presence of dextran.

The apparent diffusion coefficient of Ca ions in the extracellular space of guinea pig taenia coli was estimated from model experiments to be 3.2 x 10(-6) cm2 sec -1, while the relationship between Ca concentration in the medium and tonic tension in 40 mM K-contracture was measured in the same muscle both in the presence and absence of dextran 10. On the basis of these experiments and certain assumptions, the time course of tension decline by Ca withdrawal during K-contracture was calculated. Under all dextran concentrations tested (0--15%) the calculated time course of tension decline was in good agreement with one actually observed except in a short period of time immediately after Ca withdrawal. The results suggested that Ca distribution in the extracellular space during the loss of Ca is in good agreement with the diffusion theory in cylinder, and that each muscle fiber shows its tension without delay in response to the change in Ca concentration in the vicinity of the fiber according to the tonic tension-Ca relationship mentioned above. The discrepancy between calculation and observation was partially explained on an experimental basis.

Animals↗