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Results for “diffusion theory”

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PubMed2001

Propagation failure in discrete bistable reaction-diffusion systems: theory and experiments.

Wave front propagation failure is investigated in discrete bistable reaction-diffusion systems. We present a theoretical approach including dissipative effects and leading to an analytical expression of the critical coupling beyond which front propagation can occur as a function of the nonlinearity threshold parameter. Our theoretical predictions are confirmed by numerical simulations and experimental results on an equivalent electrical diffusive lattice.

Journal Article↗
PubMed2000

Information technology adoption in health care: when organisations and technology collide.

The implementation of advanced information systems is enabling great social and organisational changes. However, health care has been one of the slowest sectors to adopt and implement information technology (IT). This paper investigates why this is so, reviewing innovation diffusion theory and its application to both health organisations and information technology. Innovation diffusion theory identifies variables that influence the 'innovativeness' of organisations and the rate at which a technology diffuses. When analysed, these variables show why IT implementation has progressed at a slower rate in health compared with other industry sectors. The complexity of health organisations and their fragmented internal structure constrain their ability to adopt organisation wide IT. This is further impacted upon by the relative immaturity of strategic health IT which is complicated and unable to show quantifiable benefits. Both organisational and technological factors lead to the slow adoption of strategic IT. On the other hand, localised IT solutions and those providing measurable cost reductions have diffused well.

Diffusion of Innovation↗
PubMed2000

Calibration of near-infrared frequency-domain tissue spectroscopy for absolute absorption coefficient quantitation in neonatal head-simulating phantoms.

Frequency-domain tissue spectroscopy is a method to measure the absolute absorption coefficient of bulk tissues, assuming that a representative model can be found to recover the optical properties from measurements. While reliable methods exist to calculate absorption coefficients from source-detector measurements less than a few centimeters apart along a flat tissue volume, it is less obvious what methods can be used for transmittance through the larger tissue volumes typically associated with neonatal cerebral monitoring. In this study we compare the use of multiple distance frequency-domain measurements processed with (i) a modified Beer-Lambert law method, (ii) an analytic infinite-medium diffusion theory expression, and (iii) a numerical finite element solution of the diffusion equation, with the goal of recovering the absolute absorption coefficient of the medium. Based upon our observations, the modified Beer-Lambert method provides accurate absolute changes in the absorption coefficient, while analytic infinite-medium diffusion theory solutions or finite element-based numerical solutions can be used to calculate the absolute absorption coefficient, assuming that the data can be measured at multiple source-detector distances. We recommend that the infinite-medium multi-distance method or the finite element method be used across large tissue regions for calculation of the absolute absorption coefficient using frequency-domain near-infrared measurements at multiple positions along the head.

Absorption↗
PubMed2005

Instructional design strategies for health behavior change.

To help health educators build upon the best of different health behavior change theories, this paper offers a unified set of instructional design strategies for health education interventions. This set draws upon the recommendations of Rosenstock (Health Belief Model), Bandura (Social Cognitive Theory), and Dearing (Diffusion Theory), and uses a modified Events of Instruction framework (adapted from Robert Gagne): gain attention (convey health threats and benefits), present stimulus material (tailor message to audience knowledge and values, demonstrate observable effectiveness, make behaviors easy-to-understand and do), provide guidance (use trustworthy models to demonstrate), elicit performance and provide feedback (to enhance trialability, develop proficiency and self-efficacy), enhance retention and transfer (provide social supports and deliver behavioral cues). Sample applications of these strategies are provided. A brief review of research on adolescent smoking prevention enables consideration of the frequency with which these strategies are used, and possible patterns between strategy use and behavioral outcomes.

Adolescent↗
PubMed2002

A Monte Carlo EM approach for partially observable diffusion processes: theory and applications to neural networks.

We present a Monte Carlo approach for training partially observable diffusion processes. We apply the approach to diffusion networks, a stochastic version of continuous recurrent neural networks. The approach is aimed at learning probability distributions of continuous paths, not just expected values. Interestingly, the relevant activation statistics used by the learning rule presented here are inner products in the Hilbert space of square integrable functions. These inner products can be computed using Hebbian operations and do not require backpropagation of error signals. Moreover, standard kernel methods could potentially be applied to compute such inner products. We propose that the main reason that recurrent neural networks have not worked well in engineering applications (e.g., speech recognition) is that they implicitly rely on a very simplistic likelihood model. The diffusion network approach proposed here is much richer and may open new avenues for applications of recurrent neural networks. We present some analysis and simulations to support this view. Very encouraging results were obtained on a visual speech recognition task in which neural networks outperformed hidden Markov models.

Algorithms↗
PubMed1986

In vivo photometric analysis of hemoglobin.

Since virtually all the oxygen carried by blood at normal hematocrit is reversibly bound to red blood cell hemoglobin, the distribution of oxygen within the microcirculation can be determined from measurements of hemoglobin concentration and hemoglobin oxygen saturation in vessels of the network. Photometric methods that rely on light absorption and scattering properties of blood are described. Criteria for selecting the wavelengths needed to analyze hemoglobin in the microcirculation are specified. Two theoretical descriptions of light absorption and scattering, multiple scattering theory and photon diffusion theory, are applied to the problem. Practical approaches to the determination of hemoglobin concentration and oxygen saturation in the microcirculation follow from these theoretical formulations. Technical aspects of microscope photometry including light sources, microscopy, and detection systems are described with special emphasis on the problem of glare. The importance of in vitro as well as in vivo calibrations is stressed, and several recent applications of a working system are discussed. Current problems as well as future developments of this methodology are delineated as a guide to future work in this area.

Animals↗
PubMed1996

An investigation of light transport through scattering bodies with non-scattering regions.

Near-infra-red (NIR) spectroscopy is increasingly being used for monitoring cerebral oxygenation and haemodynamics. One current concern is the effect of the clear cerebrospinal fluid upon the distribution of light in the head. There are difficulties in modelling clear layers in scattering systems. The Monte Carlo model should handle clear regions accurately, but is too slow to be used for realistic geometries. The diffusion equation can be solved quickly for realistic geometries, but is only valid in scattering regions. In this paper we describe experiments carried out on a solid slab phantom to investigate the effect of clear regions. The experimental results were compared with the different models of light propagation. We found that the presence of a clear layer had a significant effect upon the light distribution, which was modelled correctly by Monte Carlo techniques, but not by diffusion theory. A novel approach to calculating the light transport was developed, using diffusion theory to analyze the scattering regions combined with a radiosity approach to analyze the propagation through the clear region. Results from this approach were found to agree with both the Monte Carlo and experimental data.

Brain↗
PubMed2004

Lattice density functional theory of molecular diffusion.

A density functional theory of diffusion is developed for lattice fluids with molecular flux as a functional of the density distribution. The formalism coincides exactly with the generalized Ono-Kondo density functional theory when there is no gradient of chemical potential, i.e., at equilibrium. Away from equilibrium, it gives Fick's first law in the absence of a potential energy gradient, and it departs from Fickian behavior consistently with the Maxwell-Stefan formulation. The theory is applied to model a nanopore, predicting nonequilibrium phase transitions and the role of surface diffusion in the transport of capillary condensate.

Journal Article↗
PubMed2005

A dual-electrode approach for highly selective detection of glucose based on diffusion layer theory: experiments and simulation.

A dual-electrode configuration for the highly selective detection of glucose in the diffusion layer of the substrate electrode is presented. In this approach, a glassy carbon electrode (GCE, substrate) modified with a conductive layer of glucose oxidase/Nafion/graphite (GNG) was used to create an interference-free region in its diffusion layer by electrochemical depletion of interfering electroactive species. A Pt microelectrode (tip, 5 microm in radius) was located in the diffusion layer of the GNG-modified GCE (GNG-G) with the help of scanning electrochemical microscopy. Consequently, the tip of the electrode could sense glucose selectively by detecting the amount of hydrogen peroxide (H2O2) formed from the oxidization of glucose on the glucose oxidase layer. The influences of parameters, including tip-substrate distance, substrate potential, and electrolyzing time, on the interference-removing efficiency of this dual-electrode approach have been investigated systematically. When the electrolyzing time was 30 s, the tip-substrate distance was 1.8 a (9.0 microm) (where a is the radius of the tip electrode), the potentials of the tip and substrate electrodes were 0.7 V and 0.4 V, respectively, and a mixture of ascorbic acid (0.3 mM), uric acid (0.3 mM), and 4-acetaminophen (0.3 mM) had no influence on the glucose detection. In addition, the current-time responses of the tip electrode at different tip-substrate distances in a solution containing interfering species were numerically simulated. The results from the simulation are in good agreement with the experimental data. This research provides a concept of detection in the diffusion layer of a substrate electrode, as an interference-free region, for developing novel microelectrochemical devices.

Carbon↗
PubMed2000

Dynamics of a double stranded DNA oligomer: mode-coupling diffusion approach and reduced rigid fragment models.

The local dynamics of a double stranded DNA fragment [d(CpGpCpApApApTpTpTpGpCpG)]2 of twelve base pairs is obtained to second order in the mode-coupling expansion of the Smoluchowski diffusion theory. The DNA is considered a fluctuating three-dimensional (3D) structure undergoing rotational diffusion. The starting structure for the calculations is the B canonical structure of the fragment, while the fluctuations are evaluated using molecular dynamics simulations, with the ensemble averages approximated by time averages along a trajectory of length 1.5 ns. The rotational dynamics of the bonds along the double strands are calculated and compared to experimental NMR relaxation rates of different 13C along the sequence: R(Cz), R(Cxy) and R(Hz-->Cz). For a fluctuating 3D structure the mode-coupling diffusion theory is found to be in good agreement with several relative characteristics of the experimental relaxation parameters, while motivations are given for the few differences which are due mainly to poor statistics or to inaccuracies in the diffusion model. With a view to application to larger DNA fragments, discussion is dedicated to the validity of reducing the number of degrees of freedom in the double helix statistics by grouping the atoms in rigid fragments (e.g. the backbone atoms, the sugar atoms and the base atoms of each nucleotide). Consideration is given to the effect on local dynamics properties of reduced descriptions that include only three or four rigid bodies per nucleotide as well as five rigid bodies per base pair. It is found that in general these approximations almost uniformly produce slight increase in the correlation time pattern, which grows as the rigidity in the model increases. The relative effects on the dynamic pattern for the most accurate rigid body models are modest. The errors in C1' and C5' mobilities are more significant if C5' is included in the backbone rigid body. These results offer new tools to analyse NMR relaxation behaviour and new perspectives in studying the role of dynamics in biological macromolecules.

Algorithms↗
PubMed2003

A diffusion-based theory of organism dispersal in heterogeneous populations.

We develop a general theory of organism movement in heterogeneous populations that can explain the leptokurtic movement distributions commonly measured in nature. We describe population heterogeneity in a state-structured framework, employing advection-diffusion as the fundamental movement process of individuals occupying different movement states. Our general analysis shows that population heterogeneity in movement behavior can be defined as the existence of different movement states and among-individual variability in the time individuals spend in these states. A presentation of moment-based metrics of movement illustrates the role of these attributes in general dispersal processes. We also present a special case of the general theory: a model population composed of individuals occupying one of two movement states with linear transitions, or exchange, between the two states. This two-state "exchange model" can be viewed as a correlated random walk and provides a generalization of the telegraph equation. By exploiting the main result of our general analysis, we characterize the exchange model by deriving moment-based metrics of its movement process and identifying an analytical representation of the model's time-dependent solution. Our results provide general and specific theoretical explanations for empirical patterns in organism movement; the results also provide conceptual and analytical bases for extending diffusion-based dispersal theory in several directions, thereby facilitating mechanistic links between individual behavior and spatial population dynamics.

Animal Migration↗
PubMed2001

Complex morphogenesis of surfaces: theory and experiment on coupling of reaction-diffusion patterning to growth.

Reaction-diffusion theory for pattern formation is considered in relation to processes of biological development in which there is continuous growth and shape change as each new pattern forms. This is particularly common in the plant kingdom, for both unicellular and multicellular organisms. In addition to the feedbacks in the chemical dynamics, there is then another loop linking size and shape changes with the reaction-diffusion patterning of growth controllers in the growing region. In studies by computation, the codes must incorporate, alongside the usual solvers of the partial differential dynamic equations, a versatile growth code, to express any kind of shape change. We have found that regulation of shape change in particular ways (e.g. to make narrow-angle branchings) demands new features in our chemical mechanisms. Our growth algorithm is for a surface growing tangentially, but moving outward and changing shape to accommodate the extra area. This is potentially applicable both to the tunica layer of multicellular plant meristems and to the growing tip of the cell surface, e.g. in the morphogenesis of single-celled chlorophyte algae which display branching processes: whorl formation in Acetabularia (Dasycladales) and repeated dichotomous branching in Micrasterias (Desmidiaceae). For computational studies, a hemispherical shell is a reasonable idealization of the initial shape. We describe results of two types of study: (1) Pattern formation by three reaction-diffusion models, with contrasted nonlinearities, on the hemispherical shell, particularly to find conditions for robust formation of annular pattern or pattern for dichotomous branching, both of which are common in plants. (2) Sequential dichotomous branchings in a system growing and changing in shape from the hemispherical start.

Journal Article↗
PubMed1999

Diffusion MR imaging. Theory and applications.

Diffusion MR imaging provides a novel way to characterize tissues based on sensitivity to the microscope molecular motion of water. Clinical implementation requires strong, fast hardware and careful post-processing of diffusion parameters. It is important to recognize that diffusion images and derivatives such as the trace of the diffusion tensor are quite specific in reflecting the physical properties of diffusion, but are non-specific for pathology. Restricted diffusion is the earliest clinically detectable sign of ischemia, but similar diffusion changes can be seen with infection and some tumors. Diffusion MR techniques are providing new ways to study problems in oncology, epilepsy, white matter disorders, and infectious diseases, both for research and clinical applications.

Animals↗
PubMed2002

Microstructural characterization using diffuse backscatter: theory and experiment.

A microstructure characterization technique is presented which utilizes the azimuthal moments of backscattered intensity to determine the Legendre moments of the microstructure's phase function. The technique is based on a late-time, diffuse approximate solution to the radiative transfer equation. Monte Carlo simulations are presented indicating that the technique is robust for the first azimuthal moment but less so for higher-order moments.

Journal Article↗
PubMed2002

Quantitative fluorescence lifetime spectroscopy in turbid media: comparison of theoretical, experimental and computational methods.

A Monte Carlo model developed to simulate time-resolved fluorescence propagation in a semi-infinite turbid medium was validated against previously reported theoretical and computational results. Model simulations were compared to experimental measurements of fluorescence spectra and lifetimes on tissue-simulating phantoms for single and dual fibre-optic probe geometries. Experiments and simulations using a single probe revealed that scattering-induced artefacts appeared in fluorescence emission spectra, while fluorescence lifetimes were unchanged. Although fluorescence lifetime measurements are generally more robust to scattering artefacts than are measurements of fluorescence spectra, in the dual-probe geometry scattering-induced changes in apparent lifetime were predicted both from diffusion theory and via Monte Carlo simulation, as well as measured experimentally. In all cases, the recovered apparent lifetime increased with increasing scattering and increasing source-detector separation. Diffusion theory consistently underestimated the magnitude of these increases in apparent lifetime (predicting a maximum increase of approximately 15%), while Monte Carlo simulations and experiment were closely matched (showing increases as large as 30%). These results indicate that quantitative simulations of time-resolved fluorescence propagation in turbid media will be important for accurate recovery of fluorophore lifetimes in biological spectroscopy and imaging applications.

Computer Simulation↗
PubMed2005

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

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

Algorithms↗