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PubMed1999

Effect of localization devices and registration methods on the accuracy of stereotactic frame systems predicted by the Gaussian approach.

A qualitative work-flow analysis of a neurosurgical procedure indicates that the resolution of the image used to plan the intervention is the major source of inaccuracy. Quantitative experimental measurements confirm this observation. They fail, however, to explain the relationship between the accuracy of the frame components involved in a stereotactic procedure and the overall application accuracy. This investigation shows that the novel Gaussian approach is a flexible framework for the calculation of the application accuracy of frame systems. Therefore, the Gaussian approach provides a detailed understanding of the interplay between the various factors affecting accuracy. The basic ideas and limitations of the Gaussian approach are briefly explained. The effect of fiducial marker distribution and registration is investigated and shown to introduce a spatial dependence to the accuracy. The results of the Gaussian approach are compared with experimental data for three stereotactic frame devices: Leksell G, Cosman-Roberts-Wells, and Brown-Roberts-Wells. Although the Gaussian approach is an approximation, it reproduces the accuracy measured in the experiment within the statistical error of that experiment.

Algorithms↗

PubMed1991

Practical identification of functional expansions of nonlinear systems submitted to non-Gaussian inputs.

Time-domain identification of nonlinear systems represented by functional expansions is considered. A general framework is defined for the analysis of three identification methods: the widely used cross-correlation method, Korenberg's method, and a suboptimal least-squares method based on a stochastic approximation algorithm. First, the major characteristics of the underlying estimation problem are pointed out. Then, the identification methods are interpreted as approximations to an optimal estimator, which helps gain insight into their internal functioning and to the investigation of their connections and differences. Examination of results previously published and of the simulations reported in this article indicate that stochastic approximation is an interesting alternative to other existing methods. Identification of a biological system stimulated by a non-Gaussian input confirms the practicality of this approach.

Binomial Distribution↗

PubMed2005

How much margin reduction is possible through gating or breath hold?

We determined the relationship between intra-fractional breathing motion and safety margins, using daily real-time tumour tracking data of 40 patients (43 tumour locations), treated with radiosurgery at Hokkaido University. We limited our study to the dose-blurring effect of intra-fractional breathing motion, and did not consider differences in positioning accuracy or systematic errors. The additional shift in the prescribed isodose level (e.g. 95 %) was determined by convolving a one-dimensional dose profile, having a dose gradient representing an 8 MV beam through either lung or water, with the probability density function (PDF) of breathing. This additional shift is a measure for the additional margin that should be applied in order to maintain the same probability of tumour control as without intra-fractional breathing. We show that the required safety margin is a nonlinear function of the peak-to-peak breathing motion. Only a small reduction in the shift of isodose curves was observed for breathing motion up to 10 mm. For larger motion, 20 or 30 mm, control of patient breathing during irradiation, using either gating or breath hold, can allow a substantial reduction in safety margins of about 7 or 12 mm depending on the dose gradient prior to blurring. Clinically relevant random setup uncertainties, which also have a blurring effect on the dose distribution, have only a small effect on the margin needed for intra-fractional breathing motion. Because of the one-dimensional nature of our analysis, the resulting margins are mainly applicable in the superior-inferior direction. Most measured breathing PDFs were not consistent with the PDF of a simple parametric curve such as cos4, either because of irregular breathing or base-line shifts. Instead, our analysis shows that breathing motion can be modelled as Gaussian with a standard deviation of about 0.4 times the peak-to-peak breathing motion.

Humans↗

PubMedDate not supplied

[Rational electrovectorcardiography].

The mental process of electrocardiographists to adjust the electrocardiogram's or vectorcardiogram's interpretation with the sequence of myocardial depolarization and repolarization can be accomplished in two ways. The first one is through vectorial analysis and the second concerns the analysis of unipolar morphologies. The most suitable is to follow both ways. The first procedure constitutes the classic method accepted since Einthoven's time and refers to the vectorial representation of the electrical charges of the heart. This representation approaches the electrical moment of a unique dipole, when cardiac exploration is performed at the smallest adequate distance. This allows to establish the spatial position of the instantaneous vectors, or of the mean vector, by using different leads. The study of unipolar morphologies permits to know the distribution of the mentioned charges in the conducting medium. The adjustment of the electrical charges depends on the location of the wave fronts. These charges produce potential variations in the physical sense--Gaussian type--at the epicardial surface and also at any point of the conducting medium surrounding the heart as far as the skin. This procedure requires the use of Poisson's integral, based on the principle of Helmholtz' electromotive surface. Thereafter, it is mandatory to perform another adjustment for the results of both procedures and to scrutinize the inner cellular metabolic process, which can explain the behavior of the observed electrical phenomena and suggest the adequate treatment.

Humans↗

PubMed2003

Impacts of environmental variability in open populations and communities: "inflation" in sink environments.

Ecological communities are typically open to the immigration and emigration of individuals, and also variable through time. In this paper we argue that interesting and potentially important effects arise when one splices together spatial fluxes and temporal variability. The particular system we examine is a sink habitat, where a species faces deterministic extinction but is rescued by recurrent immigration. We have shown, using a simple extension of the canonical exponential growth model in a time-varying environment, that variation "inflates" the average abundance of sink populations. We can analytically quantify the magnitude of this effect in several special cases (square-wave temporal variation and Gaussian stochastic variation). The inflationary effect can be large in "intermittent" sinks (where there are periods with positive growth), and when temporal variation is strongly autocorrelated. The effect appears to be robust to incorporation of demographic stochasticity (due to discrete birth-death-immigration processes), and to direct density dependence. With discrete generations, however, one can observe a wide range of effects of temporal variation, including depression as well as inflation. We argue that the inflationary effect of temporal variation in sink habitats can have important implications for community structure, because it can increase the average abundance (and hence local impacts) of species that on average are being excluded from a local community. We illustrate the latter effect using a familiar model of exploitative competition for a single limiting resource. We demonstrate that temporal variation can reverse local competitive dominance, even to the extent of allowing an inferior competitor maintained by immigration to exclude a competing species that would be locally superior in a constant environment.

Algorithms↗

PubMed2006

Effect of energy disorder in interfacial kinetics of dye-sensitized solar cells with organic hole transport material.

We describe the kinetic rates for dye regeneration in dye-sensitized solar cells with organic solid hole conductors, taking as a reference the experimental results of Haque et al. (ChemPhysChem 2003, 4, 89). Our model is based on Marcus rates for electron and hole transfer, emphasizing the Gaussian spread of energy levels in the molecular materials involved. We show that the energy disorder implies a broadening of the efficiency of hole transfer with respect to the thermodynamic driving force, as observed experimentally. The model also shows that tunning of the kinetic processes for high efficiency of energy conversion of the solar cell depends critically on the interplay between the reorganization energy and the broadening parameters of the energy distributions.

Journal Article↗

PubMed1979

[Image properties of computer tomography. III. Measurements on homogenous materials. Relationship of attenuation and scatter to various parameters (author's transl)].

The present paper investigates the variations due to various parameters on the image of homogeneous materials as obtained with the Somatom CT scanner. Sugar solutions were used with concentrations equivalent to the absorption values of soft tissues. Because of quantum noise, attentuation at various points is not uniform, but is distributed in a Gaussian manner round a mean value. Using the Evaluskop it was possible to determine the degree of scatter. It was shown that, with increasing density of the material, the scatter increased and that this can be reduced by subjecting the tomogram to a smoothing process. It was further shown how the scatter depends on various parameters, such as thickness and dose. The amount of scatter in a tomogram is important in determining the density gradation which can be a achieved.

Absorptiometry, Photon↗

PubMed2002

Statistically preserved structures in shell models of passive scalar advection.

It was conjectured recently that statiscally preserved structures underlie the statistical physics of turbulent transport processes. We analyze here in detail the time-dependent (noncompact) linear operator that governs the dynamics of correlation functions in the case of shell models of passive scalar advection. The problem is generic in the sense that the driving velocity field is neither Gaussian nor delta correlated in time. We show how to naturally discuss the dynamics in terms of an effective compact operator that displays "zero modes," which determine the anomalous scaling of the correlation functions. Since shell models have neither a Lagrangian structure nor "shape dynamics," this example differs significantly from standard passive scalar advection. Nevertheless, with the necessary modifications, the generality and efficacy of the concept of statistically preserved structures are further exemplified. In passing we point out a bonus of the present approach, in providing analytic predictions for the time-dependent correlation functions in decaying turbulent transport.

Journal Article↗

PubMed2005

Elastic neutron scattering study of water dynamics in ion-exchanged type-A zeolites.

With the aim to investigate, by means of elastic neutron scattering, the effects produced by the cation substitution on the dynamics of water in zeolites, we measured, using a neutron backscattering spectrometer, the temperature dependence of mean-square atomic displacements [u2] derived from window integrated quasielastic spectra of fully and partially hydrated Na-A and Mg50-A zeolites. The results, collected in the 20-273 K temperature range, reveal that, at low temperature, the [u2] shows a harmonic trend independent of hydration and cation substitution, and, at higher temperatures, the onset of a non-Gaussian dynamics of the elastic intensity. This latter takes place at T approximately 200 K and approximately 150 K for fully and partially hydrated samples, respectively. This behavior has been interpreted in terms of reorientational jumps of H atoms described by two-site processes within an asymmetric double-minimum potential. In spite of its simplicity, the model seems to reproduce the rearrangement of the hydrogen bond network of zeolitic water. The fit results indicate a reduced proton mobility by diminishing the water content and by the induced Na+-->Mg2+ ion exchange, in agreement with previous incoherent quasielastic neutron scattering results at higher temperatures.

Journal Article↗

PubMed2003

Multimode Hong-Ou-mandel interference.

We consider multimode two-photon interference at a beam splitter by photons created by spontaneous parametric down-conversion. The resulting interference pattern is shown to depend upon the transverse spatial symmetry of the pump beam. In an experiment, we employ the first-order Hermite-Gaussian modes in order to show that, by manipulating the pump beam, one can control the resulting two-photon interference behavior. We expect these results to play an important role in the engineering of quantum states of light for use in quantum information processing and quantum imaging.

Journal Article↗

PubMed2003

Three-dimensional symmetric shapes are discriminated more efficiently than asymmetric ones.

Objects with bilateral symmetry, such as faces, animal shapes, and many man-made objects, play an important role in everyday vision. Because they occur frequently, it is reasonable to conjecture that the brain may be specialized for symmetric objects. We investigated whether the human visual system processes three-dimensional (3D) symmetric objects more efficiently than asymmetric ones. Human subjects, having learned a symmetric wire object, discriminated which of two distorted copies of the learned object was more similar to the learned one. The distortion was achieved by adding 3D Gaussian positional perturbations at the vertices of the wire object. In the asymmetric condition, the perturbation was independent from one vertex to the next. In the symmetric condition, independent perturbations were added to only half of the object; perturbations on the other half retained the symmetry of the object. We found that subjects' thresholds were higher in the symmetric condition. However, since the perturbation in the symmetric condition was correlated, a stimulus image provided less information in the symmetric condition. Taking this in to consideration, an ideal-observer analysis revealed that subjects were actually more efficient at discriminating symmetric objects. This reversal in interpretation underscores the importance of ideal-observer analysis. A completely opposite, and wrong, conclusion would have been drawn from analyzing only human discrimination thresholds. Given the same amount of information, the visual system is actually better able to discriminate symmetric objects than asymmetric ones.

Differential Threshold↗

PubMed2004

Stalk phase formation: effects of dehydration and saddle splay modulus.

One of the earliest lipid intermediates forming in the course of membrane fusion is the lipid stalk. Although many aspects of the stalk hypothesis were elaborated theoretically and confirmed by experiments it remained unresolved whether stalk formation is always an energy consuming process or if there are conditions where the stalks are energetically favorable and form spontaneously resulting in an equilibrium stalk phase. Motivated by a recent breakthrough experiments we analyze the physical factors determining the spontaneous stalk formation. We show that this process can be driven by interplay between two factors: the elastic energy of lipid monolayers including a contribution of the saddle splay deformation and the energy of hydration repulsion acting between apposing membranes. We analyze the dependence of stalk formation on the saddle splay (Gaussian) modulus of the lipid monolayers and estimate the values of this modulus based on the experimentally established phase boundary between the lamellar and the stalk phases. We suggest that fusion proteins can induce stalk formation just by bringing the membranes into close contact, and accumulating, at least locally, a sufficiently large energy of the hydration repulsion.

Cell Surface Extensions↗

PubMed2004

Multiplicative neural noise can favor an independent components representation of sensory input.

Arguments have been advanced to support the role of principal components (e.g., Karhunen-Loéve, eigenvector) and independent components transformations in early sensory processing, particularly for color and spatial vision. Although the concept of redundancy reduction has been used to justify a principal components transformation, these transformations per se do not necessarily confer benefits with respect to information transmission in information channels with additive independent identically distributed Gaussian noise. Here, it is shown that when a more realistic source of multiplicative neural noise is present in the information channel, there are quantitative benefits to a principal components or independent components representation for Gaussian and non-Gaussian inputs, respectively. Such a representation can convey a larger quantity of information despite the use of fewer spikes. The nature and extent of this benefit depend primarily on the probability distribution of the inputs and the relative power of the inputs. In the case of Gaussian input, the greater the disparity in power between dimensions, the greater the advantage of a principal components representation. For non-Gaussian input distributions with a kurtosis that is super-Gaussian, an independent components representation is similarly advantageous. This advantage holds even for input distributions with equal power since the resulting density is still rotationally asymmetric. However, sub-Gaussian input distributions can lead to situations where maximally correlated inputs are the most advantageous with respect to transmitting the greatest quantity of information with the fewest number of spikes.

Models, Neurological↗

PubMed2003

Bimolecular processes on silica gel surfaces: energetic factors in determining electron-transfer rates.

Triplet state and radical cation formation is observed following laser excitation of anthracene, phenanthrene and naphthalene (and their derivatives) adsorbed on silica gel. Energy- and electron-transfer reactions of these compounds with co-adsorbed azulene have been studied using a time-resolved diffuse reflectance laser flash photolysis technique. Triplet energy transfer from the arene derivative to azulene and electron transfer from azulene to the arene radical cation have been investigated in order to distinguish between diffusional and energetic control in these systems. Energy and electron transfer can be studied independently due to differing absorption properties and energy dependencies of production of the triplet states and radical cations. Transient decay kinetics for both electron and energy transfer have been modelled using two different rate constant distributions: a log Gaussian and a symmetrical Levy stable distribution. The latter model has also been demonstrated to be applicable to the decay of radical cations in the absence of an electron donor, which cannot be adequately described by the Gaussian model. Energy-transfer rates between the arene derivatives and azulene have been found to be close to the diffusion-controlled limit; however, in most cases, the rate of electron transfer is considerably lower. A correlation between the bimolecular rate constant and free energy of electron transfer has been found, indicating a Marcus inverted region. Compounds with bulky substituents show a further reduction in the rate of electron transfer, suggesting that an additional steric factor is involved in this process.

Journal Article↗

PubMed2001

Various aspects of the constraints imposed on the photochemistry of systems in porous silica.

This manuscript briefly reviews the photochemistry of organic molecules on porous silica (or SiO2). To gain an understanding of the chemistry on silica, data are displayed and discussed with respect to studies in homogeneous solution. In particular, the exact dimensionality of kinetic processes on porous SiO2 is a matter for debate. Hence, units of concentration of an adsorbate on the surface are expressed as moles per nanometer squared and as moles per liter, in order to compare with solution. Many studies show that organic molecules adsorb to SiO2 via the surface silanol (or surface hydroxyl OH) groups. The adsorption is heterogeneous, due to various clusters of silanol groups and to charge transfer (CT) sites. Photophysical studies clearly show these effects. The photo-induced reactions on SiO2 may be described by 'fractal' approaches, but a 'Gaussian' approach is often more useful to the photochemist. Photo-induced reactions occur via movement of the reactants on the surface, as in the case of the Langmuir-Hinshelwood (LH) mechanism or, as in the case of the Eley-Rideal (ER) mechanism, by bombardment of a surface bound excited state by a gaseous reactant, such as O2. Quenching of excited singlet states by O2 produces excited triplet states, which in turn are quenched to give singlet molecular oxygen. At room temperature the O2 quenching process on silica occurs by both mechanisms to approximately the same extent. However, the LH mechanism is dominant at lower temperatures and the ER mechanism is dominant at higher temperatures. Some quenchers, including carbon tetrachloride and tetranitromethane only quench by the LH mechanism giving rise to static quenching and chloro or nitro derivatives of the excited state. Photo-induced electron transfer between excited arenes and amines occurs readily, but the ionic products are short-lived compared to solution. This is due to the limited diffusion of the products on the surface, which in turn promotes back-electron transfer. Photoionization of arenes occurs on SiO2 via a two-photon process and gives very long-lived ions compared to solution. This is due to trapping of the photo-produced electrons by the SiO2 itself. Finally, the effects of co-adsorbants, including solvents, surfactants, and polymers, in photoreactions at the SiO2 surface are considered. The review ends with suggestions for future studies.

Journal Article↗

PubMed2003

Early reactions of light-induced protochlorophyllide and chlorophyllide transformations analyzed in vivo at room temperature with a diode array spectrofluorometer.

The steps of protochlorophyllide (Pchlide) photoreduction and subsequent chlorophyllide (Chlide) transformations which occur in the seconds to minutes time-scale were studied using a diode array spectrofluorometer in dark-grown barley leaves. The intensity of the excitation light was varied between 3 and 2,500 micromol m(-2) s(-1) and a series of fluorescence spectra were recorded at room temperature in the seconds and minutes time scales. In certain experiments, 77-K emission spectra were measured with the same equipment. The high quality of the spectra allowed us to run spectral resolution studies which proved the occurrence, at room temperature, of multiple Pchlide and Chlide forms found previously in 77-K spectra. The comparison of the 77-K and room-temperature spectra showed that the fluorescence yields of the nonphotoactive 633-nm Pchlide form and of the Chlide product emitting at 678 nm were temperature independent. The fluorescence intensity of aggregated NADPH-pigment-POR complexes (photoactive 656-nm Pchlide and 693-nm Chlide forms) were strongly increased at 77 K, while that of the NADP(+)-Chlide-POR (684-686-nm Chlide form) was much less affected by temperature. Information was obtained also about the dynamics of the transformation of pigment forms in the light at different photon densities. At low light intensities, the phototransformation of the 642-644-nm Pchlide form was faster than that of the 654-656-nm form. The relative amplitudes of Gaussian components related to different Chlide forms found after exposure to a constant amount of photons strongly depended on the light intensity used. Strong quenching of all Chlide components occurred upon prolonged exposure to high intensity light. These effects are discussed by considering the interconversion processes between different forms of the pigment-protein complexes, their relative fluorescence yields and energy migration processes.

Chlorophyllides↗

PubMed2006

Ab initio/Rice-Ramsperger-Kassel-Marcus study of the singlet C4H4 potential energy surface and of the reactions of C2(X1 Sigmag+) with C4H4(X1A1g) and C(1D) with C3H4 (allene and methylacetylene).

Ab initio modified Gaussian-2 G2M(RCC,MP2) calculations have been performed for various isomers and transition states on the singlet C4H4 potential energy surface. The computed relative energies and molecular parameters have then been used to calculate energy-dependent rate constants for different isomerization and dissociation processes in the C4H4 system employing Rice-Ramsperger-Kassel-Marcus theory and to predict branching ratios of possible products of the C2(1Sigmag+)+C2H4, C(1D)+H2CCCH2, and C(1D)+H3CCCH reactions under single-collision conditions. The results show that C2 adds to the double C=C bond of ethylene without a barrier to form carbenecyclopropane, which then isomerizes to butatriene by a formal C2 "insertion" into the C-C bond of the C2H4 fragment. Butatriene can rearrange to the other isomers of C4H4, including allenylcarbene, methylenecyclopropene, vinylacetylene, methylpropargylene, cyclobutadiene, tetrahedrane, methylcyclopropenylidene, and bicyclobutene. The major decomposition products of the chemically activated C4H4 molecule formed in the C2(1Sigmag+)+C2H4 reaction are calculated to be acetylene+vinylidene (48.6% at Ecol = 0) and 1-buten-3-yne-2-yl radical [i-C4H3(X2A'), H2C=C=C=CH*]+H (41.3%). As the collision energy increases from 0 to 10 kcal/mol, the relative yield of i-C4H3+H grows to 52.6% and that of C2H2+CCH2 decreases to 35.5%. For the C(1D)+allene reaction, the most important products are also i-C4H3+H (55.2%) and C2H2+CCH2 (30.1%), but for C(1D)+methylacetylene, which accesses a different region of the C4H4 singlet potential energy surface, the calculated product branching ratios differ significantly: 65%-69% for i-C4H3+H, 18%-14% for C2H2+CCH2, and approximately 8% for diacetylene+H2.

Journal Article↗

PubMed2005

An interpretation of size-scale plasticity in geometrically confined systems.

The mesoscopic constitutive behavior of face-centered cubic metals as a function of the system characteristic dimension recently has been investigated experimentally. Strong size effects have been identified in both polycrystalline submicron thin films and single crystal micro pillars. The size effect is manifested as an increase in strength and hardening rate as the system dimensions are decreased. In this article, we provide a mechanistic interpretation for the observed mesoscopic behavior. By performing 3D discrete dislocation dynamics simulations of grains representative of the system microstructure and associated characteristic dimensions, we show that the experimentally observed size effects can be qualitatively described. In these simulations, a constant density of dislocation sources per unit of grain boundary area is modeled by sources randomly distributed at grain boundaries. The source length (strength) is modeled by a Gaussian distribution, in which average and standard deviation is independent of the system characteristic dimension. The simulations reveal that two key concepts are at the root of the observed plasticity size effect. First, the onset of plasticity is governed by a dislocation nucleation-controlled process (sources of various length, i.e., strengths, in our model). Second, the hardening rate is controlled by source exhaustion, i.e., sources are active only once as a result of the limited dislocation mobility arising from size and boundary effects. The model postulated here improves our understanding of why "smaller is stronger" and provides predictive capabilities that should enhance the reliable design of devices in applications such as microelectronics and micro/nano-electro-mechanical systems.

Journal Article↗