SEARCH · PubMed Health

Results for “Models, Theoretical”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,081 records · Page 60Linked to original sources

PubMed2002

Anomalous nuclear magnetic relaxation of aqueous solutions of ferritin: an unprecedented first-order mechanism.

Ferritin, the iron-storing protein, speeds up proton transverse magnetic relaxation in aqueous solutions. This T(2) shortening is used in MRI to quantify iron in the brain and liver. Current theoretical models underestimate the relaxation enhancement by ferritin at imaging fields, and they do not predict the measured dependence of the rate enhancement on the magnetization of the particles. Here it is shown that a proton exchange dephasing model (PEDM) overcomes these limitations by allowing a first-order relaxation mechanism. The PEDM considers proton exchange between bulk water and exchangeable protons located at the surface of the hydrated iron oxide nanometric core of the protein. Relaxation is shown to depend on the distribution of the frequency shifts of the adsorption sites; the observed properties agree with a Lorentzian distribution. Computer simulations utilizing recent Mössbauer spectroscopy data show that the distribution of these shifts is effectively Lorentzian.

Animals↗

PubMed2004

Pulse sequence for multislice T1rho-weighted MRI.

A 2D multislice spin-lock (MS-SL) MR pulse sequence is presented for rapid volumetric T1rho-weighted imaging. Image quality is compared with T1rho-weighted data collected using a single-slice (SS) SL sequence and T2-weighted data from a standard MS spin-echo (SE) sequence. Saturation of longitudinal magnetization by the application of nonselective SL pulses is experimentally measured and theoretically modeled as T2rho decay. The saturation data is used to correct the image data as a function of the SL pulse duration to make quantitative measurements of T1rho. Measurements of T1rho using the saturation-corrected MS-SL data are nearly identical to those measured using an SS-SL sequence. The MS-SL sequence produces quantitative T1rho maps of an entire sample volume with the high-SNR advantages conferred by SE-based sequences.

Animals↗

PubMed1992

Reduction of RF penetration effects in high field imaging.

A method of correcting for the RF inhomogeneity in the body by dielectric loading of the coil-to-shield space in an RF resonator (coil and shield assembly) is described. The presence of the RF coil and RF shield have significant effects on RF homogeneity. Based on theoretical calculations, a method for adjusting the RF homogeneity by manipulating the axial propagation constant, kappa z, is proposed. This is accomplished by loading the coil-to-shield space with dielectric material of suitable relative permittivity so as to increase kappa z and decrease the radial propagation constant, kappa rho. In this manner, the radial wavelength (lambda rho = 2 pi/kappa rho) can be increased relative to the body dimensions, and the field amplitude variations in the axial plane minimized. Theoretical calculations indicate that a value of between 30 and 40 for the relative permittivity of the dielectric material in the coil-to-shield space would reduce the RF field inhomogeneity from +/- 15% to about +/- 3% over a central 30-cm-diameter region of a homogeneous 40-cm-diameter body at both 64 and 170 MHz. The theoretical model was verified in laboratory measurements of the [formula; see text] field generated in a test coil at 170 MHz which was scaled to correspond to a body at 64 MHz. However, the improved RF field homogeneity would be accompanied by increased RF power requirements and reduced coil sensitivity.

Humans↗

PubMed1998

Off-resonance experiments and contrast agents to improve magnetic resonance imaging.

The effect of off-resonance irradiation on the water proton NMR signal intensity has been investigated as follows: (a) in the presence of a paramagnetic probe like manganese(II); (b) in the presence of bovine serum albumin (BSA) and two gadolinium(III) complexes, Gd-DTPA and Gd-BOPTA; (c) in the presence of cross-linked BSA and the two above-mentioned gadolinium(III) complexes. The experimental data have been rationalized on the basis of the available theoretical models. The effectiveness of the two complexes as contrast agents for MRI has been predicted. It is shown that contrast agents providing comparable longitudinal and transverse relaxation rate enhancements are those of general interest for off-resonance magnetization transfer-MRI.

Animals↗

PubMed2004

On the nature of the NAA diffusion attenuated MR signal in the central nervous system.

In the brain, on a macroscopic scale, diffusion of the intraneuronal constituent N-acetyl-L-aspartate (NAA) appears to be isotropic. In contrast, on a microscopic scale, NAA diffusion is likely highly anisotropic, with displacements perpendicular to neuronal fibers being markedly hindered, and parallel displacements less so. In this report we first substantiate that local anisotropy influences NAA diffusion in vivo by observing differing diffusivities parallel and perpendicular to human corpus callosum axonal fibers. We then extend our measurements to large voxels within rat brains. As expected, the macroscopic apparent diffusion coefficient (ADC) of NAA is practically isotropic due to averaging of the numerous and diverse fiber orientations. We demonstrate that the substantially non-monoexponential diffusion-mediated MR signal decay vs. b value can be quantitatively explained by a theoretical model of NAA confined to an ensemble of differently oriented neuronal fibers. On the microscopic scale, NAA diffusion is found to be strongly anisotropic, with displacements occurring almost exclusively parallel to the local fiber axis. This parallel diffusivity, ADCparallel, is 0.36 +/- 0.01 microm2/ms, and ADCperpendicular is essentially zero. From ADCparallel the apparent viscosity of the neuron cytoplasm is estimated to be twice as large as that of a temperature-matched dilute aqueous solution.

Animals↗

PubMed1995

Effects of the situational context and interactional process on the quality of family caregiving.

A staged theoretical model designed to explain the quality of elder caring by family members was tested. The model posits how the situational context, interactional process, and caregiving burden perceived by the caregiver affect the quality of elder caring. The purpose was to determine the amount of variance explained by the interactional process beyond that explained by the situational context and caregiving burden. Data were collected from 209 elder-caregiver dyads using interviews, observations, and caregiver self-reports. The strongest predictors of caregiving burden were the caregiver's stressful negative life events (situational context) and discrepancy between past and present image of elder (interactional process). The strongest predictors of quality of elder caring were the caregiver's perception of subjective burden and a monitoring role definition on the part of the caregiver (interactional process).

Activities of Daily Living↗

PubMed2006

Defining the competencies of those conducting pharmacovigilance.

PURPOSE: The absence of agreed competencies in pharmacovigilance and patient risk management is a key factor hindering the development of training and a curriculum in these interrelated disciplines. If competencies were to be developed, they could be tailored to the different roles and responsibilities of the various staff involved. PROPOSAL: We describe a simple theoretical model of functional and behavioural competencies that is based on an organisation with three levels of staff: evidence collectors and gatherers, evidence processors and distillers, and decision makers. CONCLUSIONS: To stimulate discussion about how to enhance the progress already being made in pharmacovigilance and patient risk management, we have proposed functional and behavioural competencies for pharmacovigilance as a way of identifying and measuring scientific and personal skills pertinent to pharmacovigilance. Competencies should, we believe, be agreed and adopted by all those seeking to improve pharmacovigilance. We invite discussion of these proposals.

Communication↗

PubMed2006

Entropy capacity determines protein folding.

Search and study of the general principles that govern kinetics and thermodynamics of protein folding generate a new insight into the factors controlling this process. Here, based on the known experimental data and using theoretical modeling of protein folding, we demonstrate that there exists an optimal relationship between the average conformational entropy and the average energy of contacts per residue-that is, an entropy capacity-for fast protein folding. Statistical analysis of conformational entropy and number of contacts per residue for 5829 protein structures from four general structural classes (all-alpha, all-beta, alpha/beta, alpha+beta) demonstrates that each class of proteins has its own class-specific average number of contacts (class alpha/beta has the largest number of contacts) and average conformational entropy per residue (class all-alpha has the largest number of rotatable angles phi, psi, and chi per residue). These class-specific features determine the folding rates: alpha proteins are the fastest folding proteins, then follow beta and alpha+beta proteins, and finally alpha/beta proteins are the slowest ones. Our result is in agreement with the experimental folding rates for 60 proteins. This suggests that structural and sequence properties are important determinants of protein folding rates.

Amino Acids↗

PubMed1994

Collective motions in proteins investigated by X-ray diffuse scattering.

We have developed theoretical models for analysis of X-ray diffuse scattering from protein crystals. A series of models are proposed to be used for experimental data with different degrees of precision. First, we propose the normal mode model, where conformational dynamics of a protein is assumed to occur mostly in a limited conformational subspace spanned by a small number of low-frequency normal modes in the protein. When high precision data are available, variances and covariances of the normal mode variables can be determined from experimental data using this model. For experimental data with lower degrees of precision, we introduce a series of simpler models. These models express the covariance matrix using relatively simple empirical correlation functions by assuming the correlation between a pair of atoms to be isotropic. As an application of these simpler models, we calculate diffuse-scattering patterns from a human lysozyme crystal to examine how each adjustable parameter in the models affects general features of the resulting patterns. The results of the calculation are summarized as follows. (1) The higher order scattering makes a significant contribution at high resolutions. (2) The resulting simulated patterns are sensitive to changes in correlation lengths of about 1 A, as well as to changes of the functional form of the correlation function. (3) But only the "average" value of the intra- and intermolecular correlation lengths seems to determine the gross features of the pattern. (4) The effect of the atom-dependent amplitude of fluctuations is difficult to observe.

Computer Simulation↗

PubMed2001

Post-ingestive positive controls of ingestive behavior.

Post-ingestive negative controls of ingestive behavior are well characterized. Nutrients act in the gut to inhibit meal size by direct actions on feeding and by conditioning a satiation response to orosensory (flavor) stimuli. Accumulating evidence indicates that there are also post-ingestive positive controls of ingestion that operate by conditioning flavor preference and increased acceptance. In several experiments rats trained to consume a flavored solution paired with intragastric carbohydrate infusions significantly increased their solution intake. Drinking bout size and number, and lick rates and burst size were also increased by carbohydrate infusions. Whether intake is stimulated or inhibited by post-ingestive nutrient actions depends upon several factors, including most notably nutrient concentration. Post-ingestive positive controls need to be incorporated into theoretical models of ingestive behavior.

Animals↗

PubMed2002

Estimating recreational trout fishing damages in Montana's Clark Fork River basin: summary of a natural resource damage assessment.

This paper summarizes a natural resource damage assessment for the State of Montana. Mining wastes have caused significant reductions in trout stocks in a 145-mile stretch of Montana's Silver Bow Creek and Clark Fork River. To estimate economic damages from decreases in catch rates, we develop and estimate an individual-based utility-theoretic model of where and how often an angler will fish as a function of travel costs, catch rates, and other influential characteristics of the site and individuals. The model includes resident and nonresident anglers who currently fish in Montana, and allows them to have different preferences. Demand parameters and expected catch rates are simultaneously estimated. The value of time is endogenously estimated as a proportion of the wage rate. Catch rates are linked to trout stocks through a stock-catch function. Collection of the angler data involved a three-step process: anglers were intercepted at 26 study sites, a subsample of anglers was selected to reflect the population trip-taking proportions to the study sites, and these anglers received follow-up surveys through the fishing season. Avidity weights are used to correct for the higher level of avidity inherent in intercept samples.

Animals↗

PubMed1999

Length dependence of RNA-RNA annealing.

The association of complementary nucleic acids can be described by a second order rate constant k. For extended molecules, including complex nucleic acids, values of k were shown to be proportional to the square root of the chain length L of the shorter nucleic acid strand at temperatures between tm and tm-30 degrees C. For homopolymers this is true over a wider temperature range. Below temperatures of tm-30 degrees C, annealing rate constants may sharply decrease due to the formation of intramolecular structures. It seems to be reasonable to assume that the formation of intramolecular structures of nucleic acids reduces the density of nucleation sites for annealing and, thereby, lowers the rates of association. Here, we examined the relationship between RNA chain length and the kinetics of RNA-RNA annealing at physiological ionic strength and temperature. We used a complete sequence space derived from chloramphenicol acetyltransferase (cat) sequences to average over all structures for each given length. For groups of progressively longer antisense RNA species and a 800 nucleotides long complementary RNA, the observed annealing rate constants kobs were measured in vitro. The structure-averaged values for kobs of RNA-RNA annealing were not related to the square root of the chain length. Instead, they were found to be proportional to 10(alphaL) (alpha=0.0017). Here, a theoretical model is suggested in which the observed length dependence is mainly influenced by ionic interactions between complementary RNA strands. The observed length dependence has substantial implications for the biological behavior of long-chain complementary RNA including the design of antisense RNA. The efficacy of antisense RNA in living cells is known to be related to annealing kinetics in vitro. Thus, on a statistical basis and independent of individual structures, long-chain rather than short-chain antisense RNA should lead to stronger inhibition.

Base Pairing↗

PubMed2001

Detection power, estimation efficiency, and predictability in event-related fMRI.

Experimental designs for event-related functional magnetic resonance imaging can be characterized by both their detection power, a measure of the ability to detect an activation, and their estimation efficiency, a measure of the ability to estimate the shape of the hemodynamic response. Randomized designs offer maximum estimation efficiency but poor detection power, while block designs offer good detection power at the cost of minimum estimation efficiency. Periodic single-trial designs are poor by both criteria. We present here a theoretical model of the relation between estimation efficiency and detection power and show that the observed trade-off between efficiency and power is fundamental. Using the model, we explore the properties of semirandom designs that offer intermediate trade-offs between efficiency and power. These designs can simultaneously achieve the estimation efficiency of randomized designs and the detection power of block designs at the cost of increasing the length of an experiment by less than a factor of 2. Experimental designs can also be characterized by their predictability, a measure of the ability to circumvent confounds such as habituation and anticipation. We examine the relation between detection power, estimation efficiency, and predictability and show that small increases in predictability can offer significant gains in detection power with only a minor decrease in estimation efficiency.

Artifacts↗

PubMedDate not supplied

Deuterium NMR and ab initio studies of strongly hydrogen-bonded molecules.

Two newly synthesized alkylated cyanomalonaldehyde derivatives possess very short intramolecular hydrogen bonds, which are studied by deuterium NMR. Both the dimethyl and di-tert-butyl derivatives have small deuterium quadrupole coupling constants and large asymmetry parameters. In addition, the di-tert-butyl compound, which has one of the shortest O D...O hydrogen bonds yet measured, has a quadrupole coupling which anomalously increases with temperature. The significance of these phenomena is explored using a theoretical model which employs vibrationally averaged ab initio electric field gradient tensors calculated with large basis sets and electron correlation via the multiconfigurational self-consistent field method.

Computer Simulation↗

PubMed1987

Observations of HCN in comet P/Halley.

We present observations of the HCN J = 1-0 rotational transition at 3.4 mm wavelength in comet P/Halley. The data were obtained during a total of 56 individual observing sessions between November 1985 and May 1986 and represent the first time that a cometary parent molecule has been so extensively monitored. The HCN production rate is well correlated with the total visual magnitude of the comet, and comparison of the HCN production to the total gas production of the comet indicates that it is a relatively minor constituent with 0.1% the abundance of H2O. Comparison of HCN and CN production suggests that HCN is a major parent molecule of CN, but probably not the sole parent. HCN spectra obtained by binning the data with heliocentric distance show that the line width, and thus the parent outflow velocity, increases with decreasing heliocentric distance, and that there is a tendency for the lines to be blue shifted due to anisotropic outgassing from the nucleus. Finally, there is evidence of day-to-day time variability in the total HCN emission and in the hyperfine ratios. The time variation of the total emission is consistent with the known time variable behavior of the comet, and detailed comparisons to optical data, where possible, confirm this interpretation. However, non-LTE values of the hyperfine ratios are not consistent with theoretical modeling of the excitation of these transitions.

Astronomical Phenomena↗

PubMed1991

Public pensions in transition: an optimal policy path.

"The main purpose of this paper is to analyze problems of financing an old-age insurance when birth rates are low and population declines or fertility fluctuates with time....[The author] investigates a theoretical model which analyzes the welfare optimal combination of private savings via the capital market and the state forced savings via a PAYG [pay-as-you-go] financed public pension system. Long-run effects as well as short-run implications are considered. The economic properties of an optimal steady state are determined and additionally an optimal transition path--the best feasible conversion policy--which leads to the new steady state is specified."

Demography↗

PubMed1987

The effects on Schild regressions of antagonist removal from the receptor compartment by a saturable process.

A theoretical model of the effects of a saturable removal mechanism for an antagonist diffusing into the receptor compartment of a tissue is used to calculate expected deviations in Schild regressions. At concentrations of antagonist which do not saturate the removal mechanism, there can be a deficit of antagonist in the receptor compartment as compared to the concentration of antagonist bathing the tissue. This results in a shift to the right of the Schild regression and a corresponding underestimation of antagonist potency. The model predicts that as the concentration of antagonist exceeds the Km for removal (saturation of the removal process), this concentration deficit is eliminated, resulting in a proportionate increase in antagonist concentration at the receptor and a concomitant increase in receptor antagonism. This results in a steepening of the Schild regression; the slope in the region of saturation is greater than one. Experimental evidence in support of this model was found in studies of the antagonism of responses to bethanechol by atropine in rabbit ileum; this species is known to have an atropinesterase capable of hydrolyzing atropine. The Schild regression for atropine was curvilinear with an overall slope of 1.42 (1.34-1.5) and pKB = 8.5 (8.36-8.8); in the ileum from guinea pigs, a species which does not possess this enzyme, the Schild regression for atropine was linear, had a slope not significantly different from unity (1.1; 0.95-1.2) and a pKb of 9.0 (8.9-9.2). The slope of the regression in rabbit ileum was corrected to unity by the addition of an excess concentration of methylbutyrate, an alternate substrate for atropinesterase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Periodic electric field as a biopolymer conformation switch: a possible mechanism.

A theoretical model is proposed to describe the influence of a periodic electric field (PEF) upon a biopolymer. The biopolymer is treated as a classical mechanical system consisting of subsystems (molecular groups) which interact with each other through potential forces. The PEF is treated as a periodic driving force applied to a molecular group. The energy dissipation is considered using the model of fluid (viscous) friction. Arguments for the non-linear character of the friction-velocity dependence caused by the non-Newtonian rheology of a viscous medium are formulated. A forced molecular-group motion is investigated for the situation of a small driving-force period, with oscillations overdamped and a driving force consisting of more than one harmonic. As a result, it is established that the motion always gets to a terminal stage when only a small-scale vibration about some point, X*, takes place. The terminal motion is preceded by a transient characterized by the presence of a directional velocity component and so by a drift along a potential profile. The drift goes on until a barrier is met which has a sufficiently large steepness (the barrier height is not important). As a result, the point X* may happen to be remote from the conformation potential local minimum (conformational state). The physical reasons for the drift are described.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical Phenomena↗