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At least 955 records · Page 53Linked to original sources
PubMed1976

Changes in the shape of the developing vertebrate nervous system analyzed experimentally, mathematically and by computer simulation.

Two forces are necessary and sufficient to produce the transformation of the newt neural plate from a hemispheric sheet of cells one cell thick to a keyhole shape. These forces are: (1) a regionally programmed shrinkage of the surface of the neural plate (accomplished by contraction of the apical surfaces of the neural plate cells and elongation of these cells perpendicular to the plate); and (2) displacement of the whole sheet caused by elongation of either the notochord or the overlying neural plate cells in the antero-posterior direction. A computer simulation and mathematical analysis ("morphodynamics"), together with experiments and observations on embryos, were used to deduce the morphogenesis of the neural plate from these forces.

Animals↗
PubMedDate not supplied

The use of computer simulation in teaching clinical diagnosis.

Previous studies have shown that feedback of the weighting used in making complex judgements can greatly improve learning when the correct weighting is known. This paper describes the application to teaching medical diagnosis of a microcomputer program which calculates and displays factor weighting based on answers to clinical case simulations. In one application, students learning to diagnose urinary tract infusion with computer feedback of weighting progressed much more rapidly than a control group who received outcome feedback alone. Other applications in the clinical setting suggest that this type of learning can improve diagnostic accuracy and predictive calibration through more accurate assimilation of the clinical information available.

Computer Simulation↗
PubMed1983

REM latency and depression: computer simulations based on the results of phase delay of sleep in normal subjects.

A phase advance of the circadian rhythm of rapid eye movement (REM) sleep propensity relative to the sleep-wake cycle has been hypothesized to account for the abnormalities of REM sleep in depression. One implication of this hypothesis is that an acute phase delay of sleep in normal subjects should produce the same abnormalities of REM sleep. A further implication is that changes in REM sleep that occur in normal subjects with delay shifts of sleep of progressively greater magnitude describe those that occur during the course of a depressive episode. This hypothesis was tested by computer simulation using two equations which, given the data derived from normal subjects experiencing phase delays of sleep, generated the REM latencies expected during successive stages of a depressive episode. For severely depressed patients, the computer-generated data matched those found empirically. The known correlation between severity of depression and REM latency, and the REM latencies of recovered patients, are consistent with the hypothesis.

Computers↗
PubMed2005

Hydrodynamic simulation (computational fluid dynamics) of asymmetrically positioned tablets in the paddle dissolution apparatus: impact on dissolution rate and variability.

The aim of this work was to investigate the dissolution rate from both the curved and planar surfaces of cylindrical compacts of benzoic acid, which were placed centrally and non-centrally at the base of the vessel of the paddle dissolution apparatus. The effect of fixing the compacts to a particular position on the variability of dissolution results was also examined. In addition, computational fluid dynamics (CFD) was used to simulate fluid flow around compacts in the different positions in the vessel, and the relationship between the local hydrodynamics in the region of the compacts and the dissolution rate determined. The dissolution rate was found to increase from the centre position to the off-centre positions for each surface examined. There was a corresponding increase in maximum fluid velocities calculated from the CFD fluid flow simulations at a fixed distance from the compact. There was less variability in dissolution from compacts fixed to any of the positions compared with those that were not fixed. Fluid flow around compacts in different positions could be successfully modelled, and hydrodynamic variability examined, using CFD. The effect of asymmetric fluid flow was evident visually from the change in shape of the eroded compacts.

Benzoic Acid↗
PubMed1993

Steady states in the twin-well potential oscillator: Computer simulations and approximate analytical studies.

The paper is focused on the phenomena of various steady-state oscillations exhibited by the twin-well potential system. Regions of existence of different attractors in the system parameter domain are examined and a picture book of different steady states for fixed damping and forcing is presented: 20 different combinations of single or coexisting, small orbit or large orbit, periodic and chaotic attractors are displayed. Computer simulations are followed by an approximate analytical analysis: A study of various forms of instability of periodic solutions gives close form approximate criteria for occurrence of T-periodic small orbit and large orbit oscillations, and for cross-well chaos.

Journal Article↗
PubMedDate not supplied

Computer simulation of the upper extremities.

The development of real-time, interactive, three-dimensional, computer graphic simulation of the musculoskeletal system with emphasis on the upper extremities is described. Developments in image analysis, scientific visualization, and interactive computer methods and the continuous improvement in knowledge of musculoskeletal function have combined to provide an exciting new tool for musculoskeletal research. Interactive simulation also has promising applications in medical education and clinical rehabilitation. Only those developments that fit the specific criteria of realism, real-time response, intuitive interaction, three-dimensional structures, and modeling flexibility are discussed. It is hoped that an improved understanding of this emerging tool and an appreciation for its potential applications will be gained.

Arm↗
PubMed2000

Computer simulation of backscattered alpha particles

Alpha-particle spectrometry forms an important aspect of radionuclide metrology. Accurate measurements require corrections to be made for factors such as self-absorption within the source and backscattering from the backing material. The theory of the latter phenomenon has only received limited attention. Furthermore the experimental verification of these theoretical results requires adequate counting statistics for a variety of sources with different activities. These problems could be resolved by computer simulations of the various interactions which occur as alpha-particles move through different materials. The pioneering work of Ziegler and his coworkers over several years, has provided the sophisticated software (SRIM) which has enabled us to obtain the results presented here. These results are compared with theoretical and experimental values obtained previously.

Journal Article↗
PubMed1984

Computer simulation for the stereotactic placement of interstitial radionuclide sources into computed tomography-defined tumor volumes.

This report describes a method for the preoperative determination of radioactive interstitial source placement within computed tomography (CT)-defined tumor boundaries. The method utilizes CT data obtained under stereotactic conditions. Tumor boundaries are digitized from CT slices and are retained in a three-dimensional computer matrix. A solid tumor volume is created by an interpolation program and may be sliced orthogonal to any specific stereotactic surgical view line. The surgeon may simulate radioactive source placement within the slices and view the resultant isodose configuration against tumor contours on successive slices. Once the best source placement has been determined, the computer outputs the mechanical adjustments that will be necessary on a stereotactic frame located in the operating room for the stereotactic placement of each source and gives the length of each source. Sources are stereotactically implanted utilizing a double-catheter afterloading technique.

Adult↗
PubMed2002

Flow properties of driven-diffusive lattice gases: theory and computer simulation.

We develop n-cluster mean-field theories (1 < or = n < or = 4) for calculating the flux and the gap distribution in the nonequilibrium steady states of the Katz-Lebowitz-Spohn model of the driven diffusive lattice gas, with attractive and repulsive interparticle interactions, in both one and two dimensions for arbitrary particle densities, temperature as well as the driving field. We compare our theoretical results with the corresponding numerical data we have obtained from the computer simulations to demonstrate the level of accuracy of our theoretical predictions. We also compare our results with those for some other prototype models, notably particle-hopping models of vehicular traffic, to demonstrate the qualitative features we have observed in the Katz-Lebowitz-Spohn model, emphasizing, in particular, the consequences of repulsive interparticle interactions.

Journal Article↗
PubMed1984

Theoretical studies of phospholipid bilayers and monolayers. Perturbing probes, monolayer phase transitions, and computer simulations of lipid-protein bilayers.

This paper describes some mathematical models for studying properties of lipid bilayer membranes. It is shown that there is evidence from fluorescent probe and electron spin resonance studies that some integral proteins are "randomly" distributed in the plane of a lipid bilayer for T greater than Tc, so that protein-protein "contacts" can occur. This implies that there is no permanent annulus of lipids around these proteins, so that there is no unique stoichiometric ratio of "boundary lipids" to protein. Computer simulation techniques are reviewed and comments are made about some recently introduced methods. Dynamical models of lipid--integral protein bilayers are outlined and it is shown that much differential scanning calorimetry, freeze-fracture, X-ray, and 2H nuclear magnetic resonance data can be understood. It is predicted that specific heat curves should show a rise at a temperature, TK less than Tc, at which a protein-rich phase starts to "melt"; that dipalmitoyl phosphatidylcholine (DPPC) hydrocarbon chains in such a protein-rich phase should exhibit some static disorder down to approximately -10 degrees C, around which they should freeze noncooperatively; and that the crossing of phase boundaries, as protein concentration expressed as mole fraction X changes just below Tc, should be reflected in a complicated dependence of protein lateral motion upon X. Models to study the liquid condensed-liquid expanded (LC-LE) transition of phosphatidylcholine (PC) monolayers at the air-water interface are described. The rounding in the LC phase of pressure-area isotherms are understood as the melting of microscopic gel-phase "domains" into macroscopic regions of fluid-phase lipids. There is some experimental support for this. Finally, to model the difference between a monolayer and the corresponding bilayer, an interaction is introduced between the two halves of a bilayer. It is shown to be very weak in the case of PC bilayers and it is outlined in the paper how monolayer and bilayer thermodynamics can be related. It was found that the internal lateral pressure of a DPPC bilayer is approximately 30 dyn/cm (1 dyn = 10 microN).

Fluorescent Dyes↗
PubMed2001

Acoustic nonlinearity parameter tomography for biological tissues via parametric array from a circular piston source--theoretical analysis and computer simulations.

The acoustic nonlinearity parameter B/A describes the nonlinear features of a medium and may become a novel parameter for ultrasonic tissue characterization. This paper presents a theoretical analysis for acoustic nonlinear parameter tomography via a parametric array. As two primary waves of different frequencies are radiated simultaneously from a circular piston source, a secondary wave at the difference frequency is generated due to the nonlinear interaction of the primary waves. The axial and radial distributions of sound pressure amplitude for the generated difference frequency wave in the near field are calculated by a superposition of Gaussian beams. The calculated results indicated that the difference frequency component of the parametric array grows linearly with distance from the piston source. It therefore provides a better source to do the acoustic nonlinearity parameter tomography because the fundamental and second harmonic signals both have a near field that goes through many oscillations due to diffraction. By using a finite-amplitude insert substitution method and a filtered convolution algorithm, a computer simulation for B/A tomography from the calculated sound pressure of the difference frequency wave is studied. For biological tissues, the sound attenuation is considered and compensated in the image reconstruction. Nonlinear parameter computed tomography (CT) images for several biological sample models are obtained with quite good quality in this study.

Journal Article↗
PubMed1980

Sensitivity and other factors affecting biospecific desorption in chromatography of proteins. A study by computer simulation.

Some theoretical aspects of the desorption of a column-bound protein by elution with its biospecific ligand are considered in cases where, in comparison with the unliganded protein, the protein-ligand complex has a diminished but finite affinity for the adsorbent. A quantity termed the biospecific sensitivity, B, is introduced to facilitate comparison between different systems. Biospecific sensitivity may be defined as the fractional change in standard free energy of adsorption on formation of the protein-ligand complex. The effects of a moderate-to-low biospecific sensitivity on theoretical desorption profiles have been examined by using a computer simulation of the classical multiple-plate column model. Desorption was simulated under various boundary conditions involving protein-adsorbent and protein-ligand affinities and the initial concentrations of adsorption sites, protein and ligand. These simulations suggest that, when the biospecific sensitivity is low, desorption is optimized if (a) the unliganded protein is adsorbed as weakly as possible, (b) the column is loaded to near-saturation with the required protein, (c) the free ligand concentration is many times greater than that giving near-saturation of the protein in free solution, and (d) protein contaminants with high affinity for the adsorbent, and present in large amount, are removed in preliminary purification steps.

Adsorption↗
PubMed1992

A method for computer simulation of ultrasound Doppler color flow images--I. Theory and numerical method.

Ultrasound imaging systems utilizing the pulsed Doppler principle are capable of providing images of blood flow in real time. We present a useful method for simulating flow images on a computer. Our method assumes that blood and surrounding tissue consist of many point-like scatters positioned randomly in three dimensions. The position-dependent acoustic response of each scatterer is calculated using the acoustic impulse response method. This method takes into account the spatial effects of the transducer geometry on both the amplitude and temporal response of point-scattering. Details of theory, assumptions made in the simulation, and numerical methods are described fully for a spherically focused transducer, as well as a discussion of signal processing for generation of the flow image. Motion of a single scatterer is investigated to test the performance of the simulation algorithm. This simulation method could potentially be beneficial for detailed study of current and future flow imaging systems.

Blood Flow Velocity↗
PubMed1997

Computer simulation of centration effects on corneal-topography analysis of excimer laser photorefractive keratectomy ablations.

Excimer laser photorefractive keratectomy (PRK) procedures photoablate the central optical zone, potentially altering the corneal vertex normal and thus the corneal topography (CT) system reference axis. Anterior corneal surface coordinates (x, y, and z) were computer generated to simulate spherical and PRK-ablated corneal surfaces. Power maps were reconstructed by using surface tangents relative to various assumed CT reference axes, the original and displaced vertex normals. The simulation process eliminated focus, alignment, and surface-reconstruction errors. A simulated 5.00-D myopic PRK decentered 1 mm from the vertex produced a new corneal vertex 0.13 mm from the original position. Axial power-map generation by using this new reference axis produced a spherical keratectomy with astigmatic untreated periphery. This computer simulation demonstrates that reference axis changes are significant for even modest PRK decentration and can explain apparent steepening in untreated corneal areas. CT users should be aware that pre- and postoperative CT power maps may not be directly comparable.

Computer Simulation↗
PubMed1978

Conduction through demyelinated plaques in multiple sclerosis: computer simulations of facilitation by short internodes.

Clinical and laboratory observations both suggest that it may be possible for action potentials to traverse, in a continuous manner and without interruption, demyelinated zones along some axons. This continuous mode of conduction requires the presence of sufficient numbers of sodium channels in the demyelinated region. One of the factors which will tend to prevent such conduction is the impedance mismatch at sites of focal demyelination, which may result in a reduction in current density sufficient to cause conduction failure. As part of an effort to examine the conditions which would promote conduction into, and beyond, the demyelinated region, we examined, using computer simulations, the effects of reduction in length of the proximal internodes closest to the demyelinated region. Our results indicate that reduction in length of the two internodes closest to the demyelinated region. to approximately one-third of normal length or less, will facilitate conduction beyond the plaque. The results suggest that reductions in internode length, which have been histologically observed along some demyelinated fibres, may have functional significance in terms of facilitating conduction past focally demyelinated zones.

Action Potentials↗
PubMed2003

Spatial distribution of liposome encapsulated tin etiopurpurin dichloride (SnET2) in the canine prostate: implications for computer simulation of photodynamic therapy.

Photodynamic therapy (PDT) is a minimally invasive treatment that can be employed in many human diseases including prostate cancer. PDT for prostate cancer depends on the sequestration of a photosensitizing drug within the glandular tissue. The photosensitizer is subsequently activated by light (usually from a laser) and the active drug destroys tissue. Since prostate cancer is a multifocal disease, PDT must ablate the glandular prostate completely. This will depend on the precise placement of light sources in the prostate and delivery of a therapeutic light dose to the entire gland. Also, sources of light and their spatial distribution must be tailored to each individual patient. The uniform, therapeutic light distribution can be achieved by interstitial light irradiation. In this case, the light is delivered by diffusers placed within the substance of the prostate parallel to the urethra at a distance optimized to deliver adequate levels of light and to create the desired photodynamic effect. To help achieve the uniform light distribution throughout the prostate we have developed a computer program that can determine treatment effects. The program predicts the best set of parameters and the position of light diffusers in space, and displays them in graphical or in numerical form assuming a fixed attenuation coefficient. The two parameters of greatest importance in the computer simulation are attenuation coefficient and critical fluence. Both depend on the concentration of active drug within the prostate gland. It is necessary to know the nature of the spatial distribution of photosensitizer within the prostate to execute computer modeling of PDT with high precision. We found that the concentration of SnET2 is heterogeneous in nature, and is higher in the proximity of the glandular capsule. It is clear therefore that any future attempts of computerized modeling of this procedure must take into consideration the uneven sequestration of photosensitizer and the consequential asymmetrical necrosis of the prostate.

Algorithms↗