相关论文
1997
Magnetic and thermal instabilities in a plane layer: I

Abstract A plane layer of electrically conducting Boussinesq fluid between z = ± d/2 rotating with constant angular velocity ω = ωz under a vertical gravitational field g = - gz is examined. In order to simulate the Earth's toroidal field, an imposed non-uniform magnetic field of the form is considered where B is the magnetic field strength and B0(z) is chosen to be antisymmetric about z = 0. Studying where the most unstable mode of convection neither grows or decays (neutral stability), purely magnetic instabilities are achieved when the parameter which measures the field strength - the Elsasser number Λ - exceeds a critical value Λc For a magnetic field profile B0 (z) = tanhγz, an asymptotic analysis with γ → ∞ is carried out which yields a value of Λc= 2. The magnetic nature of the bounding material is taken to be either perfectly conducting or perfectly insulating. Unstable modes are found for all combinations of the magnetic boundary conditions though the symmetry of the flow is broken when the bound...

1984
Atmospheric Dynamics of Uranus and Neptune: Theoretical Considerations

Uranus, because of its pole-on orientation and low internal heat source, is in a dynamically different atmospheric regime from Jupiter and Saturn. Neptune resembles Jupiter and Saturn in orientation and internal heating, but its extremely long radiative time constant puts Neptune in a different class. Voyager observations of seasonal temperature gradients, equator-to-pole temperature gradients, infrared emission, Bond albedo, possible cloud structures (bands, spots, eddies), and cloud motions should greatly improve our ability to classify planetary atmospheres according to their dynamical regimes.

1976
Influence of End Restraint in Cyclic Triaxial Tests

Abstract : Theoretical considerations concerning the effect of end restraint on the strength of soil in triaxial tests are briefly discussed herein. The theoretical aspects are supplemented by a review of all laboratory tests data available to the writer. Most of the data pertain to static drained tests on sands. There are also data from static undrained tests on saturated clays. A lesser amount of previously unpublished data from the writer's files on static and cyclic loading undrained tests on saturated sands are also included. The theoretical considerations and the available data are all consistent within themselves in demonstrating and explaining the significant conclusions which follow directly from basic considerations of the behavior of soil under various loading conditions. End restraints, such as are found in most ordinary triaxial test equipment, prevent lateral strains at the ends of the specimen and thereby lead to nonuniformities and concentrations in the stresses and strains throughout the specimen. As a result, volume changes in soils which dilate strongly when sheared will be affected. If tested undrained, this will be reflected by variations in pore pressure from that associated with uniform stress and strain distributions throughout.

2007 - Canadian Journal of Neurological Sciences / Journal Canadien des Sciences Neurologiques
Wallerian-Like Degeneration After Ischemic Stroke Revealed by Diffusion - Weighted Imaging

A 22-year-old female was seen in the emergency within one hour of acute onset of right sided headache followed by weakness of the left side of body. On neurological examination, she was mildly drowsy, had forced right gaze deviation, dysarthria, left hemiplegia and left hemisensory loss. Computed tomography (CT) scan revealed early ischemic changes in the right middle cerebral artery (MCA) territory. The CT angiography done showed evidence of dissection of the supraclinoid segment of the right internal carotid artery with reduced flow distally into the MCA, which was confirmed by a conventional angiogram. In view of the intracranial carotid dissection, the patient was not treated with intravenous tissue plasminogen activator. Magnetic resonance imaging (MRI) of the brain done on the next day revealed evidence of acute ischemic lesions in the right MCA and anterior cerebral artery territory on diffusion-weighted imaging (DWI), with normal brainstem. [Figure 1] A repeat MRI performed 13 days after ictus showed hyperintense signal on DWI in the right cerebral peduncle which was hypointense on apparent diffusion coefficient (ADC) map suggestive of Wallerian-like degeneration. [Figure 2] The signal changes were less conspicuous on T2-weighted images. She had antigravity strength in the left leg but remained weak in her left arm at one month.

2012 - Applied Physics Letters
Magnetic field dependence of the spin relaxation length in spin light-emitting diodes

We investigate the spin relaxation length during vertical electron transport in spin-light emitting diode devices as a function of magnetic field strength at room temperature. In most publications on spin relaxation in optoelectronic devices, strong magnetic fields are used to achieve perpendicular-to-plane magnetization of the spin injection contacts. We show experimentally that high magnetic field strengths significantly reduce spin relaxation during transport to the active region of the device. We obtain a spin relaxation length of 27(3) nm in magnetic remanence and at room temperature, which nearly doubles at 2 T magnetic field strength.

1996 - CP
A New Approach for Weighted Constraint Satisfaction: Theoretical and Computational Results

We consider the Weighted Constraint Satisfaction Problem which is a central problem in Artificial Intelligence. Given a set of variables, their domains and a set of constraints between variables, our goal is to obtain an assignment of the variables to domain values such that the weighted sum of satisfied constraints is maximized. In this paper, we present a new approach based on randomized rounding of semidefinite programming relaxation. Besides having provable worst-case bounds, our algorithm is simple and efficient in practice, and produces better solutions than other polynomial-time algorithms such as greedy and randomized local search.

2003 - Int. J. Bifurc. Chaos
Comparison of Cardiac Magnetic Field Distributions during Depolarization and Repolarization

Multichannel magnetocardiography measures the magnetic field distribution of the human heart noninvasively from many sites over the body surface. Multichannel magnetocardiogram (MCG) analysis enables regional temporal differences in the distribution of cardiac magnetic field strength during depolarization and repolarization to be identified, allowing estimation of the global and local inhomogeneity of the cardiac activation process. The aim of this study was to compare the spatial distribution of cardiac magnetic field strength during ventricular depolarization and repolarization in both normal subjects and patients with cardiac abnormalities, obtaining amplitude measurements by magnetocardiography. MCGs were recorded at 49 sites over the heart from three normal subjects and two patients with inverted T-wave conditions. The magnetic field intensity during depolarization and repolarization was measured automatically for each channel and displayed spatially as contour maps. A Pearson correlation was used to determine the spatial relationship between the variables. For normal subjects, magnetic field strength maps during depolarization (R-wave) showed two asymmetric regions of magnetic field strength with a high positive value in the lower half of the chest and a high negative value above this. The regions of high R-wave amplitude corresponded spatially to concentrated asymmetric regions of high magnetic field strength during repolarization (T-wave). Pearson-r correlation coefficients of 0.7 (p<0.01), 0.8 (p<0.01) and 0.9 (p<0.01) were obtained from this analysis for the three normal subjects. A negative correlation coefficient of -0.7 (p<0.01) was obtained for one of the subjects with inverted T-wave abnormalities, suggesting similar but inverted magnetic field and current distributions to normal subjects. Even with the high correlation values in these four subjects, the MCG was able to identify differences in the distribution of magnetic field strength, with a shift in the T-wave relative to the R-wave. The measurement of cardiac magnetic field distribution during depolarization and repolarization of normal subjects and patients with clinical abnormalities should enable the improvement of theoretical models for the explanation of the cardiac depolarization and repolarization processes.

2016 - Journal of Cerebral Blood Flow & Metabolism
A general model to calculate the spin-lattice (T1) relaxation time of blood, accounting for haematocrit, oxygen saturation and magnetic field strength

Many MRI techniques require prior knowledge of the T1-relaxation time of blood (T1bl). An assumed/fixed value is often used; however, T1bl is sensitive to magnetic field (B0), haematocrit (Hct), and oxygen saturation (Y). We aimed to combine data from previous in vitro measurements into a mathematical model, to estimate T1bl as a function of B0, Hct, and Y. The model was shown to predict T1bl from in vivo studies with a good accuracy (±87 ms). This model allows for improved estimation of T1bl between 1.5–7.0 T while accounting for variations in Hct and Y, leading to improved accuracy of MRI-derived perfusion measurements.

1995 - arXiv: Astrophysics
Observational Searches for Solar g-modes: Some Theoretical Considerations

We argue that the solar g-modes are unlikely to have caused the discrete peaks in the power spectrum of the solar wind flux observed by Thomson et al. (1995). The lower limit to the energy of individual g-modes, using the amplitudes given by Thomson et al., is estimated to be at least 10$^{36}$ erg for low order g-modes; the resulting surface velocity amplitude is at least 50 cm s$^{-1}$, larger than the observational upper limit (5 cm s$^{-1}$). We suggest that the most likely source for the excitation of solar g-modes is turbulent stresses in the convection zone. The surface velocity amplitude of low degree and low order g-modes resulting from this process is estimated to be of order 10$^{-2}$ cm s$^{-1}$. This amplitude is interestingly close to the detection threshold of the SOHO satellite. The long lifetime of g-modes ($\sim 10^6$ years for low order modes) should be helpful in detecting these small amplitude pulsations.

2012 - Journal of Physics D
Numerical analysis of magnetic nanoparticle transport in microfluidic systems under the influence of permanent magnets

A finite element technique was employed for analysing the transport behaviour of magnetic nanoparticles (MNPs) under the gradient magnetic field generated by rectangular permanent magnets with different configurations. To predict the exact particle dynamic behaviour, the governing non-linear differential equations, Navier–Stokes and convection–diffusion were coupled with the magnetic field equation. The MNP concentration distribution was calculated and taken as an evaluation parameter to show where MNPs are preferentially captured in a microchannel. Since the dynamic behaviour of MNPs in the flow was dependent on the competition between magnetic and fluidic forces, the effects of the flow velocity and magnetic field strength on the MNP concentration distribution were analysed. Meanwhile, the effects of magnetic design parameters for permanent magnets on the magnetic force and MNP concentration distribution were analysed. Results showed that the MNP concentration in the capture region increased with magnetic field strength and decreased with increasing flow velocity. And the shape and position of the high concentration regions were related to the applied inlet velocity, magnetic field strength, geometry of the magnets and the orientation of the remanent flux density. The simulations performed can be used as a tool for the design and optimization of millimetre-sized rectangular magnets for developing efficient lab-on-a-chip systems.

1993 - Journal of Theoretical Politics
Penalty and Crime: Further Theoretical Considerations and Empirical Evidence

Two different models of the game between police and public, one with two-sided incomplete information and the other with multiple strategies, lead to the same results as my previous research: changes in penalties at equilibrium do not affect crime but instead affect police behavior. I compare these models with alternative models of the police-public game which come to different conclusions. I provide reasons for the differences in the conclusions, as well as empirical evidence against which the different models can be evaluated.

1997
Increase of PCO2 during blooms of Emiliania huxleyi: Theoretical considerations on the asymmetry between acquisition of HCO3‐ and respiration of free CO2

The principal source of dissolved inorganic carbon (CT) of most marine phytoplankton is thought to be free CO2, but Emiliania huxleyi can utilize HCO3− ion both for calcification (C) and photosynthesis (P) . We present a simple model that simulates continuous diurnal removal of CT as either free CO2 or HCO3− across the range of CT typically encountered in seawater (2,100–1,800 µmol kgsw−1). Removal as free CO2 forces a drawdown in partial pressure of dissolved carbon dioxide (Pco2), at given CT, of up to 170 µatm greater than that forced by removal as HCO3− by E. huxleyi (assuming Cnet : Pnet = 1.0). Diurnal Cnet : Pnet must exceed 1.5 for Pco2 to increase during removal of HCO3−. When nocturnal respiration of free CO2 (R) is incorporated into the model, the respective drawdowns in Pco2 forced by diurnal removal of free CO2 and HCO3 are balanced by diel ratios R : Pnet = 1.0 and R : (P + C)net ≈ 0.15. Therefore, with diel R : (P + C)net > 0.15 but diurnal Cnet : Pnet = 1.0, E. huxleyi can theoretically grow with net diel increase in Pco2; however, diel Cnet : Pnet > 1.0 is forced by nocturnal loss of particulate organic carbon but not particulate inorganic carbon. Published values for C : P do not necessarily represent diel Cnet : Pnet and caution is required when extrapolating these ratios to predict changes in CT and Pco2 during natural blooms of E. huxleyi.

2003 - astro-ph/0310017
General Relativistic Magnetohydrodynamic Simulations of Collapsars: Rotating Black Hole Cases

We have performed 2.5-dimensional general relativistic magnetohydrodynamic (MHD) simulations of the gravitational collapse of a magnetized rotating massive star as a model of gamma-ray bursts (GRBs). The current calculation focuses on general relativistic MHD with simplified microphysics (we ignore neutrino cooling, physical equation of state, and photodisintegration). Initially, we assume that the core collapse has failed in this star. A few M☉ black hole is inserted by hand into the calculation. The simulations presented in the paper follow the accretion of gas into a black hole that is assumed to have formed before the calculation begins. The simulation results show the formation of a disklike structure and the generation of a jetlike outflow inside the shock wave launched at the core bounce. We have found that the jet is accelerated by the magnetic pressure and the centrifugal force and is collimated by the pinching force of the toroidal magnetic field amplified by the rotation and the effect of geometry of the poloidal magnetic field. The maximum velocity of the jet is mildly relativistic (~0.3c). The velocity of the jet becomes larger as the initial rotational velocity of stellar matter gets faster. On the other hand, the dependence on the initial magnetic field strength is a bit more complicated: the velocity of the jet increases with the initial field strength in the weak field regime, then is saturated at some intermediate field strength, and decreases beyond the critical field strength. These results are related to the stored magnetic energy determined by the balance between the propagation time of the Alfvén wave and the rotation time of the disk (or twisting time).

2017 - NeuroImage
An open, multi-vendor, multi-field-strength brain MR dataset and analysis of publicly available skull stripping methods agreement

ABSTRACT This paper presents an open, multi‐vendor, multi‐field strength magnetic resonance (MR) T1‐weighted volumetric brain imaging dataset, named Calgary‐Campinas‐359 (CC‐359). The dataset is composed of images of older healthy adults (29–80 years) acquired on scanners from three vendors (Siemens, Philips and General Electric) at both 1.5 T and 3 T. CC‐359 is comprised of 359 datasets, approximately 60 subjects per vendor and magnetic field strength. The dataset is approximately age and gender balanced, subject to the constraints of the available images. It provides consensus brain extraction masks for all volumes generated using supervised classification. Manual segmentation results for twelve randomly selected subjects performed by an expert are also provided. The CC‐359 dataset allows investigation of 1) the influences of both vendor and magnetic field strength on quantitative analysis of brain MR; 2) parameter optimization for automatic segmentation methods; and potentially 3) machine learning classifiers with big data, specifically those based on deep learning methods, as these approaches require a large amount of data. To illustrate the utility of this dataset, we compared to the results of a supervised classifier, the results of eight publicly available skull stripping methods and one publicly available consensus algorithm. A linear mixed effects model analysis indicated that vendor (Symbol) and magnetic field strength (Symbol) have statistically significant impacts on skull stripping results. Symbol. No caption available. Symbol. No caption available. HIGHLIGHTSA public multi‐vendor, multi‐field‐strength brain MR dataset is proposed and it is now available for download at http://miclab.fee.unicamp.br/tools.Consensus masks are used as “silver‐standards” to assess agreement between different skull stripping methods.Influences of scanner magnetic field strength and scanner vendor on skull stripping results are analyzed.

1971
A theoretical and experimental study of nonpropagating free-burning fires

Theoretical and experimental studies are reported on stationary free-burning fires of cross piles (cribs) of wood. Based on the various experimental results which are presented, simplified models of the burning behavior for each of two fairly distinct regimes are developed. For densely packed configurations, the mass, momentum, and energy transfers involved in the flow through the structure are considered. The resultant theoretical model provides expressions for the maximum burning rate in terms of the basic geometric and physical properties of the fuel structure and is found to agree with the experimental results that show that, in the densely packed region, the specific burning rate is a strong function of the crib height and packing density. For the loosely packed regime, heat and mass transfer ideas are combined with experimental results to show that the burning rate per unit exposed surface area is inversely proportional to the square root of the fuel element thickness with the constant of proportionality being a function only of the physical properties of the fuel elements and not the fuel bed geometry. The generalization of the results to other structures is discussed and the predictions are compared with wooden pallet data.

1999 - AIAA Journal
Nonlinear Structural Response of Laminated Composite Plates Subjected to Blast Loading

We are concerned with the theoretical analysis of the laminated composite plates exposed to normal blast shock waves as well as presenting correlation between the theoretical analysis and the experimental results of the strain time histories. The laminated composite plate is clamped at all edges. On the theoretical side of the study, dynamic equations of the plates are derived by the use of the virtual work principle within the framework of the Love theory of plates. The geometric nonlinearity effects are taken into account with von Karman assumptions. Then the governing equations of the laminated plate are solved by the Runge-Kutta-Verner method. A new displacement function is considered for the theoretical solution of the blast-loaded clamped plate. Furthermore, finite element modeling and analysis for the blast-loaded composite plates are presented. On the experimental side of the study, tests have been carried out on the laminated composite plates with clamped edges for two different blast loads. The results of theoretical and finite element methods are compared with the experimental results. Theoretical and finite element analyses results are in a good agreement. There is a qualitative agreement between the analyses and experimental results in the first load case. The predicted peak strains and response frequency are in an agreement with the experimental results for first load case. Thus the theoretical solution may be used for providing material in the preliminary design stage. There is a difference between the analysis and experimental results in the second load case because of the extremely large deflections. In this study the effects of loading conditions, geometrical properties, and material properties are separately examined on the dynamic behavior, as well.

2011 - Journal of Cerebral Blood Flow & Metabolism
T1- and T*2-Dominant Extravasation Correction in DSC-MRI: Part I—Theoretical Considerations and Implications for Assessment of Tumor Hemodynamic Properties

We present a novel contrast agent (CA) extravasation-correction method based on analysis of the tissue residue function for assessment of multiple hemodynamic parameters. The method enables semiquantitative determination of the transfer constant and can be used to distinguish between T1- and T*2-dominant extravasation effects, while being insensitive to variations in tissue mean transit time (MTT). Results in 101 patients with confirmed glioma suggest that leakage-corrected absolute cerebral blood volume (CBV) values obtained with the proposed method provide improved overall survival prediction compared with normalized CBV values combined with an established leakage-correction method. Using a standard gradient-echo echo-planar imaging sequence, ∼60% and 10% of tumors with detectable CA extravasation mainly exhibited T1- and T*2-dominant leakage effects, respectively. The remaining 30% of leaky tumors had mixed T1- and T*2-dominant effects. Using an MTT-sensitive correction method, our results show that CBV is underestimated when tumor MTT is significantly longer than MTT in the reference tissue. Furthermore, results from our simulations suggest that the relative contribution of T1-versus T*2-dominant extravasation effects is strongly dependent on the effective transverse relaxivity in the extravascular space and may thus be a potential marker for cellular integrity and tissue structure.

1986 - Magnetic Resonance Imaging
The magnetic field dependence of the breathing artifact

Abstract Breathing causes ghost artifacts in magnetic resonance (MR) images. These ghosts are more conspicuous at high magnetic field strength due to (i) operational factors that affect the relative intensity of the individual ghosts and (ii) factors that affect the image intensity of the moving structures producing the ghosts. Both types of factors are identified and illustrated with images of a human subject. A brief theoretical analysis of the noiseless MR image of a point object in sinusoidal motion shows that the intensity structure of each ghost depends on the number of pixels over which the object moves. Generally, images with narrower pixels have ghosts with greater relative intensity. However, the breathing artifact is more apparent at high magnetic field strength primarily because the signal-to-noise ratio and the cotnrast between fat and muscle tissues are increased.

1985 - Aerosol Science and Technology
Cyclone Collection Efficiency: Comparison of Experimental Results with Theoretical Predictions

This paper describes the results of tests conducted on a Stairmand high-efficiency cyclone. The cyclone was pilot-plant scale with a design air flow of 0.139 m3/s (300 cfm). Collection efficiency and pressure drop were measured over a range of air flows at ambient temperature and pressure. An oil mist was used as a test aerosol because it consisted of spherical drops of uniform density unlikely to bounce or reentrain after striking the cyclone wall. At each air flow, a fractional efficiency curve (collection efficiency versus particle diameter) was determined. Experimental curves were compared with fractional efficiency curves generated by several cyclone efficiency theories. Over the range of particle sizes measured (1 to 7 μm), the predictions of a modified version of Barth's theory and the Leith-Licht theory were closest to experimental results.

2008 - Journal of Magnetic Resonance Imaging
MR susceptibility weighted imaging (SWI) complements conventional contrast enhanced T1 weighted MRI in characterizing brain abnormalities of Sturge‐Weber Syndrome

To evaluate the efficacy of susceptibility weighted imaging (SWI) in comparison to standard T1 weighted postgadolinium contrast (T1‐Gd) MRI in patients with Sturge‐Weber Syndrome (SWS).

Susceptibility weighted imaging at ultra high magnetic field strengths: Theoretical considerations and experimental results

Jan Sedlacik
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Alexander Rauscher
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Jörg Stadler
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Andreas Deistung
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Jürgen R Reichenbach
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Stephan Witoszynskyj
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J. Reichenbach
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J. Sedlacik
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A. Rauscher
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J. Stadler
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A. Deistung
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S. Witoszynskyj

Abstract:We present numerical simulations and experimental results for susceptibility weighted imaging (SWI) at 7 T. Magnitude, phase, and SWI contrast were simulated for different voxel geometries and imaging parameters, resulting in an echo time of 14 msec for optimum contrast between veins and surrounding tissue. Slice thickness of twice the in‐plane voxel size or more resulted in optimum vessel visibility. Phantom and in vivo data are in very good agreement with the simulations and the delineation of vessels at 7 T was superior compared to lower field strengths. The phase of the complex data reveals anatomical details that are complementary to the corresponding magnitude images. Susceptibility weighted imaging at very high field strengths is a promising technique because of its high sensitivity to tissue susceptibility, its low specific absorption rate, and the phase's negligible sensitivity to B1 inhomogeneities. Magn Reson Med 60:1155–1168, 2008. © Wiley‐Liss, Inc.

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引用
Accelerated mapping of magnetic susceptibility using 3D planes‐on‐a‐paddlewheel (POP) EPI at ultra‐high field strength
NMR in biomedicine
2017
High‐resolution human cervical spinal cord imaging at 7 T
NMR in biomedicine
2012
Using magnetic field simulation to study susceptibility-related phase contrast in gradient echo MRI
NeuroImage
2009
Rapid brain MRI acquisition techniques at ultra‐high fields
NMR in biomedicine
2016
Diffusion-weighted imaging of the brain at 7 T with echo-planar and turbo spin echo sequences: preliminary results.
Magnetic resonance imaging
2011
Optimisation of T₂*-weighted MRI for the detection of small veins in multiple sclerosis at 3 T and 7 T.
European journal of radiology
2013
A new susceptibility‐weighted image reconstruction method for the reduction of background phase artifacts
Magnetic resonance in medicine
2014
ToF‐SWI: Simultaneous time of flight and fully flow compensated susceptibility weighted imaging
Journal of magnetic resonance imaging : JMRI
2009
Dual‐echo arteriovenography imaging with 7T MRI
Journal of magnetic resonance imaging : JMRI
2010
Tracking discrete off‐resonance markers with three spokes (trackDOTS) for compensation of head motion and B0 perturbations: Accuracy and performance in anatomical imaging
Magnetic resonance in medicine
2018
Detecting lesions in multiple sclerosis at 4.7 tesla using phase susceptibility‐weighting and T2‐weighting
Journal of magnetic resonance imaging : JMRI
2009
Ultra-high-field magnetic resonance: Why and when?
World journal of radiology
2010
Improved susceptibility‐weighted imaging for high contrast and resolution thalamic nuclei mapping at 7T
Magnetic resonance in medicine
2020
A Simulation Tool for Dynamic Contrast Enhanced MRI
PloS one
2013
Human brain atlas for automated region of interest selection in quantitative susceptibility mapping: Application to determine iron content in deep gray matter structures
NeuroImage
2013
Recent applications of UHF‐MRI in the study of human brain function and structure: a review
NMR in biomedicine
2016
High-Resolution MR Imaging of the Human Brainstem In vivo at 7 Tesla
Front. Hum. Neurosci.
2013
Development, testing, and application of quantitative oxygenation imaging from magnetic susceptibility by MRI
2014
Human imaging at 9.4 T using T2*‐, phase‐, and susceptibility‐weighted contrast
Magnetic resonance in medicine
2011
Validation of quantitative estimation of tissue oxygen extraction fraction and deoxygenated blood volume fraction in phantom and in vivo experiments by using MRI
Magnetic resonance in medicine
2010