bluuebunny/crossref_metadata_2025_split huggingface dataset:
Six is enough? Comparison of diffusion parameters measured using six or more diffusion‐encoding gradient directions with deterministic tractography
Authors: Lebel Catherine , Benner Thomas , Beaulieu Christian 
Published: December 2011

AbstractDiffusion tensor imaging tractography is commonly used to quantify white matter tracts in the human brain via parameters such as fractional anisotropy and mean diffusivity. Simulation studies recommend the use of more than six directions for robust parameter estimates; however, no study has examined the impact of the number of gradient directions on deterministic tractography‐derived diffusion parameters in human brain. Here, for 10 major white matter tracts in 11 healthy volunteers at 1.5 T, six‐direction diffusion tensor imaging data were compared to 30‐ or 60‐direction data, keeping scan time and number of b = 0 images constant within each test. Mean diffusivity was systematically lower for six‐direction protocols (20/40 comparisons); six‐direction data had higher fractional anisotropy in the superior longitudinal fasciculus and smaller tract volume for the genu of the corpus callosum. In general, parameter differences due to the number of directions were smaller than those from intersubject variation or signal‐to‐noise ratio. Despite some absolute differences, standard deviations were significantly different for only one of 160 comparisons. Thus, six‐direction data provide diffusion measures with comparable robustness to 30‐ or 60‐direction data and yield appropriate parameter values for most white matter tracts, although there are clear advantages in acquiring higher angular resolution data. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Comparison of NMR lipid profiles in mitotic arrest and apoptosis as indicators of paclitaxel resistance in cervical cell lines
Authors: Zietkowski Dominik. , Payne Geoffrey S. , Nagy Eszter , Mobberley Margaret A. , Ryder Timothy A. , deSouza Nandita M. 
Published: December 2011

AbstractThis study aimed to characterize changes in lipid saturation using magnetic resonance spectroscopy of sensitive (HeLa) and resistant (C33A; Me180) cervical cancer cell lines following exposure to paclitaxel to explore lipid profiles as biomarkers of drug resistance. Spectra were acquired at 11.74 T. Flow cytometry, electron, and confocal microscopy assessed cellular morphology. Western blots assessed cytoplasmic phospholipase A2, fatty acid synthase, and acyl‐CoA synthetase1 expression. After 24 h of paclitaxel exposure, >60% of cells showed mitotic arrest. At 48 h, HeLa cells showed apoptosis while C33A/Me180 cells showed normal morphology indicating resistance. MR‐visible lipids increased significantly in all lines at 24 h with further increases at 48 h; resistant lines showed smaller increases than HeLa. Cytoplasmic phospholipase A2 and fatty acid synthase levels were unchanged at 24 h and dropped at 48 h in HeLa; acyl‐CoA synthetase1 was higher in Me180/C33A than in HeLa controls but did not increase significantly. The percentage of cells displaying lipid droplets increased significantly at 24 and 48 h in all lines; droplet size increased only in HeLa cells. Droplet number was >3–4× greater in apoptotic compared with mitotic‐arrested cells. Apoptotic cells accumulate unsaturated fatty acids in large (relative to control) droplets; resistant lines accumulated smaller droplets with less triglycerides. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Nonuniform and multidimensional Shinnar‐Le Roux RF pulse design method
Authors: Grissom William A. , McKinnon Graeme C. , Vogel Mika W. 
Published: December 2011

AbstractThe Shinnar‐Le Roux (SLR) radiofrequency (RF) pulse design algorithm is widely used for designing slice‐selective RF pulses due to its intuitiveness, optimality, and speed. SLR is limited, however, in that it is only capable of designing one‐dimensional pulses played along constant gradients. We present a nonuniform SLR RF pulse design framework that extends most of the capabilities of classical SLR to nonuniform gradient trajectories and multiple dimensions. Specifically, like classical SLR, the new method is a hard pulse approximation‐based technique that uses filter design relationships to produce the lowest power RF pulse that satisfies target magnetization ripple levels. The new method is validated and compared with methods conventionally used for nonuniform and multidimensional large‐tip‐angle RF pulse design. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
R mapping in the presence of macroscopic B0 field variations
Authors: Hernando Diego , Vigen Karl K. , Shimakawa Ann , Reeder Scott B. 
Published: December 2011

Abstract
$R_2^{\ast ab}$
mapping has important applications in MRI, including functional imaging, tracking of super‐paramagnetic particles, and measurement of tissue iron levels. However, $R_2^{\ast ab}$
measurements can be confounded by several effects, particularly the presence of fat and macroscopic $B_0$
field variations. Fat introduces additional modulations in the signal. Macroscopic field variations introduce additional dephasing that results in accelerated signal decay. These effects produce systematic errors in the resulting $R_2^{\ast ab}$
maps and make the estimated $R_2^{\ast ab}$
values dependent on the acquisition parameters. In this study, we develop a complex‐reconstruction, confounder‐corrected $R_2^{\ast ab}$
mapping technique, which addresses the presence of fat and macroscopic field variations for both 2D and 3D acquisitions. This technique extends previous chemical shift‐encoded methods for $R_2^{\ast ab}$, fat and water mapping by measuring and correcting for the effect of macroscopic field variations in the acquired signal. The proposed method is tested on several 2D and 3D phantom and in vivo liver, cardiac, and brain datasets. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Generalized stochastic approach for constitutive equation in linear elasticity: a random matrix model
Authors: Guilleminot J. , Soize C. 
Published: December 2011

SUMMARYThis work is concerned with the construction of stochastic models for random elasticity matrices, allowing either for the generation of elasticity tensors exhibiting some material symmetry properties almost surely (integrating the statistical dependence between the random stiffness components) or for the modeling of random media that requires the mean of a stochastic anisotropy measure to be controlled apart from the level of statistical fluctuations. To this aim, we first introduced a decomposition of the stochastic elasticity tensor on a deterministic tensor basis and considered the probabilistic modeling of the random components, having recourse to the MaxEnt principle. Strategies for random generation and estimation were further reviewed, and the approach was exemplified in the case of a material that was transversely isotropic almost surely. In a second stage, we made use of such derivations to propose a generalized model for random elasticity matrices that took into account, almost separately, constraints on both the level of stochastic anisotropy and the level of statistical fluctuations. An example was finally provided and showed the efficiency of the approach. Copyright © 2011 John Wiley & Sons, Ltd.
When can it be not optimal to adopt a new technology? A viability theory solution to a two‐stage optimal control problem of new technology adoption
Authors: Krawczyk Jacek B. , Serea Oana‐Silvia 
Published: December 2011

SUMMARYPoliticians often deplore economic agents’ behaviour when they do not accept new technologies. For a new technology to be adopted, the new technology value function needs to dominate the old technology value function. If this is the case, a technology switch will occur. We characterise the value functions, without computing them, using the fact that their hypographs are viability kernels of some auxiliary control problems and study whether the graphs intersect. If they do not, the corresponding value functions do not dominate each other, and the switch cannot occur at a positive time. Using this characterisation, we analyse a technology adoption problem and show how to recognise the models, for which the switch will occur at time zero or never, without solving an optimal control problem. We conclude that the current control regime may not change if the economic agents’ preferences are modelled as an integral of discounted differences between a reward from the flow variable (control) and a penalty from the stock variable (state).Copyright © 2011 John Wiley & Sons, Ltd.
Two‐Grid hp‐Version DGFEMs for Strongly Monotone Second‐Order Quasilinear Elliptic PDEs
Authors: Congreve Scott , Houston Paul , Wihler Thomas P. 
Published: December 2011

AbstractIn this article we develop the a priori error analysis of so‐called two‐grid hp‐version discontinuous Galerkin finite element methods for the numerical approximation of strongly monotone second‐order quasilinear partial differential equations. In this setting, the fully nonlinear problem is first approximated on a coarse finite element space V(𝒯H,P). The resulting ‘coarse’ numerical solution is then exploited to provide the necessary data needed to linearize the underlying discretization on the finer space V(𝒯h,p); thereby, only a linear system of equations is solved on the richer space V(𝒯h,p). Numerical experiments confirming the theoretical results are presented. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Automated parameter selection for rational Arnoldi approximation of Markov functions
Authors: Güttel Stefan , Knizhnerman Leonid 
Published: December 2011

AbstractRational Arnoldi is a powerful method for approximating functions of large sparse matrices times a vector. The selection of asymptotically optimal parameters for this method is crucial for its fast convergence. We present a heuristic for the automated pole selection when the function to be approximated is of Markov type, such as the matrix square root. The performance of this approach is demonstrated at several numerical examples. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Advanced finite element formulations for modeling thin piezoelectric structures
Authors: Klinkel Sven , Legner Dieter , Wagner Werner 
Published: December 2011

AbstractThis contribution is concerned with mixed finite element formulations for modeling piezoelectric beam and shell structures. Due to the electromechanical coupling, specific deformation modes are joined with electric field components. In bending dominated problems incompatible approximation functions of these fields cause incorrect results. These effects occur in standard finite element formulations, where interpolation functions of lowest order are used. A mixed variational approach is introduced to overcome these problems. The mixed formulation allows for a consistent approximation of the electromechanical coupled problem. It utilizes six independent fields and could be derived from a Hu‐Washizu variational principle. Displacements, rotations and the electric potential are employed as nodal degrees of freedom. According to the Timoshenko theory (beam) and the Reissner‐Mindlin theory (shell), the formulations account for constant transversal shear strains. To incorporate three dimensional constitutive relations all transversal components of the electric field and the strain field are enriched by mixed finite element interpolations. Thus the complete piezoelectric coupling is appropriately captured. The common assumption of vanishing transversal stress and dielectric displacement components is enforced in an integral sense. Some numerical examples will demonstrate the capability of the presented finite element formulation. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modelling and remodelling of biological tissue in the framework of continuum biomechanics
Authors: Ehlers Wolfgang , Krause Robert , Markert Bernd 
Published: December 2011

AbstractA biological tissue in general is formed by cells, extracellular matrix (ECM) and fluids. Consequently, its overall material behaviour results from its components and their interaction among each other. Furthermore, in case of living tissues, the material properties do not remain constant but naturally change due to adaptation processes or diseases.In the context of the Theory of Porous Media (TPM), a continuum‐mechanical model is introduced to describe the complex fluid‐structure interaction in biological tissue on a macroscopic scale. The tissue is treated as an aggregate of two immiscible constituents, where the cells and the ECM are summarised to a solid phase, whereas the fluid phase represents the extracellular and interstitial liquids as well as necrotic debris and cell or matrix precursors in solution.The growth and remodelling processes are described by a distinct mass exchange between the fluid and solid phase, which also results in a change of the constituent material behaviour. To furthermore guarantee the compliance with the entropy principle, the growth energy is introduced as an additional quantity. It measures the average of chemical energy available for cell metabolism, and thus, controls the growth and remodelling processes.To set an example, the presented model is applied for the simulation of the early stages of avascular tumour growth in the framework of the finite element method (FEM). (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparing exact inversion and singular perturbation approaches for a serial flexible manipulator
Authors: Gorius Thomas , Seifried Robert , Eberhard Peter 
Published: December 2011

AbstractThis paper reviews the singular perturbation control theory in the context of flexible multibody systems. The theory is motivated and explained by a simple, but sufficiently complex model. It is explained that based on the singular perturbed model an end‐effector trajectory tracking can be achieved by an integral manifold controller. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Synthesis of Adequate Mathematical Description of Rolling Mill Motion
Authors: Menshikov Yuri 
Published: December 2011

AbstractSynthesis of adequate mathematical description of the main mechanical line of rolling mill are considered. The four‐mass model with weightless elastic connections is chosen as mathematical model of dynamic system of the main mechanical line of the rolling mill. The problem was reduced to solution of integral equation of the first kind (to unsteady problem). The methods of obtaining of the steady solutions are suggested. Synthesis of the adequate mathematical descriptions with unitary model of external load are suggested. The metal rolling was executed with using of real experimental measurements as an example. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the description and measurement of the friction coefficient in models of multibody dynamics
Authors: Perzborn Nils , Ostermeyer Georg‐Peter 
Published: December 2011

AbstractIn many fields of the multi‐body world friction must be taken into account. Friction is usually reduced to a ratio: the coefficient of friction. Newer theories also consider dynamic friction laws. For the use in multi‐body models, a stepped complexity and measurably hedged description of the friction is necessary. Therefore, different experimental designs that allow dynamic measurements are discussed. An example friction pair is investigated by measurements and the quality of the results is pointed out. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A consistent direct transcription method for optimal control problems in multibody dynamics
Authors: Siebert Ralf , Betsch Peter 
Published: December 2011

AbstractThe present work deals with optimal control problems governed by differential‐algebraic equations (DAEs). In particular, the control effort, which is necessary for moving a multibody system from one configuration to another, will be minimized. The orientation of the rigid bodies will be described using directors, which facilitates the integration of the equations of motion with an energy‐momentum consistent time‐stepping scheme [1]. This type of structure‐preserving integrators offer outstanding numerical stability and robustness properties in comparison to the often applied generalized coordinates formulation. In the context of optimal control, other kinds of consistent integrators have been applied previously in [2] and [3]. We will test the different formulations with two numerical examples, a 3‐link manipulator and a satellite. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Piezoelectric control of flexible vibrations in rotating beams: An experimental study
Authors: Zehetner Christian , Zenz Georg , Gerstmayr Johannes 
Published: December 2011

AbstractActive control of flexible vibrations by distributed piezoelectric actuators and sensors plays an increasing role in engineering, especially in light‐weight structures. Exemplarily, in this contribution a rotating beam is studied which can be found in many practical applications, e.g. as robot arms or flexible manipulators in production processes. It has been intensively shown in the literature that it is possible to completely suppress the flexible vibrations by an appropriate distribution of piezoelectric actuation strains. In order to compensate the inertial forces in the considered rotating beam, a complex distribution is obtained, such that a practical realisation would be very extensive. To overcome the problem, a discrete approximation by piezoelectric patches is applied. In order to find an optimal configuration for an experimental setup, and to investigate several control strategies, a numerical simulation model has been implemented based on Bernoulli‐Euler beam theory. The numerical results are verified by an experimental set‐up, in which 48 piezoelectric patches have been attached on a beam with rectangular hollow cross‐section. Each patch can be used either as an actuator or a sensor. Additionally, strain gauges can be used as sensors. For monitoring, acceleration sensors are used. The control system is implemented within a dSpace environment. The results show a significant reduction of the flexible vibrations. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Experimental and numerical investigations on engineering tissue enhanced by mechanical stimulation
Authors: Yi Jeong‐Hun , Stoffel Marcus , Weichert Dieter , Nebelung Sven , Rath Björn 
Published: December 2011

AbstractThe aim of the paper is to investigate the remodeling phenomenon of a cell‐seeded material (collagen type‐I) due to collagen type‐II newly synthesized by the cells. For the experiments, a cell‐seeded condensed collagen gel is mechanically stimulated in a bioreactor for four weeks. The remodeled stiffness of the cell‐seeded gel is measured by a compression test and is explained with an evolution law. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Homogenisation method to capture the non‐linear behaviour of intervertebral discs in multi‐body systems
Authors: Karajan Nils , Ehlers Wolfgang , Röhrle Oliver , Schmitt Syn 
Published: December 2011

AbstractThe present contribution is motivated by the desire to compute physiological loads on the intervertebral discs (IVD) of a human lumbar spine during activities like standing, bending and falling. Following this, a mechanical multi‐body system (MBS) is utilised to capture the overall mechanical behaviour of a human, whereas an inhomogeneous, anisotropic, multi‐phasic finite‐element model (FEM) is applied to resolve the resulting field quantities inside an IVD. In order to couple the FEM of the IVD with the numerically diverse MBS, a homogenisation procedure has to be applied such that field quantities can be converted into discrete quantities. In particular, the MBS captures the mechanical behaviour of an IVD using a bushing element, which provides discrete force‐displacement and moment‐rotation relations.The goal of this contribution is to present a homogenisation method for the IVD as well as a possibility to include the homogenised results in the MBS without the need for embedded FE computations in the MBS. Instead, certain deformation modes of the IVD are pre‐computed and represented using a non‐linear constitutive equations. This task becomes even more challenging, as the resulting discrete DOF of a motion segment appear in a coupled fashion due to the structure of the IVD, i. e., a rotation in the sagittal plane triggers a resulting moment and a resulting force. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Forward dynamics applied to a three‐dimensional continuum‐mechanical model of the upper limb
Authors: Röhrle Oliver , Ramasamy Ellankavi , Schmitt Syn 
Published: December 2011

AbstractThis paper introduces, for the first time, a methodology to achieve a forward dynamics simulation of the musculoskeletal system using three‐dimensional continuum‐mechanical skeletal muscle models. This is achieved by coupling one‐ and three‐dimensional skeletal muscle models. The feasibility of this methodology is demonstrated through a forward dynamics simulation of the upper limb involving the biceps and triceps muscle. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Active strain and activation models in cardiac electromechanics
Authors: Rossi Simone , Ruiz‐Baier Ricardo , Pavarino Luca F. , Quarteroni Alfio 
Published: December 2011

AbstractWe present a model for mechanical activation of the cardiac tissue depending on the evolution of the transmembrane electrical potential and certain gating/ionic variables that are available in most of electrophysiological descriptions of the cardiac membrane. The basic idea consists in adding to the chosen ionic model one ordinary differential equation for the kinetics of the mechanical activation function. A relevant example illustrates the desired properties of the proposed model, such as delayed muscle contraction and correct magnitude of the muscle fibers' shortening. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the Mathematical Simulation of the Measuring of the Intraocular Pressure by Applanation Method after Refractive Surgery
Authors: Bauer Svetlana , Semenov Boris , Kornikov Vladimir , Kachanov Andrey 
Published: December 2011

AbstractApplanation tonometry estimates intraocular pressure (IOP) by quantifying the force needed to create a defined amount of deformation of the cornea (Goldmann tonometer) or by estimating the diameter of the circular contact area of the cornea and flat tonometer of defined load (Maklakov tonometer). The first simplest models of the applanation method for measurement of the IOP were based on approach, in which an eyeball is modelled as a thin‐walled spherical liquid‐filled soft shell with corneal biomechanical properties. It was usually supposed that these properties were the same for all patients. In this work numerical simulation have been carried out using finite element code ANSYS. The eye shell is modeled as two joint shells (cornea and sclera) with different mechanical properties. The results are obtained for numerous sets of parameters and were compared to clinical data. For statistics the measurements of IOP for both eyes of 120 patients before and one month after refractive surgery are used. All parameter of refractive surgery (depth, the width and the place of ablation ‐ refractive surgery for myopia or hypermetropia) have effect on IOP reading obtained with both Goldmann and Maklakov tonometry. The results obtained by Goldmann tonometer are significantly more sensitive to all parameters of refractive surgery than those found with Maklakov tonometer with load 10 g. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)