bluuebunny/crossref_metadata_2025_split huggingface dataset:
Numerical simulation of SAR induced around Co‐Cr‐Mo hip prostheses in situ exposed to RF fields associated with 1.5 and 3 T MRI body coils
Authors: Powell John , Papadaki Annie , Hand Jeff , Hart Alister , McRobbie Donald 
Published: December 2011

AbstractWhen patients with metallic prosthetic implants undergo an MR procedure, the interaction between the RF field and the prosthetic device may lead to an increase in specific absorption rate (SAR) in tissues surrounding the prosthesis. In this work, the distribution of SAR10g around bilateral CoCrMo alloy hip prostheses in situ in anatomically realistic voxel models of an adult male and female due to RF fields from a generic birdcage coil driven at 64 or 128 MHz are predicted using a time‐domain finite integration technique. Results indicate that the spatial distribution and maximum values of SAR10g are dependent on body model, frequency, and the position of the coil relative to the body. Enhancement of SAR10g close to the extremities of a prosthesis is predicted. Values of SAR10g close to the prostheses are compliant with recommended limits if the prostheses are located outside the coil. However, caution is required when the prostheses are within the coil since the predicted SAR10g close to an extremity of a prosthesis exceeds recommended limits when the whole body averaged SAR is 2 W kg−1. Compliance with recommended limits is likely to require a reduction in the time averaged input power. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Free‐breathing inner‐volume black‐blood imaging of the human heart using two‐dimensionally selective local excitation at 3 T
Authors: Abd‐Elmoniem Khaled Z. , Barmet Christoph , Stuber Matthias 
Published: December 2011

AbstractBlack‐blood fast spin‐echo imaging is a powerful technique for the evaluation of cardiac anatomy. To avoid fold‐over artifacts, using a sufficiently large field of view in phase‐encoding direction is mandatory. The related oversampling affects scanning time and respiratory chest motion artifacts are commonly observed. The excitation of a volume that exclusively includes the heart without its surrounding structures may help to improve scan efficiency and minimize motion artifacts. Therefore, and by building on previously reported inner‐volume approach, the combination of a black‐blood fast spin‐echo sequence with a two‐dimensionally selective radiofrequency pulse is proposed for selective “local excitation” small field of view imaging of the heart. This local excitation technique has been developed, implemented, and tested in phantoms and in vivo. With this method, small field of view imaging of a user‐specified region in the human thorax is feasible, scanning becomes more time efficient, motion artifacts can be minimized, and additional flexibility in the choice of imaging parameters can be exploited. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Overcoming the low relaxivity of gadofosveset at high field with spin locking
Authors: Richardson O. C. , Scott M. L. J. , Tanner S. F. , Waterton J. C. , Buckley D. L. 
Published: December 2011

AbstractThe contrast agent gadofosveset, which binds reversibly to serum albumin, has a high longitudinal relaxivity at lower magnetic fields (≤3.0 T) but a much lower relaxivity at high fields. Spin locking is sensitive to macromolecular content; it is hypothesized that combining this technique with the albumin‐binding properties of gadofosveset may enable increased relaxivity at high fields. In vitro measurements at 4.7 T found significantly higher spin‐lock relaxation rates, R1ρ (1/T1ρ), when gadofosveset was serum albumin‐bound than when unbound. R1ρ values for a nonbinding contrast agent (gadopentetate dimeglumine) in serum albumin were similar to those for unbound gadofosveset. R2 (1/T2) values were also significantly higher at 4.7 T for serum albumin‐bound gadofosveset than for unbound. Spin locking at high field generates significantly higher relaxation rates for gadofosveset than conventional contrast agents and may provide a method for differentiating free and bound molecules at these field strengths. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Fast cardiac T1 mapping in mice using a model‐based compressed sensing method
Authors: Li Wen , Griswold Mark , Yu Xin 
Published: December 2011

AbstractDirect measurement of the longitudinal relaxation time T1 provides objective and quantitative diagnostic information. However, current T1 mapping methods are generally time consuming without the aid of fast imaging. This study used a model‐based compressed sensing method for fast cardiac T1 mapping in small animals. Based on the physics of magnetization recovery, the aliasing artifact associated with under‐sampling was removed by exploiting the sparsity of the signals in the T1 recovery direction. Simulation study was performed to evaluate the reconstruction accuracy under various experimental conditions. Several approaches that accounted for phase variations were compared for optimized reconstruction in the phantom study. In vivo validation was performed on a cardiac manganese‐enhanced MRI study using mice. Accurate reconstruction of the under‐sampled images and the resulting T1 maps were achieved in both simulation and MRI studies on phantom and in vivo mice. These results suggest that the current compressed sensing method allows fast (<80 s) T1 mapping of the mouse heart at high spatial resolution (234 × 469 μm2). Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.
Challenging Evidence‐based Decision‐making: A Hypothetical Case Study about Return to Work
Authors: Aas Randi W. , Alexanderson Kristina 
Published: December 2011

AbstractA hypothetical case study about return to work was used to explore the process of translating research into practice. The method involved constructing a case study derived from the characteristics of a typical, sick‐listed employee with non‐specific low back pain in Norway. Next, the five‐step evidence‐based process, including the Patient, Intervention, Co‐Interventions and Outcome framework (PICO), was applied to the case study. An inductive analysis produced 10 technical and more fundamental challenges to incorporate research into intervention decisions for an individual with comorbidity. A more dynamic, interactive approach to the evidence‐based practice process is proposed. It is recommended that this plus the 10 challenges are validated with real life cases, as the hypothetical case study may not be replicable. Copyright © 2011 John Wiley & Sons, Ltd.
Loss Coefficients in Laminar Flows: Essential for the Design of Micro Flow Systems
Authors: Schmandt Bastian , Herwig Heinz 
Published: December 2011

AbstractThe concept of head loss coefficients K for the determination of losses in conduit components is discussed in detail. While so far it has mainly been applied to fully turbulent flows it is extended here to also cover the laminar flow regime which is relevant for micro systems due to the low Reynolds numbers of these flows. Specific numbers of K can be determined by integration of the entropy generation field (second law analysis) obtained from a numerical simulation. It will be shown that a definition of K based on entropy generation is superior to a widely used definition that refers to a pressure drop caused by the conduit component. With the second law analysis details of the physics become available. For example it can be shown that often the main part of the entropy generation occurs downstream of the component. This aspect becomes important when several conduit components are combined in close proximity, like two 90 degree bends that are close to each other. Often in such situations the combination as a whole has to be looked upon as one new complex component. The general approach is discussed and illustrated for various conduit components and combinations of them. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The Lightweight Robot ElRob: Interesting Aspects in Modeling and Control
Authors: Höbarth Wolfgang , Gattringer Hubert , Staufer Peter , Bremer Hartmut 
Published: December 2011

AbstractThe articulated robot ElRob, consisting of flexible links and joints, is considered in several publications. Recent developments are presented in this work.The overall goal of the research is to decrease the effects of structural elasticities in lightweight robots. For this purpose model‐based control concepts are investigated and very accurate and efficient kinematic and dynamic models are necessary. The robot is split into groups of bodies, the so called subsystems, with separated describing velocities and coordinate systems. To obtain structured equations of motion the Projection Equation is used. The beams are modelled using the floating frame of reference formulation and a Ritz‐approach.Because of its flexibility, the examined robot is an underactuated system leading to special difficulties. As an example is it not possible to compute the desired joint angles with respect to a reference path in task space for the flexible system (inverse kinematic problem).Different methods to solve this drawback and other problems resulting from flexibility are discussed with special focus on feed forward control and different feedback control concepts. The resulting end point error, the necessary control input and other interesting results for the laboratory experiment are presented and compared. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimal Control of Multibody Dynamics with Contact
Authors: Koch Michael W. , Leyendecker Sigrid 
Published: December 2011

AbstractThis work discusses two different structure preserving integrators in the framework of optimal control simulations with contact. The first one is a variational integrator, based on the constrained version of the Lagrange‐D'Alembert. The resulting scheme preserves the symplecticity and the momentum maps of the simulated multibody dynamics. The second integrator is an energy momentum scheme and it is based on the augmented Hamiltonian equations, which are discretised using the discrete derivative in [2]. Both integrators are applied to simulate the optimal control of compass gait, for which the contact between the foot and the ground is modelled as perfectly plastic contact. The second example represents a monopedal jumper and it is used to examine the dynamical behaviour of the perfectly elastic and perfectly plastic contact formulation. The resulting differential algebraic equations (DAEs) are solved by the aforementioned symplectic momentum method. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Variational multirate integration of constrained dynamics
Authors: Leyendecker Sigrid , Ober‐Blöbaum Sina 
Published: December 2011

AbstractMechanical systems with dynamics on varying time scales, in particular those including highly oscillatory motion, impose challenging questions for numerical integration schemes. Tiny step sizes are required to guarantee a stable integration of the fast frequencies. However, for the simulation of the slow dynamics, integration with a larger time step is accurate enough. Small time steps increase integration times unnecessarily, especially for costly function evaluations. For systems comprising fast and slow dynamics, multirate methods integrate the slow part of the system with a relatively large step size while the fast part is integrated with a small time step. Main challenges are the identification of fast and slow parts (e.g. by separating the energy or by distinguishing sets of variables), the synchronisation of their dynamics and in particular the treatment of mixed parts that often appear when fast and slow dynamics are coupled by constraints. In this contribution, a multirate integrator is derived in closed form via a discrete variational principle on a time grid consisting of macro and micro time nodes. Variational integrators (based on a discrete version of Hamilton's principle) lead to symplectic and momentum preserving integration schemes that also exhibit good energy behavior. The resulting multirate variational integrator has the same preservation properties. An example demonstrates the performance of the multirate integrator for constrained multibody dynamics. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the Suitability of Different Online Gait Generation Methods for Pre‐Defined Footsteps
Authors: Mayr Johannes , Gattringer Hubert , Bremer Hartmut 
Published: December 2011

AbstractDetailed dynamical modeling is the basis for simulation and model based control. In this contribution the Projection Equation is used for the modeling of a biped walking machine, resulting in the equations of motion which are needed for gait generation and verification of its stability. For biped robots one main issue is the generation of stable trajectories for the center of mass (CoM). Several different approaches based on the Zero Moment Point (ZMP) scheme have been presented in the past. Due to the complex dynamic structure of bipedal robots most of the considered algorithms use a linear inverted pendulum as a simplified model. This results in a decoupling of the ZMP equations in lateral and forward direction, but limits the trajectories to a constant height of the CoM. An extension of the well known LQR theory by future reference values has been proposed. This model based approach seems to perform quite well, but does not allow the consideration of constraints on the position of the ZMP. This limitation is removed by the use of Model Predictive Control (MPC) with inequality constraints. By extending this approach to a time invariant one the restriction to a constant height of the CoM is no longer necessary. Both methods as well as the time invariant approach for variable CoM heights have been evaluated in simulations and will be experimentally verified on a real robot soon. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A contribution to efficient calculation of complex drill string dynamics for deep hole drilling
Authors: Shi Minzhe , Schiefer Frank , Ostermeyer Georg‐Peter 
Published: December 2011

AbstractThis paper presents a hybrid model to describe drill string dynamics for deep hole drilling. Generally, a typical rotary drill string has a length of several kilometers, but the diameter is less than half a meter. Due to the large ratio of length to diameter, a drill string is a very flexible system. Consequently, an operating drill string is always affected by axial, torsional and lateral vibrations, which potentially induce serious failures. In order to avoid fatal defects, simulations to forecast vibrations are necessary. The simulation should be capable to exhibit the complex dynamical phenomena, e.g. sick‐slip, forward whirl and backward whirl, and interactions between drill string and borehole. Usually, these simulations are very time‐consuming. In this work, a hybrid model consisting of lumped masses connected with weightless beam elements representing the drill string is developed. The interaction between the drill string and the borehole is implemented by unilateral constraints to describe the nonlinear contact behavior. It was shown that accuracy and simulating time were improved by this model with respect to classical finite‐element models. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Hand Vibrations of the Human Operator in the Working Space
Authors: Arghir Mariana , Rus Mariana , Macovescu Sorin Constantin 
Published: December 2011

AbstractThe paper contains the identification and the taking into account the vibrations produced by the machines, the lathe with different number of rotation, and them action over the human bodies inside the working space. For the identification of vibrations is applied a new method, the Moiré projection method, that did not used until this moment regarding the vibrations action over the human body. Our research was to apply the Moiré projection method to the human hand. They were compared with the measured vibrations using a classic vibrometer with three‐axial accelerometer. The results in the booth situation were in the same order of the unit scale, and the optical method named Moiré projection method can be considered a valid method for the human vibrations measurements without touch of the surface. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A continuum‐mechanical analysis of the influence of mechanical stimuli on biological tissue
Authors: Avci Arzu , Markert Bernd , Ehlers Wolfgang 
Published: December 2011

AbstractMechanical stimuli play a crucial role in the differentiation process of mesenchymal stem cells (MSC). The resulting mechanical signals are important in the regulation of various cell functions and maintenance of many tissues. The underlying molecular and biophysical mechanisms of the differentiation process are poorly understood. Present remodelling and growth models are purely phenomenological without linkage to cell mechanisms.The presented macroscopic model of MSC mechanics is based on a multiphasic‐multicomponent formulation within the framework of Theory of Porous Media (TPM), where a single cell is considered as a mixture of interacting constituents. In particular, the constituents are the solid cytoskeleton saturated by a fluid phase (cytoplasm), which itself consists of a liquid solvent and mobile components, e. g., chemical messengers, proteins, etc. To demonstrate the capabilities of the developed model, first qualitative numerical simulations of the impact of external forces on MSC are presented. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A finite element approach for modelling hip joint contact
Authors: Fietz Kristin , Nackenhorst Udo 
Published: December 2011

AbstractIn this contribution a finite element model for the three dimensional investigation of hip joint contact is described. A shell‐like interface element with variable thickness is developed for modelling fluid flow in the synovial gap. For this purpose the Taylor‐Hood element is extended in order to take a spatial thickness distribution and local thickness changes into account. The interaction between the synovial fluid and the cartilage layers is solved by a staggered iteration using an artificial compressibility method. Cartilage is modelled using the theory of porous media and three dimensional geometries are reconstructed from medical imaging data. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Numerical investigations on the fixation of cemented and uncemented endoprostheses
Authors: Hürkamp André , Lutz André , Nackenhorst Udo 
Published: December 2011

AbstractIn this contribution, a constitutive model adopted from the computational plasticity‐models of Drucker‐Prager and von Mises is presented. This model captures the material behavior of osseointegration and the curing‐process of bone cement. With this basic model, both simulations of bone‐ingrowth of uncemented implants and simulations of the curing process of bone cement for cemented implants are carried out in a bone‐implant interface. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Bone remodelling: A combined biomechanical and systems‐biological challenge
Authors: Krause Robert , Schittler Daniella , Reiter Michael , Waldherr Steffen , Allgöwer Frank , Karastoyanova Dimka , Leymann Frank , Markert Bernd , Ehlers Wolfgang 
Published: December 2011

AbstractBone tissues are formed by cells, bone matrix and fluids. The bone matrix is a porous structure, which is remodelled by bone‐cells. To describe this process, a coupled systems‐biological and biomechanical model is presented.The macroscopic mechanical behaviour of bone is specified by a biphasic model embedded in the Theory of Porous Media, where the solid phase represents cells and bone matrix, and the fluid phase summarises the extracellular fluids and its components. In this context, the bone remodelling process is described on the macro‐scale by a distinct mass exchange that also results in a change of the constituents'material properties.On the micro‐scale, the description of the bone remodelling process stems from a systems‐biological cell interaction model. Therein, the bone matrix formation and resorption are a result of the stress‐regulated activity of cells.Here, a staggered solutions strategy is presented. Therein, snapshots of the mechanical stress distribution are calculated on the macro‐scale by use of the finite element method. The snapshots are locally evaluated on the micro‐scale by use of a cell interaction model, which calculates the long term remodelling process. The evaluation results of the micro‐scale are then used to update the reference configuration of the mechanical simulation. As an integrative modelling platform scientific workflows and web‐service technologies are employed. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim
Model reduction for nonlinear biomechanical structural analysis
Authors: Radermacher Annika , Reese Stefanie 
Published: December 2011

AbstractFor surgery training and on‐line support during surgeries a method is required which is able to reduce simulation time possibly even to realtime. This contribution compares the effectiveness of three model reduction methods, all of which are widely used for linear problems, in the context of nonlinear structural mechanics. Three reduction methods will be extended to nonlinear elasticity including large deformations. The performance of the extended concepts is investigated for a simplified model of a human inferior turbinate in the context of the functional endoscopic sinus surgery. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Knee joint kinematics: comparison of two optimization models with respect to data noise
Authors: Reichl Irene , Auzinger Winfried , Schmiedmayer Heinz‐Bodo , Weinmüller Ewa 
Published: December 2011

AbstractThe interpretation of human knee joint kinematics in terms of displacements is an outcome of the underlying model of the joint and the measurement technique. Measurement errors and noise challenge the development of optimization procedures which, based on a reduction in degrees of freedom, aim for the reproducibility of joint displacements by computational techniques. So far, optimization algorithms have been applied which are based on a kinematic model of the healthy human tibio‐femoral joint (TFJ) as a compound hinge with two fixed orthogonal axes. On the other hand, empirical studies find non‐orthogonal rotational axes. Therefore, it was the aim of the present study to investigate the implications of a refined kinematic model on the accuracy of computed joint rotation angles. For the purpose of quantitative comparison, kinematic data of a TFJ with two axes intersecting at an arbitrary angle were simulated. The joint rotations were optimized for the assumption of (a) two orthogonal and intersecting axes (model A), and (b) two axes intersecting at an arbitrary angle (model B). Model B recovers the original input data closer in case of low noise level as encountered in invasive measurement techniques. Skin mounted markers tracking involves non‐normally distributed noise which is typically larger by on order of magnitude. In this case, model A exhibits a more favorable performance. These observations motivate the search for alternative kinematic descriptions of the TFJ. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
On the Coupling of 3D‐1D Muscle Models for Lumbar Spine Mechanics
Authors: Sprenger Michael , Karajan Nils , Schmitt Syn , Röhrle Oliver 
Published: December 2011

AbstractThe aim of this research is to represent, within one modelling framework, selected parts of the musculoskeletal system using principles of continuum mechanics, while other parts are modelled using lumped‐parameter models and principles of Multi‐Body Dynamics. The most challenging part within such a framework will be to properly model the transition from 3D to 1D models for skeletal muscles as many of the skeletal muscles extend beyond the selected part. Hence, this paper focuses on an interface condition for the 3D‐1D transition within a skeletal muscle. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)