Anh Quách Văn - Academia.edu (original) (raw)

Papers by Anh Quách Văn

Introduction Polycyclic aromatic hydrocarbons (PAHs) are classified as persistent organic polluta... more Introduction Polycyclic aromatic hydrocarbons (PAHs) are classified as persistent organic pollutants. They exist ubiquitously in various environmental components. In a multi-phases system of water environment, hydrophobic organic contaminants like PAHs exist in several different forms causes PAHs to exhibit a complicated behavior in their distribution and transport. For these reasons, understanding the distribution, behavior and transport of PAHs in water environment is the essential to evaluate the dispersion of the contamination as well as the pollution of PAHs and to manage and to control PAHs level in environment. To understand, evaluate, and control the level of PAHs in water environment as well as propose a prediction for future condition, monitoring is the foundation of creating the data base for clarifying the distribution and analyzing dynamics of PAHs in water environment. We chose the rivers in Kanazawa as a case study to investigate the dynamic of PAHs in water environment. Kanazawa locates in Hokkuriku Area of Japan, and has two main rivers, Asano River and Sai River, flowing into the Sea of Japan. As the Sea of Japan is a relative closed water environment surrounded by the Asian continent, Korea peninsula and Japanese archipelago, it is considered to be vulnerable to the anthropogenic pollution. However, no data are currently available. Monitoring PAHs in these rivers have been carried since Nov. 2004 to create a database of PAHs level in these two rivers. This study focuses on the distribution and partition of PAHs in water, particulate matter and sediment at the estuaries of these two rivers. Sampling and analysis Water and surface sediment at the estuaries of the rivers were collected from Asano and Sai Rivers. Sampling period was from Nov. 2004 to Aug. 2006 in monthly frequency except for Mar. and Apr. 2005. SPM was separated from water by filtration through 0.5 m glass fiber filter. PAHs in water, SPM, sediment were quantified by HPLC/Fluorescence detector. Partition coefficients K d were calculated as Eq.(1)

Journal of neural transmission (Vienna, Austria : 1996), Jan 2, 2015

Mild parkinsonian features can be observed in patients with essential tremor (ET). Although dopam... more Mild parkinsonian features can be observed in patients with essential tremor (ET). Although dopamine transporter (DAT) imaging is usually normal in ET, some studies found mild dopaminergic deficit in ET patients compared to healthy controls (HC). We analyzed clinical and DAT imaging data in ET patients with and without parkinsonian features. Thirty-nine ET patients with and without parkinsonian features and 13 HC underwent detailed examination by a movement disorders neurologist and 123-I ioflupane SPECT. Two independent radiologists "blinded" to the clinical diagnosis analyzed images visually and by semi-quantitative calculation of striatal binding ratios in different volumes of interests. ET patients were divided into pure ET group (no parkinsonian features, n = 22), ET-P [one parkinsonian feature not sufficient for the clinical diagnosis of Parkinson's disease (PD), n = 9], and ET + PD (two or more parkinsonian features meeting diagnostic criteria for PD, n = 8). As...

International Journal of Non-Linear Mechanics, 2011

The paper presents two new results in the domain of the elastoplastic buckling and post-buckling ... more The paper presents two new results in the domain of the elastoplastic buckling and post-buckling of beams under axial compression. (i) First, the tangent modulus critical load, the buckling mode and the initial slope of the bifurcated branch are given for a Timoshenko beam (with the transverse shear effects). The result is derived from the 3D J 2 flow plastic bifurcation theory with the von Mises yield criterion and a linear isotropic hardening. (ii) Second, use is made of a specific method in order to provide the asymptotic expansion of the post-critical branch for a Euler-Bernoulli beam, exhibiting one new non-linear fractional term. All the analytical results are validated by finite element computations.

International Journal of Solids and Structures, 2008

In this paper, a shell finite element is designed within the total Lagrangian formulation framewo... more In this paper, a shell finite element is designed within the total Lagrangian formulation framework to deal with the plastic buckling and post-buckling of thin structures, such as cylindrical shells. First, the numerical formulation is validated using available analytical results. Then it is shown to be able to provide the bifurcation modes-possibly the secondary ones-and describe the complex advanced post-critical state of a cylinder under axial compression, where the theory is no longer operative.

Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2008

Motion during diffusion encodings leads to phase errors in different shots of a multishot acquisi... more Motion during diffusion encodings leads to phase errors in different shots of a multishot acquisition. Phase differences in k-space data among shots result in phase cancellation artifacts in the reconstructed image. Due to aliasing of the phase from under-sampled regions of the shot, correction of the phase error using direct low-resolution phase subtraction is incomplete. We introduce a new k-space and image-space combination (KICT) method for motion artifacts cancellation that avoids incomplete phase error correction. Further, the method preserves the phase of the object, which is important for parallel imaging applications.

Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2008

Diffusion Tensor Imaging (DTI) of localized anatomical regions (i.e brainstem, cervical spinal co... more Diffusion Tensor Imaging (DTI) of localized anatomical regions (i.e brainstem, cervical spinal cord, and optic nerve) is challenging because of the existence of significant susceptibility differences in the surrounding tissues, their high motion sensitivity and the need for high spatial resolution to resolve the underlying complex histoarchitecture. The aim of the present methodology is to achieve high resolution DTI with motion compensating capability in localized regions of the central nervous system. We accomplish this by implementing self-navigated multi-shot variable density spiral encoding with outer volume suppression. In vivo application of the technique on the human brainstem demonstrates a clear delineation of the multiple local neural tracts. We also investigate the partial volume effect on the extracted diffusion anisotropy metrics by varying the in-plane resolution while maintaining a constant signal-to-noise ratio.

Thin-Walled Structures, 2015

An inflatable beam is an airtight structure made of a soft technical fabric and subjected to an i... more An inflatable beam is an airtight structure made of a soft technical fabric and subjected to an internal pressure which gives it a final cylindrical shape, a pre-stress in the membrane and a bearing capacity. Against all appearances, it is not a standard beam and it requires a specific formulation in order to take account of the internal pressure which plays a key role in its mechanical response. This work deals with inflatable beams made of orthotropic materials. The first part of the paper is concerned with the inflation of the membrane tube, an important stage which is often neglected so far in the literature. As preliminaries of the bending problem studied in the next part of the paper, the constitutive law related to the inflated state of the tube-not the natural state-is investigated. It will be shown that the constitutive law related to the inflated pre-stressed state is not the same as the constitutive law related to the natural state. Expressions of the material coefficients involved in the former constitutive law will be established from the material coefficients defined on the natural reference configuration which are the only ones supposed to be known. The second part of the paper deals with the bending of the inflatable beam. The Timoshenko beam kinematics will be chosen because of the significant shear effect in the tube wall and the problem will be formulated in finite deformations in order to accounts for all the nonlinear effects, in particular the action due to the internal pressure which is a follower load. The nonlinear system of equations obtained will then be linearized around the pre-stressed configuration and will result in a more tractable linear system.

Thin-Walled Structures, 2007

The discretized nonlinear equations for bending and buckling of inflatable beams are written by u... more The discretized nonlinear equations for bending and buckling of inflatable beams are written by use of the virtual work principle with Timoshenko's kinematics, finite rotations and small strains. The linearized equations around a pre-stressed reference configuration are then deduced, giving rise to a new inflatable beam finite element. The stiffness matrix contains the shear coefficient and the internal pressure. Use is made of the particular 3-node beam element to investigate the bending and the buckling of a cantilever beam, the deflection of a pinched torus and the buckling of a torus submitted to a radial compressive force. The numerical results obtained with the beam element are shown to be close to analytical and three-dimensional (3D) membrane finite element results. The validity of the numerical results is discussed, in connection with the concepts of the crushing force or the wrinkling pressure of the inflated beam.

... cases do not indicate time and tense precisely, which causes considerable problems when the l... more ... cases do not indicate time and tense precisely, which causes considerable problems when the law is applied. According to Anh Luu (2006), despite significant improvements, Vietnamese legislation work is still weak resulting in the legal system being inadequate and unstable. ...

Thin-Walled Structures, 2005

The non linear and linearized equations are derived for the in plane stretching and bending of th... more The non linear and linearized equations are derived for the in plane stretching and bending of thin walled cylindrical beams made of a membrane and inflated by an internal pressure. The Timoshenko beam model combined with the finite rotation kinematics enables one to correctly account for the shear effect and all the non linear terms in the governing equations. The linearization is carried out around a pre stressed reference configuration which has to be defined as opposed to the so called natural state. Two examples are then investigated: the bending and the buckling of a cantilever beam. Their analytical solutions show that the inflation has the effect of increasing the material properties in the beam solution. This solution is compared with the three dimensional finite element analysis, as well as the so called wrinkling pressure for the bent beam and the crushing force for the buckled beam. New theoretical and numerical results on the buckling of inflatable beams are displayed.

Thin-Walled Structures, 2013

Inflatable tubes must be pressurized before they are able to resist to external loadings. While t... more Inflatable tubes must be pressurized before they are able to resist to external loadings. While there are many studies dedicated to the behavior of inflated tubes under bending or twisting, few results are available on the inflating stage when the beam is only submitted to the internal pressure. In this paper, an exact solution is proposed for a cylindrical tube made of an orthotropic membrane, with the orthotropy axes parallel to the circumferential and longitudinal directions of the cylinder, and undergoing finite deformations under internal pressure. The solution of the problem amounts to solving a cubic equation and analytical relations are given for the inflated geometry as well as the resultant stresses in the membrane. Numerical results are shown to compare very well with those obtained from a finite element code.

European Journal of Computational Mechanics, 2006

Magnetic Resonance in Medicine, 2013

The main obstacle to high-resolution (<1.5 mm isotropic) 3D diffusion-weighted MRI is the differe... more The main obstacle to high-resolution (<1.5 mm isotropic) 3D diffusion-weighted MRI is the differential motion-induced phase error from shot-to-shot. In this work, the phase error is addressed with a hybrid 3D navigator approach that corrects motioninduced phase in two ways. In the first, rigid-body motion is corrected for every shot. In the second, repeatable nonrigid-body pulsation is corrected for each portion of the cardiac cycle. These phase error corrections were implemented with a 3D diffusionweighted steady-state free precession pulse sequence and were shown to mitigate signal dropouts caused by shot-to-shot phase inconsistencies compared to a standard gridding reconstruction in healthy volunteers. The proposed approach resulted in diffusion contrast more similar to the contrast observed in the reference echo-planer imaging scans than reconstruction of the same data without correction. Fractional anisotropy and Color fractional anisotropy maps generated with phase-corrected data were also shown to be more similar to echo-planer imaging reference scans than those generated without phase correction.

IEEE Transactions on Medical Imaging, 2009

Motion during diffusion encodings leads to different phase errors in different shots of multishot... more Motion during diffusion encodings leads to different phase errors in different shots of multishot diffusion-weighted acquisitions. Phase error incoherence among shots results in undesired signal cancellation when data from all shots are combined. Motion-induced phase error correction for multishot diffusion-weighted imaging (DWI) has been studied extensively and there exist multiple phase error correction algorithms. A commonly used correction method is the direct phase subtraction (DPS). DPS, however, can suffer from incomplete phase error correction due to the aliasing of the phase errors in the high spatial resolution phases. Furthermore, improper sampling density compensation is also a possible issue of DPS. Recently, motion-induced phase error correction was incorporated in the conjugate gradient (CG) image reconstruction procedure to get a nonlinear phase correction method that is also applicable to parallel DWI. Although the CG method overcomes the issues of DPS, its computational requirement is high. Further, CG restricts to sensitivity encoding (SENSE) for parallel reconstruction. In this paper, a new time-efficient and flexible k-space and image-space combination (KICT) algorithm for rigid body motion-induced phase error correction is introduced. KICT estimates the motion-induced phase errors in image space using the self-navigated capability of the variable density spiral trajectory. The correction is then performed in k -space. The algorithm is shown to overcome the problem of aliased phase errors. Further, the algorithm preserves the phase of the imaging object and receiver coils in the corrected k -space data, which is important for parallel imaging applications. After phase error correction, any parallel reconstruction method can be used. The KICT algorithm is tested with both simulated and in vivo data with both multishot single-coil and multishot multicoil acquisitions. We show that KICT correction results in diffusion-weighted images with higher signal-to-noise ratio (SNR) and fractional anisotropy (FA) maps with better resolved fiber tracts as compared to DPS. In peripheral-gated acquisitions, KICT is comparable to the CG method.

Computers & Structures, 2009

In this work, a weighted residual relationship is proposed as an extension of the standard virtua... more In this work, a weighted residual relationship is proposed as an extension of the standard virtual work principle to deal with the large deformation contact problem with Coulomb friction. This weak form is a mixed relationship involving the displacements and the multipliers defined on the reference contact surface of the contactor and is shown to be equivalent to the strong form of the initial/boundary value contact problem. The discretization in space by means of the finite element method is carried out on the mixed relationship in a simple way in order to obtain the semi-discrete equation system. The contact tangent stiffness is derived and numerical examples are presented to assess the efficiency of the formulation.

Computers & Structures, 2004

Computers & Structures, 2003

This paper describes a general method for deriving the plastic corrections and the consistent tan... more This paper describes a general method for deriving the plastic corrections and the consistent tangent modulus for a wide range of arbitrary non-linear hardening models within the framework of standard small strains elastoplasticity. The features of the proposed formulation are: (i) the local solution is obtained through an iterative procedure. The plastic corrections are given in closed forms exhibiting one scalar function denoted by G alg and three fourth-order tensors D alg , G alg , L alg , which are shown to be the algorithmic discrete counterparts of usual theoretical continuum quantities, (ii) the consistent tangent modulus has a symmetrical expression involving the same quantities. Finite element computations are performed using a particular non-linear kinematic hardening model and allow to exhibit the ratcheting phenomenon usually observed on mechanical components subjected to cyclic loadings.

IEEE Transactions on Medical Imaging, 2011

A multishot data acquisition strategy is one way to mitigate B0 distortion and T2 blurring for hi... more A multishot data acquisition strategy is one way to mitigate B0 distortion and T2 blurring for high-resolution diffusion-weighted magnetic resonance imaging experiments. However, different object motions that take place during different shots cause phase inconsistencies in the data, leading to significant image artifacts. This work proposes a maximum likelihood estimation and-space correction of motion-induced phase errors in 3D multishot diffusion tensor imaging. The proposed error estimation is robust, unbiased, and approaches the Cramer-Rao lower bound. For rigid body motion, the proposed correction effectively removes motion-induced phase errors regardless of the-space trajectory used and gives comparable performance to the more computationally expensive 3D iterative nonlinear phase error correction method. The method has been extended to handle multichannel data collected using phased-array coils. Simulation and in vivo data are shown to demonstrate the performance of the method.

Introduction Polycyclic aromatic hydrocarbons (PAHs) are classified as persistent organic polluta... more Introduction Polycyclic aromatic hydrocarbons (PAHs) are classified as persistent organic pollutants. They exist ubiquitously in various environmental components. In a multi-phases system of water environment, hydrophobic organic contaminants like PAHs exist in several different forms causes PAHs to exhibit a complicated behavior in their distribution and transport. For these reasons, understanding the distribution, behavior and transport of PAHs in water environment is the essential to evaluate the dispersion of the contamination as well as the pollution of PAHs and to manage and to control PAHs level in environment. To understand, evaluate, and control the level of PAHs in water environment as well as propose a prediction for future condition, monitoring is the foundation of creating the data base for clarifying the distribution and analyzing dynamics of PAHs in water environment. We chose the rivers in Kanazawa as a case study to investigate the dynamic of PAHs in water environment. Kanazawa locates in Hokkuriku Area of Japan, and has two main rivers, Asano River and Sai River, flowing into the Sea of Japan. As the Sea of Japan is a relative closed water environment surrounded by the Asian continent, Korea peninsula and Japanese archipelago, it is considered to be vulnerable to the anthropogenic pollution. However, no data are currently available. Monitoring PAHs in these rivers have been carried since Nov. 2004 to create a database of PAHs level in these two rivers. This study focuses on the distribution and partition of PAHs in water, particulate matter and sediment at the estuaries of these two rivers. Sampling and analysis Water and surface sediment at the estuaries of the rivers were collected from Asano and Sai Rivers. Sampling period was from Nov. 2004 to Aug. 2006 in monthly frequency except for Mar. and Apr. 2005. SPM was separated from water by filtration through 0.5 m glass fiber filter. PAHs in water, SPM, sediment were quantified by HPLC/Fluorescence detector. Partition coefficients K d were calculated as Eq.(1)

Journal of neural transmission (Vienna, Austria : 1996), Jan 2, 2015

Mild parkinsonian features can be observed in patients with essential tremor (ET). Although dopam... more Mild parkinsonian features can be observed in patients with essential tremor (ET). Although dopamine transporter (DAT) imaging is usually normal in ET, some studies found mild dopaminergic deficit in ET patients compared to healthy controls (HC). We analyzed clinical and DAT imaging data in ET patients with and without parkinsonian features. Thirty-nine ET patients with and without parkinsonian features and 13 HC underwent detailed examination by a movement disorders neurologist and 123-I ioflupane SPECT. Two independent radiologists "blinded" to the clinical diagnosis analyzed images visually and by semi-quantitative calculation of striatal binding ratios in different volumes of interests. ET patients were divided into pure ET group (no parkinsonian features, n = 22), ET-P [one parkinsonian feature not sufficient for the clinical diagnosis of Parkinson's disease (PD), n = 9], and ET + PD (two or more parkinsonian features meeting diagnostic criteria for PD, n = 8). As...

International Journal of Non-Linear Mechanics, 2011

The paper presents two new results in the domain of the elastoplastic buckling and post-buckling ... more The paper presents two new results in the domain of the elastoplastic buckling and post-buckling of beams under axial compression. (i) First, the tangent modulus critical load, the buckling mode and the initial slope of the bifurcated branch are given for a Timoshenko beam (with the transverse shear effects). The result is derived from the 3D J 2 flow plastic bifurcation theory with the von Mises yield criterion and a linear isotropic hardening. (ii) Second, use is made of a specific method in order to provide the asymptotic expansion of the post-critical branch for a Euler-Bernoulli beam, exhibiting one new non-linear fractional term. All the analytical results are validated by finite element computations.

International Journal of Solids and Structures, 2008

In this paper, a shell finite element is designed within the total Lagrangian formulation framewo... more In this paper, a shell finite element is designed within the total Lagrangian formulation framework to deal with the plastic buckling and post-buckling of thin structures, such as cylindrical shells. First, the numerical formulation is validated using available analytical results. Then it is shown to be able to provide the bifurcation modes-possibly the secondary ones-and describe the complex advanced post-critical state of a cylinder under axial compression, where the theory is no longer operative.

Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2008

Motion during diffusion encodings leads to phase errors in different shots of a multishot acquisi... more Motion during diffusion encodings leads to phase errors in different shots of a multishot acquisition. Phase differences in k-space data among shots result in phase cancellation artifacts in the reconstructed image. Due to aliasing of the phase from under-sampled regions of the shot, correction of the phase error using direct low-resolution phase subtraction is incomplete. We introduce a new k-space and image-space combination (KICT) method for motion artifacts cancellation that avoids incomplete phase error correction. Further, the method preserves the phase of the object, which is important for parallel imaging applications.

Conference proceedings : ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference, 2008

Diffusion Tensor Imaging (DTI) of localized anatomical regions (i.e brainstem, cervical spinal co... more Diffusion Tensor Imaging (DTI) of localized anatomical regions (i.e brainstem, cervical spinal cord, and optic nerve) is challenging because of the existence of significant susceptibility differences in the surrounding tissues, their high motion sensitivity and the need for high spatial resolution to resolve the underlying complex histoarchitecture. The aim of the present methodology is to achieve high resolution DTI with motion compensating capability in localized regions of the central nervous system. We accomplish this by implementing self-navigated multi-shot variable density spiral encoding with outer volume suppression. In vivo application of the technique on the human brainstem demonstrates a clear delineation of the multiple local neural tracts. We also investigate the partial volume effect on the extracted diffusion anisotropy metrics by varying the in-plane resolution while maintaining a constant signal-to-noise ratio.

Thin-Walled Structures, 2015

An inflatable beam is an airtight structure made of a soft technical fabric and subjected to an i... more An inflatable beam is an airtight structure made of a soft technical fabric and subjected to an internal pressure which gives it a final cylindrical shape, a pre-stress in the membrane and a bearing capacity. Against all appearances, it is not a standard beam and it requires a specific formulation in order to take account of the internal pressure which plays a key role in its mechanical response. This work deals with inflatable beams made of orthotropic materials. The first part of the paper is concerned with the inflation of the membrane tube, an important stage which is often neglected so far in the literature. As preliminaries of the bending problem studied in the next part of the paper, the constitutive law related to the inflated state of the tube-not the natural state-is investigated. It will be shown that the constitutive law related to the inflated pre-stressed state is not the same as the constitutive law related to the natural state. Expressions of the material coefficients involved in the former constitutive law will be established from the material coefficients defined on the natural reference configuration which are the only ones supposed to be known. The second part of the paper deals with the bending of the inflatable beam. The Timoshenko beam kinematics will be chosen because of the significant shear effect in the tube wall and the problem will be formulated in finite deformations in order to accounts for all the nonlinear effects, in particular the action due to the internal pressure which is a follower load. The nonlinear system of equations obtained will then be linearized around the pre-stressed configuration and will result in a more tractable linear system.

Thin-Walled Structures, 2007

The discretized nonlinear equations for bending and buckling of inflatable beams are written by u... more The discretized nonlinear equations for bending and buckling of inflatable beams are written by use of the virtual work principle with Timoshenko's kinematics, finite rotations and small strains. The linearized equations around a pre-stressed reference configuration are then deduced, giving rise to a new inflatable beam finite element. The stiffness matrix contains the shear coefficient and the internal pressure. Use is made of the particular 3-node beam element to investigate the bending and the buckling of a cantilever beam, the deflection of a pinched torus and the buckling of a torus submitted to a radial compressive force. The numerical results obtained with the beam element are shown to be close to analytical and three-dimensional (3D) membrane finite element results. The validity of the numerical results is discussed, in connection with the concepts of the crushing force or the wrinkling pressure of the inflated beam.

... cases do not indicate time and tense precisely, which causes considerable problems when the l... more ... cases do not indicate time and tense precisely, which causes considerable problems when the law is applied. According to Anh Luu (2006), despite significant improvements, Vietnamese legislation work is still weak resulting in the legal system being inadequate and unstable. ...

Thin-Walled Structures, 2005

The non linear and linearized equations are derived for the in plane stretching and bending of th... more The non linear and linearized equations are derived for the in plane stretching and bending of thin walled cylindrical beams made of a membrane and inflated by an internal pressure. The Timoshenko beam model combined with the finite rotation kinematics enables one to correctly account for the shear effect and all the non linear terms in the governing equations. The linearization is carried out around a pre stressed reference configuration which has to be defined as opposed to the so called natural state. Two examples are then investigated: the bending and the buckling of a cantilever beam. Their analytical solutions show that the inflation has the effect of increasing the material properties in the beam solution. This solution is compared with the three dimensional finite element analysis, as well as the so called wrinkling pressure for the bent beam and the crushing force for the buckled beam. New theoretical and numerical results on the buckling of inflatable beams are displayed.

Thin-Walled Structures, 2013

Inflatable tubes must be pressurized before they are able to resist to external loadings. While t... more Inflatable tubes must be pressurized before they are able to resist to external loadings. While there are many studies dedicated to the behavior of inflated tubes under bending or twisting, few results are available on the inflating stage when the beam is only submitted to the internal pressure. In this paper, an exact solution is proposed for a cylindrical tube made of an orthotropic membrane, with the orthotropy axes parallel to the circumferential and longitudinal directions of the cylinder, and undergoing finite deformations under internal pressure. The solution of the problem amounts to solving a cubic equation and analytical relations are given for the inflated geometry as well as the resultant stresses in the membrane. Numerical results are shown to compare very well with those obtained from a finite element code.

European Journal of Computational Mechanics, 2006

Magnetic Resonance in Medicine, 2013

The main obstacle to high-resolution (<1.5 mm isotropic) 3D diffusion-weighted MRI is the differe... more The main obstacle to high-resolution (<1.5 mm isotropic) 3D diffusion-weighted MRI is the differential motion-induced phase error from shot-to-shot. In this work, the phase error is addressed with a hybrid 3D navigator approach that corrects motioninduced phase in two ways. In the first, rigid-body motion is corrected for every shot. In the second, repeatable nonrigid-body pulsation is corrected for each portion of the cardiac cycle. These phase error corrections were implemented with a 3D diffusionweighted steady-state free precession pulse sequence and were shown to mitigate signal dropouts caused by shot-to-shot phase inconsistencies compared to a standard gridding reconstruction in healthy volunteers. The proposed approach resulted in diffusion contrast more similar to the contrast observed in the reference echo-planer imaging scans than reconstruction of the same data without correction. Fractional anisotropy and Color fractional anisotropy maps generated with phase-corrected data were also shown to be more similar to echo-planer imaging reference scans than those generated without phase correction.

IEEE Transactions on Medical Imaging, 2009

Motion during diffusion encodings leads to different phase errors in different shots of multishot... more Motion during diffusion encodings leads to different phase errors in different shots of multishot diffusion-weighted acquisitions. Phase error incoherence among shots results in undesired signal cancellation when data from all shots are combined. Motion-induced phase error correction for multishot diffusion-weighted imaging (DWI) has been studied extensively and there exist multiple phase error correction algorithms. A commonly used correction method is the direct phase subtraction (DPS). DPS, however, can suffer from incomplete phase error correction due to the aliasing of the phase errors in the high spatial resolution phases. Furthermore, improper sampling density compensation is also a possible issue of DPS. Recently, motion-induced phase error correction was incorporated in the conjugate gradient (CG) image reconstruction procedure to get a nonlinear phase correction method that is also applicable to parallel DWI. Although the CG method overcomes the issues of DPS, its computational requirement is high. Further, CG restricts to sensitivity encoding (SENSE) for parallel reconstruction. In this paper, a new time-efficient and flexible k-space and image-space combination (KICT) algorithm for rigid body motion-induced phase error correction is introduced. KICT estimates the motion-induced phase errors in image space using the self-navigated capability of the variable density spiral trajectory. The correction is then performed in k -space. The algorithm is shown to overcome the problem of aliased phase errors. Further, the algorithm preserves the phase of the imaging object and receiver coils in the corrected k -space data, which is important for parallel imaging applications. After phase error correction, any parallel reconstruction method can be used. The KICT algorithm is tested with both simulated and in vivo data with both multishot single-coil and multishot multicoil acquisitions. We show that KICT correction results in diffusion-weighted images with higher signal-to-noise ratio (SNR) and fractional anisotropy (FA) maps with better resolved fiber tracts as compared to DPS. In peripheral-gated acquisitions, KICT is comparable to the CG method.

Computers & Structures, 2009

In this work, a weighted residual relationship is proposed as an extension of the standard virtua... more In this work, a weighted residual relationship is proposed as an extension of the standard virtual work principle to deal with the large deformation contact problem with Coulomb friction. This weak form is a mixed relationship involving the displacements and the multipliers defined on the reference contact surface of the contactor and is shown to be equivalent to the strong form of the initial/boundary value contact problem. The discretization in space by means of the finite element method is carried out on the mixed relationship in a simple way in order to obtain the semi-discrete equation system. The contact tangent stiffness is derived and numerical examples are presented to assess the efficiency of the formulation.

Computers & Structures, 2004

Computers & Structures, 2003

This paper describes a general method for deriving the plastic corrections and the consistent tan... more This paper describes a general method for deriving the plastic corrections and the consistent tangent modulus for a wide range of arbitrary non-linear hardening models within the framework of standard small strains elastoplasticity. The features of the proposed formulation are: (i) the local solution is obtained through an iterative procedure. The plastic corrections are given in closed forms exhibiting one scalar function denoted by G alg and three fourth-order tensors D alg , G alg , L alg , which are shown to be the algorithmic discrete counterparts of usual theoretical continuum quantities, (ii) the consistent tangent modulus has a symmetrical expression involving the same quantities. Finite element computations are performed using a particular non-linear kinematic hardening model and allow to exhibit the ratcheting phenomenon usually observed on mechanical components subjected to cyclic loadings.

IEEE Transactions on Medical Imaging, 2011

A multishot data acquisition strategy is one way to mitigate B0 distortion and T2 blurring for hi... more A multishot data acquisition strategy is one way to mitigate B0 distortion and T2 blurring for high-resolution diffusion-weighted magnetic resonance imaging experiments. However, different object motions that take place during different shots cause phase inconsistencies in the data, leading to significant image artifacts. This work proposes a maximum likelihood estimation and-space correction of motion-induced phase errors in 3D multishot diffusion tensor imaging. The proposed error estimation is robust, unbiased, and approaches the Cramer-Rao lower bound. For rigid body motion, the proposed correction effectively removes motion-induced phase errors regardless of the-space trajectory used and gives comparable performance to the more computationally expensive 3D iterative nonlinear phase error correction method. The method has been extended to handle multichannel data collected using phased-array coils. Simulation and in vivo data are shown to demonstrate the performance of the method.