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Search Results: 1 - 10 of 14069 matches for " Jan Sijbers "
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Force Feedback to Assist Active Contour Modelling for Tracheal Stenosis Segmentation
Lode Vanacken,R?mulo Pinho,Jan Sijbers,Karin Coninx
Advances in Human-Computer Interaction , 2012, DOI: 10.1155/2012/632498
Abstract: Manual segmentation of structures for diagnosis and treatment of various diseases is a very time-consuming procedure. Therefore, some level of automation during the segmentation is desired, as it often significantly reduces the segmentation time. A typical solution is to allow manual interaction to steer the segmentation process, which is known as semiautomatic segmentation. In 2D, such interaction is usually achieved with click-and-drag operations, but in 3D a more sophisticated interface is called for. In this paper, we propose a semi-automatic Active Contour Modelling for the delineation of medical structures in 3D, tomographic images. Interaction is implemented with the employment of a 3D haptic device, which is used to steer the contour deformation towards the correct boundaries. In this way, valuable haptic feedback is provided about the 3D surface and its deformation. Experiments on simulated and real tracheal CT data showed that the proposed technique is an intuitive and effective segmentation mechanism. 1. Introduction Image segmentation in the medical field is an important step for the diagnosis and treatment of various diseases. In many cases, this task is performed manually [1, 2]. However, manual segmentation is widely acknowledged as being time consuming and intra- and interoperator dependent. Hence, some level of automation during the segmentation is desired, as it often significantly reduces the segmentation time. Medical image segmentation, in particular, is a very complex task, given the necessary precision required for object extraction and boundary delineation. A typical solution is to allow users to provide extra knowledge to or interfere with the segmentation process in order to refine the results yielded by the automatic steps, which is known as semiautomatic (or interactive) segmentation. The Active Contour Model (ACM) [3] is a well-known shape deformation algorithm to delineate structures in images, and several semiautomatic versions of this algorithm have been proposed in the literature [4]. ACMs minimise an energy function that controls the bending and stretching of a given initial contour and the attraction by image features. The expected result is that the contour matches the boundary of the structure of interest in the image. In 2D, the interface between user and algorithm is usually established with click/drag processes. However, if the data being segmented is three-dimensional, such as in 3D Computed Tomography (CT) images, a more refined interface is called for. The present work sets forth a 3D segmentation interface for
Isotropic non-white matter partial volume effects in constrained spherical deconvolution
Timo Roine,Ben Jeurissen,Daniele Perrone,Alexander Leemans,Jan Sijbers
Frontiers in Neuroinformatics , 2014, DOI: 10.3389/fninf.2014.00028
Abstract: Diffusion-weighted (DW) magnetic resonance imaging (MRI) is a non-invasive imaging method, which can be used to investigate neural tracts in the white matter (WM) of the brain. Significant partial volume effects (PVEs) are present in the DW signal due to relatively large voxel sizes. These PVEs can be caused by both non-WM tissue, such as gray matter (GM) and cerebrospinal fluid (CSF), and by multiple non-parallel WM fiber populations. High angular resolution diffusion imaging (HARDI) methods have been developed to correctly characterize complex WM fiber configurations, but to date, many of the HARDI methods do not account for non-WM PVEs. In this work, we investigated the isotropic PVEs caused by non-WM tissue in WM voxels on fiber orientations extracted with constrained spherical deconvolution (CSD). Experiments were performed on simulated and real DW-MRI data. In particular, simulations were performed to demonstrate the effects of varying the diffusion weightings, signal-to-noise ratios (SNRs), fiber configurations, and tissue fractions. Our results show that the presence of non-WM tissue signal causes a decrease in the precision of the detected fiber orientations and an increase in the detection of false peaks in CSD. We estimated 35–50% of WM voxels to be affected by non-WM PVEs. For HARDI sequences, which typically have a relatively high degree of diffusion weighting, these adverse effects are most pronounced in voxels with GM PVEs. The non-WM PVEs become severe with 50% GM volume for maximum spherical harmonics orders of 8 and below, and already with 25% GM volume for higher orders. In addition, a low diffusion weighting or SNR increases the effects. The non-WM PVEs may cause problems in connectomics, where reliable fiber tracking at the WM–GM interface is especially important. We suggest acquiring data with high diffusion-weighting 2500–3000 s/mm2, reasonable SNR (~30) and using lower SH orders in GM contaminated regions to minimize the non-WM PVEs in CSD.
A Generalized Bidiagonal-Tikhonov Method Applied To Differential Phase Contrast Tomography
Nick Schenkels,Jan Sijbers,Wim van Aarle,Wim Vanroose
Mathematics , 2015,
Abstract: Phase contrast tomography is an alternative to classic absorption contrast tomography that leads to higher contrast reconstructions in many applications. We review how phase contrast data can be acquired by using a combination of phase and absorption gratings. Using algebraic reconstruction techniques the object can be reconstructed from the measured data. In order to solve the resulting linear system we propose the Generalized Bidiagonal Tikhonov (GBiT) method, an adaptation of the generalized Arnoldi-Tikhonov method that uses the bidiagonal decomposition of the matrix instead of the Arnoldi decomposition. We also study the effect of the finite difference operator in the model by examining the reconstructions with either a forward difference or a central difference approximation. We validate our conclusions with simulated and experimental data.
On parameter estimation in the physics lab based on inverting a slope regression coefficient
W. Jacquet,E. Ir. Nyssen,J. Sijbers
Physics , 2012,
Abstract: Measurement uncertainty is a non trivial aspect of the laboratory component of most undergraduate physics courses. Confusion about the application of statistical tools calls for the elaboration of guidelines and the elimination of inconsistencies were possible. Linear regression is one of the fundamental statistical tool often used in a first year physics laboratory setting. In what follows we present an argument that leads to an unambiguous choice of (a) variable(s) to be used as predictor(s) and variable to be predicted.
Imprecise k-space sampling and central brightening
R. A. Hanel,S. De Backer,J. Sijbers,P. Scheunders
Physics , 2006,
Abstract: In real-world sampling of k-space data, one generally makes a stochastic error not only in the value of the sample but in the effective position of the drawn sample. We refer to the latter as imprecise sampling and apply this concept to the fourier-based acquisition of magnetic resonance data. The analysis shows that the effect of such imprecisely sampled data accounts for contributions to noise, blurring, and intensity-bias in the image. Under general circumstances, the blur and the bias may depend on the scanned specimen itself. We show that for gaussian distributed imprecision of k-vector samples the resulting intensity inhomogeneity can be explicitly computed. The presented mechanism of imprecise k-space sampling (IKS) provides a complementary explanation for the phenomenon of central brightening in high-field magnetic resonance imaging. In computed experiments, we demonstrate the adequacy of the IKS effect for explaining central brightening. Furthermore, the experiments show that basic properties of IKS can in principle be inferred from real MRI data by the analysis on the basis of bias fields in magnitude images and information contained in the phase-images.
A multi-level preconditioned Krylov method for the efficient solution of algebraic tomographic reconstruction problems
Siegfried Cools,Pieter Ghysels,Wim van Aarle,J. Sijbers,Wim Vanroose
Mathematics , 2013,
Abstract: Classical iterative methods for tomographic reconstruction include the class of Algebraic Reconstruction Techniques (ART). Convergence of these stationary linear iterative methods is however notably slow. In this paper we propose the use of Krylov solvers for tomographic linear inversion problems. These advanced iterative methods feature fast convergence at the expense of a higher computational cost per iteration, causing them to be generally uncompetitive without the inclusion of a suitable preconditioner. Combining elements from standard multigrid (MG) solvers and the theory of wavelets, a novel wavelet-based multi-level (WMG) preconditioner is introduced, which is shown to significantly speed-up Krylov convergence. The performance of the WMG-preconditioned Krylov method is analyzed through a spectral analysis, and the approach is compared to existing methods like the classical Simultaneous Iterative Reconstruction Technique (SIRT) and unpreconditioned Krylov methods on a 2D tomographic benchmark problem. Numerical experiments are promising, showing the method to be competitive with the classical Algebraic Reconstruction Techniques in terms of convergence speed and overall performance (CPU time) as well as precision of the reconstruction.
A Longitudinal Analysis of the Stability of Household Money Demand  [PDF]
Jan Tin
Modern Economy (ME) , 2011, DOI: 10.4236/me.2011.23046
Abstract: Past aggregate time-series studies, conducted under the assumption of a representative economic agent, frequently show that the demand for narrowly defined M1, especially non-interest-yielding demand deposit, is unstable during periods of financial innovations. Whether this is longitudinally the case among life-cycle savers is unclear. This study utilizes longitudinal data to take another look and find that volatility in the demand for non-interest-earning checking accounts in the mid and late 1990s is attributable solely to the portion held for the transactions motive. When the conventional Baumol-Tobin model is extended to include human capital and family formation variables representing the life-cycle motive, equilibrium money demand is a stable function of both economic and demographic variables.
A Spectral Method in Time for Initial-Value Problems  [PDF]
Jan Scheffel
American Journal of Computational Mathematics (AJCM) , 2012, DOI: 10.4236/ajcm.2012.23023
Abstract: A time-spectral method for solution of initial value partial differential equations is outlined. Multivariate Chebyshev series are used to represent all temporal, spatial and physical parameter domains in this generalized weighted residual method (GWRM). The approximate solutions obtained are thus analytical, finite order multivariate polynomials. The method avoids time step limitations. To determine the spectral coefficients, a system of algebraic equations is solved iteratively. A root solver, with excellent global convergence properties, has been developed. Accuracy and efficiency are controlled by the number of included Chebyshev modes and by use of temporal and spatial subdomains. As examples of advanced application, stability problems within ideal and resistive magnetohydrodynamics (MHD) are solved. To introduce the method, solutions to a stiff ordinary differential equation are demonstrated and discussed. Subsequently, the GWRM is applied to the Burger and forced wave equations. Comparisons with the explicit Lax-Wendroff and implicit Crank-Nicolson finite difference methods show that the method is accurate and efficient. Thus the method shows potential for advanced initial value problems in fluid mechanics and MHD.
Operationalizing Sustainability Principles in the Engineering Profession  [PDF]
Jan Adamowski
Natural Resources (NR) , 2012, DOI: 10.4236/nr.2012.34024
Abstract: The engineering profession has responded to the issue of sustainable development in two main ways. It has responded through public policy statements that acknowledge the magnitude of the problem in addition to pledging to steer engineering towards a more sustainable future, and it has also responded more directly through technological innovation. In this paper, these two responses will be explored with respect to the debate on how to operationalize sustainability principles in practical terms. This paper also attempts to provide the rationale for a philosophy of engineering ethics grounded in the notion of sustainable development. It is hoped that this would lead to a revised “social contract” that would enable engineers to engage more actively in political, technical, economic and social discussions and processes.
Chaos in Planar, Circular, Restricted Three-Body Problem  [PDF]
Jan Vrbik
Applied Mathematics (AM) , 2013, DOI: 10.4236/am.2013.41008

In this article we analyze the motion of a test particle of a planar, circular, restricted three-body problem in resonance, using the Kustaanheimo-Stiefel formalism. We show that a good qualitative description of the motion can be reduced to three simple equations for semi-major axis, eccentricity and resonance angle. Studying these equations reveals the onset of chaos, and sheds a new light on its weak nature. The 7:4 resonance is used as an example.

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