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| 3. |
- Bergvall, Erik, et al.
(författare)
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A fast and highly automated approach to myocardial motion analysis using phase contrast magnetic resonance imaging.
- 2006
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Ingår i: Journal of Magnetic Resonance Imaging. - : Wiley. - 1522-2586 .- 1053-1807. ; 23:5, s. 652-661
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: To develop a fast and highly automated method for calculating two-dimensional myocardial motion and deformation using velocity encoded magnetic resonance imaging. Materials and Methods: Two-dimensional phase contrast magnetic resonance imaging was used to acquire time resolved velocity maps of the myocardium. Cardiac motion was calculated by an iterative integration-regularization scheme of low computational cost. Image segmentation was performed using active appearance models. Results: Validation of motion tracking was performed in N = 47 subjects using saturation grid-tagging and closely followed "tag-lines." Image segmentation was validated vs. manual delineation. Conclusion: The speed and limited user interaction gives the method good potential for use in clinical practice.
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| 4. |
- Bergvall, Erik, et al.
(författare)
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Regularization of phase contrast magnetic resonance images using optical flow and smoothness constraints
- 2005
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Ingår i: Computers in Cardiology. - 0276-6574. ; 32, s. 33-36
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Konferensbidrag (refereegranskat)abstract
- This paper presents a post processing strategy for myocardial velocity fields obtained by phase contrast magnetic resonance imaging. Such data can be used to track cardiac motion and to calculate strain. The method combines data regularization with optical flow estimation to overcome the partial volume effect in the image acquisition. Validation is performed both in vitro and in vivo and it is shown that the method improves the accuracy of cardiac motion tracking.
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| 5. |
- Bergvall, Erik, et al.
(författare)
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Spline-based cardiac motion tracking using velocity-encoded magnetic resonance imaging.
- 2008
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Ingår i: IEEE Transactions on Medical Imaging. - 1558-254X. ; 27:8, s. 1045-1053
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Tidskriftsartikel (refereegranskat)abstract
- This paper deals with the problem of tracking cardiac motion and deformation using velocity-encoded magnetic resonance imaging. We expand upon an earlier described method and fit a spatiotemporal motion model to measured velocity data. We investigate several different spatial elements both qualitatively and quantitatively using phantom measurements and data from human subjects. In addition, we also use optical flow estimation by the Horn-Schunk method as complementary data in regions where the velocity measurements are noisy. Our results show that it is possible to obtain good motion tracking accuracy in phantoms with relatively few spatial elements, if the type of element is properly chosen. The use of optical flow can correct some measurement artifacts but may give an underestimation of the magnitude of the deformation. In human subjects the different spatial elements perform quantitatively in a similar way but qualitative differences exists, as shown by a semiquantitative visual scoring of the different methods.
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| 6. |
- Berthilsson, Rikard, et al.
(författare)
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Recursive Structure and Motion from Image Sequences using Shape and Depth Spaces
- 1997
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Ingår i: [Host publication title missing]. - 1063-6919. ; , s. 444-449
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper a novel recursive method for estimating structure and motion from image sequences is presented. The novelty lies in the fact that the output of the algorithm is independent of the chosen coordinate systems in the images as well as the ordering of the points. It relies on subspace methods and is derived from both ordinary coordinate representations and camera matrices and from a so called depth and shape analysis. Furthermore, no initial phase is needed to start up the algorithm. It starts directly with the first two images and incorporates new images as soon as new corresponding points are obtained. The performance of the algorithm is shown on simulated data. Moreover, the two different approaches, one using camera matrices and the other using the concepts of affine shape and depth, are unified into a general theory of structure and motion from image sequences.
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| 7. |
- Cwikel, Michael, et al.
(författare)
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Jaak Peetre, the man and his work
- 2002
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Ingår i: Function spaces, interpolation theory and related topics. - Berlin : Walter de Gruyter. - 3110171171 ; , s. 1-22
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Konferensbidrag (refereegranskat)
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| 8. |
- Diehl, Stefan, et al.
(författare)
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Analytical and numerical description of the settling process in the activated sludge operation
- 1990
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Ingår i: Instrumentation, control, and automation of water and wastewater treatment and transport systems : proceedings of the 5th IAWPRC Workshop held in Yokohama and Kyoto, Japan, 26 July-3 August 1990. - 0080407765 ; , s. 471-478
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Konferensbidrag (refereegranskat)abstract
- The secondary clarifier or settler is crucial for the whole activated sludge operation. Consequently, it is important to obtain a reliable analytical model as well as a useful numerical method, which can be used in the automatic control of the settling process. Discontinuities (shocks) appear physically, and an analytical description as well as a stable numerical algorithm must be able to handle these discontinuities. A model based on the Kynch theory of sedimentation is used, where the settling flux is a function only of the local concentration and is assumed to have one inflexion point. For such non-convex flux functions, the settling process is qualitatively the same, independent of further assumptions on the shapes of the settling functions. Using the theory of nonlinear conservation laws, the main results obtained are: how to calculate transient and asymptotic behavior; how to control the concentration profile of the settler, in particular the depth of the sludge blanket, for a given load; and, a numerical algorithm which automatically preserves shocks and gives the physically correct solution according to the analytical treatment. The algorithm can be applied on any flux function (with more than one inflexion point).
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| 11. |
- Hedström, Erik, et al.
(författare)
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Model based cardiac motion tracking using velocity encoded magnetic resonance imaging
- 2007
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Ingår i: Image Analysis (Lecture Notes in Computer Science). - Berlin, Heidelberg : Springer Berlin Heidelberg. - 0302-9743 .- 1611-3349. - 9783540730392 ; 4522, s. 82-91
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Konferensbidrag (refereegranskat)abstract
- This paper deals with model based regularization of velocity encoded cardiac magnetic resonance images (MRI). We extend upon an existing spatiotemporal model of cardiac kinematics by considering data certainty and regularity of the model in order to improve its performance. The method was evaluated using a computer simulated phantom and using in vivo gridtag MRI as gold standard. We show, both quantitatively and qualitatively, that our modified model performs better than the original one.
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| 13. |
- Heyden, Anders, et al.
(författare)
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Perception and Action using Multilinear Forms
- 1997
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Ingår i: Proceedings of the International Workshop on Algebraic Frames for the Perception-Action Cycle ; Lecture Notes In Computer Science; Vol. 1315. - 3540635173 ; , s. 54-65
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 19. |
- Jansson, Tomas, et al.
(författare)
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Ultrasound Doppler Vector Tomography - Measurements of Directional Blood Flow
- 1997
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Ingår i: Ultrasound in Medicine and Biology. - 0301-5629. ; 23:1, s. 47-57
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Tidskriftsartikel (refereegranskat)abstract
- An experimental system has been developed to verify the possibility of detecting flow activity using a technique called ultrasound Doppler vector tomography. A tomography algorithm is used to reconstruct blood flow vector fields using data from computer-controlled ultrasound continuous-wave Doppler scanning equipment. The result is a picture in which the brightness variations represent the reconstructed values of the curl of the velocity field (del x v). Continuous ultrasound is transmitted into a region with flow activity and the Doppler-shifted signals are received. To obtain measurement data suited for fan beam tomography, the scanning is performed in a plane from points encircling the region. Reconstructions have been achieved using measurement data from two different flow phantoms. A comparison between the experimental results and simulations shows good conformity. Copyright (C) 1997 World Federation for Ultrasound in Medicine & Biology.
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| 25. |
- Nielsen, Lars, et al.
(författare)
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Projective Area-invariants as an extension of the Cross-Ratio
- 1991
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Ingår i: Graphical Models. - : Elsevier BV. - 1524-0703. ; 54:1, s. 145-159
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Tidskriftsartikel (refereegranskat)abstract
- Projective invariants provide a framework for computer vision where the image of an object is described by its intrinsic properties, independently of the particular view. It is advantageous if these intrinsic properties are defined in terms of computationally simple features. An area-measurement provides a good candidate that is easy to reliably compute from a particular image of the object. The main contributions of this paper are the definition and justification of area-invariants in projective geometry and the indication of its relevance in image analysis. A framework that covers one-dimensional intervals and two-dimensional figures has been developed. In the linear case, the invariants are linear only in two cases. The first case is the well known cross-ratio, and the second case is called the polar case. The generalization to the plane can be done in different directions. One can use either points (on the line or in the plane) or the geometric figures (intervals, triangles, circles) as the basic entities involved. The first view was adopted already by Möbius, who generalized the cross-ratio in various directions. The second view used here leads to another generalization of the cross-ratio, where the invariants are relations between the areas of a class of geometric figures, related to each other in a certain manner. Remarkably enough, these invariants turn out to be linear if the figures involved are related in a pole/ polar configuration
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| 28. |
- Sparr, Gunnar, et al.
(författare)
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A Common Framework for Kinetic Depth, Reconstruction and Motion for Deformable Objects
- 1994
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Ingår i: Computer Vision - ECCV '94. Third European Conference on Computer Vision. Proceedings. Vol.II. - 3540579575 ; 2, s. 471-482
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, problems related to depth, reconstruction and motion from a pair of projective images are studied under weak assumptions. Only relative information within each image is used, nothing about their interrelations or about camera calibration. Objects in the scene may be deformed between the imaging instants, provided that the deformations can be described locally by affine transformations. It is shown how the problems can be treated by a common method, based on a novel interpretation of a theorem in projective geometry of M. Chasles, and the notion of “affine shape”. No epipolar geometry is used. The method also enables the computation of the “depth flow”, i.e. a relative velocity in the direction of the ray of sight
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| 29. |
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| 30. |
- Sparr, Gunnar, et al.
(författare)
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An algebraic/analytic method for reconstruction from image correspondences
- 1991
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Ingår i: Theory & applications of image analysis (Series in machine perception and artificial intelligence). - 9810209452 ; , s. 274-281
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A method for the reconstruction and motion problems is presented, under the assumption that a number of point correspondences in a pair of images are known. A geometric and algebraic theory is based on invariance properties of point configuration under affine and projective transformations. In particular, a characterization of those image pairs which correspond to the same point configuration in an unknown scene is given. An algorithm along these lines is presented, yielding complete sets of solutions to the reconstruction problem. A main idea is to exploit the affine structure of the problem, and work with relative locations, before turning to the metrical structure and absolute locations. It turns out that relative depth information can be achieved at a low computational cost
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| 33. |
- Sparr, Gunnar, et al.
(författare)
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Depth-Computations from Polyhedral Images
- 1992
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Ingår i: Computer Vision - ECCV '92. Second European Conference on Computer Vision Proceedings. - 3 540 55426 2 ; 10:10, s. 683-688
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A method is developed for the computation of depth maps, modulo scale, from one single image of a polyhedral scene. Only affine shape properties of the scene and image are used, hence no metrical information. Results from simple experiments show good performance, with exactness and robustness. It is also shown how the underlying theory may be used to single out and characterise certain singular situations that may occur in machine interpretation of line drawings
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| 34. |
- Sparr, Gunnar, et al.
(författare)
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Discontinuity preserving visual reconstruction by means of potential theory
- 1990
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Ingår i: Pattern Recognition Letters. - 0167-8655. ; 11:2, s. 117-122
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Tidskriftsartikel (refereegranskat)abstract
- Discontinuity preserving interpolation methods are often desirable in image reconstruction from sparse visual data. Two such schemes are constructed by interpreting the image as the potentialcause by some distribution of charges, and then detecting certain patterns (notably dipole curves) among the charges. Experiments have been performed on an ink-jet plotter, showing good performance.
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| 37. |
- Sparr, Gunnar
(författare)
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Euclidean and Affine Structure/Motion for Uncalibrated Cameras from Affine Shape and Subsidiary Information
- 1998
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Ingår i: 3D Structure from Multiple Images of Large-Scale Environments. European Workshop, SMILE'98. Proceedings. - 3 540 65310 4 ; , s. 187-207
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The paper deals with the structure-motion problem for uncalibrated cameras, in the case that subsidiary information is available, consisting, for example, in known coplanarities or parallelities among points in the scene, or known positions of some focal points (hand-eye calibration). Despite unknown camera calibrations, it is shown that in many instances the subsidiary information makes affine or even Euclidean reconstruction possible. A parametrization by affine shape and depth is used, providing a simple framework for the incorporation of apriori knowledge, and enabling the development of iterative, rapidly converging algorithms. Any number of points in any number of images are used in a uniform way, with equal priority, and independently of coordinate representations. Moreover, occlusions are allowed
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| 45. |
- Sparr, Gunnar
(författare)
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Simultaneous Reconstruction of Scene Structure and Camera Locations from Uncalibrated Image Sequences
- 1996
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Ingår i: Proceedings of the 13th International Conference on Pattern Recognition. - 0 8186 7282 X ; 1, s. 328-333
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The paper deals with the structure-motion problem for images of point configurations taken by uncalibrated cameras. Using a parametrisation by affine shape and kinetic depth, a complete and explicit characterisation of the imaging geometry is given, including the shape of the object configuration and the positions of the cameras relative to the scene. No epipolar geometry is used. It is shown that not only the projective but also the affine structure of the scene can be recovered when knowing the relative placement of five of the camera centres (four if they are coplanar). Variational algorithms for reconstruction and motion are presented, thus avoiding numerically unstable solving of algebraic equations. Any number of points in any number of images can be treated simultaneously and uniformly, without preselection of reference points. The performances of the algorithms are illustrated on simulations and experiments
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| 48. |
- Sparr, Gunnar
(författare)
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Ultrasound Doppler Tomography
- 1996
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Ingår i: Mathematics and physics of emerging biomedical imaging. - 0309053870
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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