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Sökning: WFRF:(Åström Karl)

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1.
  • Eriksson, Anders P, et al. (författare)
  • On the bijectivity of thin-plate splines
  • 2012
  • Ingår i: Analysis for Science, Engineering and Beyond, The Tribute Workshop in Honour of Gunnar Sparr held in Lund, May 8-9, 2008. - Springer. - 978-3-642-20236-0 (online). - 978-3-642-20235-3 (print) ; 6, s. 93-141
  • Bokkapitel (övrigt vetenskapligt)abstract
    • The thin-plate spline (TPS) has been widely used in a number of areas such as image warping, shape analysis and scattered data interpolation. Introduced by Bookstein (IEEE Trans. Pattern Anal. Mach. Intell. 11(6):567–585 1989), it is a natural interpolating function in two dimensions, parameterized by a finite number of landmarks. However, even though the thin-plate spline has a very intuitive interpretation as well as an elegant mathematical formulation, it has no inherent restriction to prevent folding, i.e. a non-bijective interpolating function. In this chapter we discuss some of the properties of the set of parameterizations that form bijective thin-plate splines, such as convexity and boundness. Methods for finding sufficient as well as necessary conditions for bijectivity are also presented. The methods are used in two settings (a) to register two images using thin-plate spline deformations, while ensuring bijectivity and (b) group-wise registration of a set of images, while enforcing bijectivity constraints.
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2.
  • Karlsson, Johan, et al. (författare)
  • Shape Modeling by Optimising Description Length Using Gradients and Parameterisation Invariance
  • 2012
  • Ingår i: Analysis for Science, Engineering and Beyond, The Tribute Workshop in Honour of Gunnar Sparr held in Lund, May 8-9, 2008. - Springer. - 978-3-642-20236-0 (online) - 978-3-642-20235-3 (print) ; 6, s. 51-91
  • Bokkapitel (övrigt vetenskapligt)abstract
    • In Statistical Shape Modeling, a dense correspondence between the shapes in the training set must be established. Traditionally this has been done by hand, a process that commonly requires a lot of work and is difficult, especially in 3D. In recent years there has been a lot of work on automatic construction of Shape Models. In recent papers (Davies et al., Medical Image Computing and Computer-Assisted Intervention MICCAI’2001, pp. 57–65, 2001; Davies et al., IEEE Trans. Med. Imaging. 21(5):525–537 2002; Kotcheff and Taylor, Med. Image Anal. 2:303–314 1998) Minimum Description Length, (MDL), is used to locate a dense correspondence between shapes. In this paper the gradient of the description length is derived. Using the gradient, MDL is optimised using steepest descent. The optimisation is therefore faster and experiments show that the resulting models are better. To characterise shape properties that are invariant to similarity transformations, it is first necessary to normalise with respect to the similarity transformations. This is normally done using Procrustes analysis. In this paper we propose to align shapes using the MDL criterion. The MDL based algorithm is compared to Procrustes on a number of data sets. It is concluded that there is improvement in generalisation when using MDL to align the shapes. In this paper novel theory to prevent the commonly occurring problem of clustering under correspondence optimisation is also presented. The problem is solved by calculating the covariance matrix of the shapes using a scalar product that is invariant to mutual reparameterisations. An algorithm for implementing the ideas is proposed and compared to Thodberg’s state of the art algorithm for automatic shape modeling. The suggested algorithm is more stable and the resulting models are of higher quality according to the generalisation measure and according to visual inspection of the specificity.
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3.
  • Astrom, K. J., et al. (författare)
  • Design of decoupled PI controller for two-by-two systems
  • 2002
  • Ingår i: IEE Proceedings - Control Theory & Applications. - IET. - 1350-2379. ; 149:1, s. 74-81
  • Tidskriftsartikel (refereegranskat)abstract
    • The design of PI controllers for systems with interacting loops is discussed. It is advantageous to deal with the interaction at the loop level, because supervisory control seldom has sufficient bandwidth. A new scheme based on modified scalar PI design and static decoupling is developed, in which the frequency characteristic of the coupling between the lower-level loops is taken into account. This leads to a design method emphasising the trade-off between the individual loop performances and the interaction indices introduced in the paper. The controller is easily implemented, due to its simple configuration based on standard components. A useful observation is that the interaction can be reduced substantially by using set-point weighting. The method is applied to three examples, including a model of a new laboratory system called the quadruple-tank process
4.
  • Färnström, Fredrik, et al. (författare)
  • Computer Vision for determination of Fridge Content
  • 2002
  • Ingår i: Proceedings SSAB'02, Symposium on Image Analysis. - Swedish Society for Automated Image Analysis. ; s. 45-48
  • Konferensbidrag (övrigt vetenskapligt)
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5.
  • Johansson, Karl Henrik, et al. (författare)
  • Limit cycles with chattering in relay feedback systems
  • 2002
  • Ingår i: IEEE Transactions on Automatic Control. - IEEE. - 0018-9286. ; 47:9, s. 1414-1423
  • Tidskriftsartikel (refereegranskat)abstract
    • Relay feedback has a large variety of applications in control engineering. Several interesting phenomena occur in simple relay systems. In the paper, scalar linear systems with relay feedback are analyzed. It is shown that a limit cycle where part of the limit cycle consists of fast relay switchings can occur. This chattering is analyzed in detail and conditions for approximating it by a sliding mode are derived. A result on existence of limit cycles with chattering is given, and it is shown that the limit cycles can have arbitrarily many relay switchings each period. Limit cycles with regular sliding modes are also discussed. Examples illustrate the results.
6.
  • Landgren, Matilda, et al. (författare)
  • An Automated System for the Detection and Diagnosis of Kidney Lesions in Children from Scintigraphy Images
  • 2011
  • Ingår i: Lecture Notes in Computer Science. - Springer Berlin / Heidelberg. - 1611-3349 (online) .- 0302-9743 (print). - 978-3-642-21226-0 (print) - 978-3-642-21227-7 (online) ; 6688, s. 489-500
  • Konferensbidrag (refereegranskat)abstract
    • Designing a system for computer aided diagnosis is a complex procedure requiring an understanding of the biology of the disease, insight into hospital workflow and awareness of available technical solutions. This paper aims to show that a valuable system can be designed for diagnosing kidney lesions in children and adolescents from 99m Tc-DMSA scintigraphy images. We present the chain of analysis and provide a discussion of its performance. On a per-lesion basis, the classification reached an ROC-curve area of 0.96 (sensitivity/specificity e.g. 97%/85%) measured using an independent test group consisting of 56 patients with 730 candidate lesions. We conclude that the presented system for diagnostic support has the potential of increasing the quality of care regarding this type of examination.
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9.
  • Ståhl, Daniel, et al. (författare)
  • Automatic Compartment Modelling and Segmentation for Dynamical Renal Scintigraphies
  • 2011
  • Ingår i: Lecture Notes in Computer Science. - Springer Berlin / Heidelberg. - 0302-9743 (print) .- 1611-3349 (online). - 978-3-642-21227-7 (online) - 978-3-642-21226-0 (print) ; 6688, s. 557-568
  • Konferensbidrag (refereegranskat)abstract
    • Time-resolved medical data has important applications in a large variety of medical applications. In this paper we study automatic analysis of dynamical renal scintigraphies. The traditional analysis pipeline for dynamical renal scintigraphies is to use manual or semiautomatic methods for segmentation of pixels into physical compartments, extract their corresponding time-activity curves and then compute the parameters that are relevant for medical assessment. In this paper we present a fully automatic system that incorporates spatial smoothing constraints, compartment modelling and positivity constraints to produce an interpretation of the full time-resolved data. The method has been tested on renal dynamical scintigraphies with promising results. It is shown that the method indeed produces more compact representations, while keeping the residual of fit low. The parameters of the time activity curve, such as peak-time and time for half activity from peak, are compared between the previous semiautomatic method and the method presented in this paper. It is also shown how to obtain new and clinically relevant features using our novel system.
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10.
  • Aanaes, H, et al. (författare)
  • Robust factorization
  • 2002
  • Ingår i: IEEE Transactions on Pattern Analysis and Machine Intelligence. - IEEE Computer Soc. - 0162-8828. ; 24:9, s. 1215-1225
  • Tidskriftsartikel (refereegranskat)abstract
    • Factorization algorithms for recovering structure and motion from an image stream have many advantages, but they usually require a set of well-tracked features. Such a set is in generally not available in practical applications. There is thus a need for making factorization algorithms deal effectively with errors in the tracked features. We propose a new and computationally efficient algorithm for applying an arbitrary errorfunction in the factorization scheme. This algorithm enables the use of robust statistical techniques and arbitrary noise models for the individual features. These techniques and models enable the factorization scheme to deal effectively with mismatched features, missing features, and noise on the individual features. The proposed approach further includes a new method for Euclidean reconstruction that significantly improves convergence of the factorization algorithms. The proposed algorithm has been implemented as a modification of the Christy-Horaud factorization scheme, which yields a perspective reconstruction. Based on this implementation, a considerable increase in error tolerance is demonstrated on real and synthetic data. The proposed scheme can, however, be applied to most other factorization algorithms.
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