Browsing by Author "Guesalaga, A"

Now showing 1 - 7 of 7
  • Thumbnail Image
    Item
    A simple technique for the autocalibration of stereo systems using the fundamental matrix
    (CAMBRIDGE UNIV PRESS, 2004) Acevedo, C; Guesalaga, A
    This paper presents a method of autocalibrating a stereo system using the full parameterization of the fundamental matrix. The technique is an improvement of classical autocalibration methods that use corresponding points and it is aimed to reduce the number of processes required to achieve a satisfactory calibration. An analysis of the matching points used in the calibration process is carried Out in order to select them based upon their quality or reliability. Using, this technique. a significant reduction of the convergence errors in the calibration process is obtained.
  • Thumbnail Image
    Item
    Computer reconstruction of pine growth rings using MRI
    (ELSEVIER SCIENCE INC, 2004) Morales, S; Guesalaga, A; Fernandez, MP; Guarini, M; Irarrazaval, P
    This work explores the use of magnetic resonance imaging (MRI) for nondestructive determination of wood characteristics and for 3D wood modeling. In this context, one of the applications under development is the automatic recognition and reconstruction of rings from transversal images obtained from MRI scanners. The algorithm analyzes a set of transversal MRI images, detecting and reconstructing growth ring edges. The information generated is then interpolated in order to obtain an accurate 3D picture of the log and its fundamental constituents (individual rings, knots, defects, etc). Results also show that the technique has potential for defect recognition, providing a powerful tool for future developments in wood analysis. The results are encouraging and further research is needed to develop automatic detection not only of rings, but also of different types of defects that are of paramount importance in the sawmill and plywood industries. (C) 2004 Elsevier Inc. All rights reserved.
  • Thumbnail Image
    Item
    Fast three-dimensional k-space trajectory design using missile guidance ideas
    (WILEY, 2004) Mir, R; Guesalaga, A; Spiniak, J; Guarini, M; Irarrazaval, P
    Three-dimensional (3D) k-space trajectories are needed to acquire volumetric images in MRI. While scan time is determined by the trajectory efficiency, image quality and distortions depend on the shape of the trajectories. There are several 3D trajectory strategies for sampling the k-space using rectilinear or curve schemes. Since there is no evidence about their optimality in terms of image quality and acquisition time, a new design method based on missile guidance ideas is explored. Since air-to-air missile guidance shares similar goals and constraints with the problem of k-space trajectory design, a control approach for missiles is used to design a 3D trajectory. The k-space is divided into small cubes, and each one is treated as a target to be sampled. The main goal is to cover the entire space as quickly and efficiently as possible, with good performance under different conditions. This novel design method is compared to other trajectories using simulated and real data. As an example, a trajectory that requires 0.11 times the number of shots needed by the cylindrical 3DFT acquisition was designed. This trajectory requires more shots (1.66 times) than the stack of spirals, but behaves better under nonideal conditions, such as off-resonance and motion. (C) 2004 Wiley-Liss, Inc.
  • Thumbnail Image
    Item
    Measurement of the glaucomatous cup using sequentially acquired stereoscopic images
    (ELSEVIER SCI LTD, 2003) Guesalaga, A; Irarrazabal, P; Guarini, M; Alvarez, R
    This paper describes a new method for recovering the three-dimensional surface of the optic disc based on stereoscopic techniques. The images that form the stereo pair are obtained sequentially from standard ophthalmoscopes. The method is based on calibration algorithms for an optical set-up and uses a novel algorithm for the search of correspondences in the stereo pair that is unique for this kind of applications. It is proposed as a quantitative tool in the diagnosis of the primary open angle glaucoma. The technique shows a robust performance, is simple to embed in an clinical ophthalmoscope, and is a low-cost solution to diagnose and control the glaucoma pathology. (C) 2003 Elsevier Ltd. All rights reserved.
  • Thumbnail Image
    Item
    Modelling end-of-roll dynamics in positioning servos
    (PERGAMON-ELSEVIER SCIENCE LTD, 2004) Guesalaga, A
    End-of-roll dynamics is a characteristic of pointing systems and positioning servos when change in movement direction occurs. Factors such as backlash and friction cannot be easily controlled nor predicted under these circumstances, and although an accurate model for friction exists, the literature is still lacking of a representation for the integrated effect backlash and friction dynamics in these loops. This paper proposes a new mathematical model to represent backlash effects in positioning servos demonstrating its accuracy to predict tracking errors of a naval gun mount. The model introduces a continuous hysteresis representation, different from classical discontinuous models, showing a behaviour closer to what has been observed empirically. The new model can be useful for high performance positioning servo-control applications. (C) 2003 Elsevier Science Ltd. All rights reserved.
  • Thumbnail Image
    Item
    Three dimensional k-space trajectory design using genetic algorithms
    (ELSEVIER SCIENCE INC, 2003) Sabat, S; Mir, R; Guarini, M; Guesalaga, A; Irarrazaval, P
    Image quality and total scan time in MRI are determined in large part by the trajectory employed to sample the Fourier space. Each trajectory has different properties like coverage of k-space, scan time, sensitivity to off-resonance conditions, etc. These properties are often contradictory, therefore a universal optimal trajectory does not exist and ultimately, it will depend on the image characteristics sought. Most trajectories used today are designed based on intuition and k-space analysis more than with optimization methods. This work presents a 3D k-space trajectory design method based on Genetic Algorithm optimization. Genetic Algorithms have been chosen because they are particularly good for searching large solution spaces. They emulate the natural evolutionary process allowing better offsprings to survive. The objective function searches the maximum of the trajectory's k-space coverage subject to hardware constraints for a fixed scanning time using the trajectory's torsion as its optimization variable.
  • Thumbnail Image
    Item
    Undersampling k-space using fast progressive 3D trajectories
    (WILEY, 2005) Spiniak, J; Guesalaga, A; Mir, R; Guarini, M; Irarrazaval, P
    In 3D MRI, sampling k-space with traditional trajectories can be excessively time-consuming. Fast imaging trajectories are used in an attempt to efficiently cover the k-space and reduce the scan time without significantly affecting the image quality. In many applications, further reductions in scan time can be achieved via undersampling of the k-space; however, no clearly optimal method exists. In most 3D trajectories the k-space is divided into regions that are sampled with shots that share a common geometry (e.g., spirals). A different approach is to design trajectories that gradually but uniformly cover the k-space. In the current work, successive shots progressively add sampled regions to the 3D frequency space. By cutting the sequence short, a natural undersampled method is obtained. This can be particularly efficient because in these types of trajectories the contribution of new information by later shots is less significant. In this work the performance of progressive trajectories for different degrees of undersampling is assessed with trajectories based on missile guidance (MG) ideas. The results show that the approach can be efficient in terms of reducing the scan time, and performs better than the stack of spirals (SOS) technique, particularly under nonideal conditions.