1,720,962 research outputs found
The DIRECTT algorithm: A promising alternative for Computed Tomography reconstructions
No full textThe DIRECTT (Direct Iterative REconstruction of Computed Tomography Trajectories) algorithm represents a promising alternative to conventional algorithms, such as the Filtered Backprojection (FBP) or the Algebraic Reconstruction Technique (ART), by overcoming restrictions associated with them. Such restrictions include the limited spatial resolution achieved through FBP due to Nyquist‘s sampling theorem, the inability of FBP to perform a quality reconstruction when projections are missing, and the excessive computing time needed for ART
Advanced algorithms for image reconstruction in Computed Tomography
No full textThis PhD project is part of the Marie Skłodowska-Curie International Training Network MUMMERING. The overarching goal of MUMMERING is the creation of a research tool that exploits the wealth of 3D imaging modalities applied in materials engineering.
The focus of this work is advanced reconstruction algorithms. The primary objective is the development and implementation of the DIRECTT algorithm. Due to restrictions introduced by the sample or the imaging setup during a measurement, it is not always possible to acquire data that fulfil the Nyquist sampling criterion. The DIRECTT algorithm is intended to produce reconstructions of superior quality for such cases of limited data sets, compared to those of other available algorithms, such as algebraic iterative ones. Although DIRECTT has been developed primarily for the reconstruction of volumes from data acquired by an X-ray Computed Tomography scanner, it can also be applied to the different modalities, such as laminography.
The talk describes the reconstruction principle of DIRECTT. Furthermore, reconstructions of measured data are presented and compared to reconstructions produced by other established algorithms
Iterative Rekonstruktion aus unterabgetasteten Computertomographiedaten
No full textIn X-ray computed tomography (XCT), an X-ray beam of intensity I0 is transmitted through an object and its attenuated intensity I is measured when it exits the object. The attenuation of the beam depends on the attenuation coefficients along its path. The attenuation coefficients provide information about the structure and composition of the object and can be determined through mathematical operations that are referred to as reconstruction.
The standard reconstruction algorithms are based on the filtered backprojection (FBP) of the measured data. While these algorithms are fast and relatively simple, they do not always succeed in computing a precise reconstruction, especially from under-sampled data.
Alternatively, an image or volume can be reconstructed by solving a system of linear equations. Typically, the system of equations is too large to be solved but its solution can be approximated by iterative methods, such as the Simultaneous Iterative Reconstruction Technique (SIRT) and the Conjugate Gradient Least Squares (CGLS).
This dissertation focuses on the development of a novel iterative algorithm, the Direct Iterative Reconstruction of Computed Tomography Trajectories (DIRECTT). After its reconstruction principle is explained, its performance is assessed for real parallel- and cone-beam CT (including under-sampled) data and compared to that of other established algorithms. Finally, it is demonstrated how the shape of the measured object can be modelled into DIRECTT to achieve even better reconstruction results.Bei der Röntgen-Computertomographie (XCT) wird ein Röntgenstrahl der Intensität I0 durch ein Objekt gesendet und seine geschwächte Intensität I hinter dem Objekt gemessen. Die Schwächung des Strahls hängt von den Abschwächungskoeffizienten entlang seines Weges innerhald des Objekts ab. Die Schwächungskoeffizienten liefern Informationen uber die Struktur und Zusammensetzung des Objekts und können durch mathematische Operationen, die als Rekonstruktion bezeichnet werden, bestimmt werden.
Die Standard-Rekonstruktionsalgorithmen basieren auf der gefilterten Rückprojektion (FBP) der Messdaten. Diese Algorithmen sind zwar schnell und relativ einfach, doch gelingt es ihnen nicht immer, eine präzise Rekonstruktion zu berechnen, vor allem bei unzureichend abgetasteten Daten.
Alternativ kann ein Bild oder ein Volumen auch durch die Lösung eines Systems linearer Gleichungen rekonstruiert werden. In der Regel ist das Gleichungssystem zu groß, um gelöst zu werden, aber seine Lösung kann durch iterative Methoden angenähert werden, wie die Simultaneous Iterative Reconstruction Technique (SIRT) und die Conjugate Gradient Least Squares (CGLS).
Im Mittelpunkt dieser Dissertation steht die Entwicklung eines neuartigen iterativen Algorithmus, des Direct Iterative Reconstruction of Computed Tomography Trajectories (DIRECTT). Nach der Erläuterung seines Rekonstruktionsprinzips wird seine Leistung für reale Parallel- und Kegelstrahl-CT-Daten (einschließlich unterabgetasteter Daten) bewertet und mit Rekonstruktionen anderer etablierter Algorithmen verglichen. Schließlich wird gezeigt, wie die Form des gemessenen Objekts in DIRECTT modelliert werden kann, um noch bessere Rekonstruktionsergebnisse zu erzielen
Iterative reconstruction from under-sampled Computed Tomography data
No full textIn X-ray computed tomography (XCT), an X-ray beam of intensity I0 is transmitted through an object and its attenuated intensity I is measured when it exits the object. The attenuation of the beam depends on the attenuation coefficients along its path. The attenuation coefficients provide information about the structure and composition of the object and can be determined through mathematical operations that are referred to as reconstruction. The standard reconstruction algorithms are based on the filtered backprojection (FBP) of the measured data. While these algorithms are fast and relatively simple, they do not always succeed in computing a precise reconstruction, especially from under-sampled data. Alternatively, an image or volume can be reconstructed by solving a system of linear equations. Typically, the system of equations is too large to be solved but its solution can be approximated by iterative methods, such as the Simultaneous Iterative Reconstruction Technique (SIRT) and the Conjugate Gradient Least Squares (CGLS). This dissertation focuses on the development of a novel iterative algorithm, the Direct Iterative Reconstruction of Computed Tomography Trajectories (DIRECTT). After its reconstruction principle is explained, its performance is assessed for real parallel- and cone-beam CT (including under-sampled) data and compared to that of other established algorithms. Finally, it is demonstrated how the shape of the measured object can be modelled into DIRECTT to achieve even better reconstruction results
Reconstruction of limited-angle projection data with DIRECTT
No full textDirect Iterative Reconstruction of Computed Tomography
Trajectories is a fully three-dimensional algorithm that
prioritizes the reconstruction of voxels corresponding to
dominant trajectories across the projections
Suppression of cone-beam artefacts with DIRECTT
No full textA model image of the artefacts is computed just once, but it is used during each iteration of DIRECTT to “correct” the back-projected volume. The final reconstruction is a clear improvement over the one computed by the conventional FDK algorithm
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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