1,721,083 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Total variation regularization in tomography with Poisson distributed data
No full textInternational audienceTomography in nuclear medicine requires resolution of a linear inverse problem. As the radioactive decay follows a Poisson law, the projections of gamma emission from inside the body are also Poisson distributed. In this talk we will introduce a dual algorithm for total variation (TV) Poisson denoising. When combined with MLEM (maximum likelihood expectation maximization) iterations, a fast and convergent algorithm for the estimation of the TV maximum-a-posteriori solution is obtained
Challenges in Compton camera imaging for medical applications
No full textInternational audienceCompton cameras are a promising alternative to collimated cameras in single particleemission computed tomography (SPECT). They meet the challenge of imaging sources having poly-chromatic spectrum and low photon yields. Currently used SPECT cameras rely on mechanicalcollimation, imposing a trade-off between efficiency and resolution. As the photon energy increases,the radiation can penetrate the collimator and thus degrade the resolution. A solution is then toincrease the thickness of the septa which further reduces the detection efficiency. Based on acoincidence detection system and on the kinematics of Compton scattering, Compton camerasrestrict to a half-cone the set of possible origins of a detected photon. They allow an increase inefficiency of one or two orders of magnitude, at the cost of more complex image reconstructiontechniques. Specific tomographic algorithms capable to invert conical Radon transforms are thenrequired to produce the image of the source.In this talk we focus on iterative reconstruction methods. We will first highlight the importanceof choosing a data model well adapted to the measurement uncertainties. Then we will discusssome regularization strategies in statistical image reconstruction. In particular we will present afast and convergent algorithm for the non-differentiable total variation prior. We will illustrateits performances on low-statistics data issued from a complex-shaped mono-energetic source. Inaddition, this example will allow us to compare the performances of collimated and Comptoncameras. We will end with an example from proton-therapy, where treatment monitoring couldbe realized by imaging the secondary prompt-gamma radiation. In this application, extremely lowphoton counts are expected in a wide energy range.This work was done in collaboration with Yuemeng Feng (PhD student, CREATIS), Elie Bretin(Institut Camille Jordan, Lyon), Anne Etxebeste (post-doc, CREATIS), David Sarrut and JeanMichel Létang (CREATIS), Étienne Testa and Mattia Fontana (Institut de Physique Nucléaire deLyon) and Denis Dauvergne (Laboratoire de Physique Subatomique et de Cosmologie, Grenoble)
Practical considerations for Compton camera analytic image reconstruction
No full textInternational audienceCompton camera imaging of gamma rays relies on the inversion of a Radon transform on conical surfaces. Algebraic reconstruction methods have been proposed since the invention of the camera in late 1970s. More recently, iterative algorithms based on statistical modeling of the acquisition have been established as a reference. Although versatile and well adapted to best account for acquisition geometry and for the statistical model of the data, these algorithms require relatively heavy computing resources and implementation on specific hardware to avoid prohibitive computing delays. On the opposite, analytic methods have limited capability to account for model alterations, for low statistics and for noise in the data, but they can provide reliable results in few seconds with standard computing resources. In this work we focus on numerical aspects of the analytic inversion of a conical Radon transform. We discuss some ways to improve the quality of images and we show that a semi-iterative TV-regularized approach allows strong improvement of results with negligible additional computation costs. The proposed method will be illustrated on Monte-Carlo data simulating a low-dose SPECT acquisition
Reconstruction tomographique pour l'imagerie SPECT avec une caméra Compton
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Filtered Backprojection Reconstruction and Redundancy in Compton Camera Imaging
No full textInternational audienceDuring the acquisition process with the Compton gamma-camera, integrals of the intensity distribution of the source on conical surfaces are measured. They represent the Compton projections of the intensity. The inversion of the Compton transform reposes on a particular Fourier-Slice theorem. This paper proposes a filtered backprojection algorithm for image reconstruction from planar Compton camera data. We show how different projections are related together and how they may be combined in the tomographical reconstruction step. Considering a simulated Compton imaging system, we conclude that the proposed method yields accurate reconstructed images for simple sources. An elongation of the source in the direction orthogonal to the camera may be observed and is to be related to the truncation of the projections induced by the finite extent of the device. This phenomenon was previously observed with other reconstruction methods, e.g., iterative maximum likelihood expectation maximization. The redundancy of the Compton transform is thus an important feature for the reduction of noise in Compton images, since the ideal assumptions of infinite width and observation time are never met in practice. We show that a selection operated on the set of data allows to partially get around projection truncation, at the expense of an enhancement of the noise in the images
Total variation regularization in tomography with Poisson distributed data
No full textInternational audienceTomography in nuclear medicine requires resolution of a linear inverse problem. As the radioactive decay follows a Poisson law, the projections of gamma emission from inside the body are also Poisson distributed. In this talk we will introduce a dual algorithm for total variation (TV) Poisson denoising. When combined with MLEM (maximum likelihood expectation maximization) iterations, a fast and convergent algorithm for the estimation of the TV maximum-a-posteriori solution is obtained
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