1,721,160 research outputs found
Improving tomographic images with a filtered projection in real space
No full textA certain quantity of noise, which reduces the diagnostic power of the tomographic technique, is always present in a reconstructed image. This is due to various reasons. On the one side, the finite precision of computers and the acquisition system geometry generate artefacts that degrade the image quality. On the other side, there are errors due to the experimental acquisition, the mathematical modelling of the physical problem and, last but not least, those caused by the choice of the reconstruction algorithm. In the present work this last aspect is improved using a reconstruction method known as filtered backprojection (FBP) in which the projections are filtered in real space rather than in Fourier space. In this way some of the so called aliasing artefacts are reduced, in particular those due to interperiodical interference of the F-transformed tails which represent one of the main problems while working in transformed space. The algorithm proposed in this paper represents an alternative to the usual filtering technique which consists of applying a filter, typically the Hamming filter, which manipulates the projections in transformed space in order to cut the high spatial frequency components. This is an important source of noise. The main limitation of such a procedure is loss of correspondence with real space and, consequently, has the possibility of introducing non-physical effects in the reconstructed image. © 2001 Elsevier Science Ltd. All rights reserved
A new approach to image reconstruction in positron emission tomography using artificial neural networks
No full textThis study investigates the possibility of using an Artificial Neural Network (ANN) for reconstructing Positron Emission Tomography (PET) images. The network is trained with simulated data which include physical effects such as attenuation and scattering. Once the training ends, the weights of the network are held constant. The network is able to reconstruct every type of source distribution contained inside the area mapped during the learning. The reconstruction of a simulated brain phantom in a noiseless case shows an improvement if compared with Filtered Back-Projection reconstruction (FBP). In noisy cases there is still an improvement, even if we do not compensate for noise fluctuations. These results show that it is possible to reconstruct PET images using ANNs. Initially we used a Dec Alpha; then, due to the high data parallelism of this reconstruction problem, we ported the learning on a Quadrics (SIMD) machine, suited for the realization of a small medical dedicated system. These results encourage us to continue in further studies that will make possible reconstruction of images of bigger dimension than those used in the present work (32 × 32 pixels)
Comparison between a germanium orthogonal strip detector and an anger camera through a simulation and modeling study
No full textProgress in detector technology has led to the development of a new generation of pixel-based imaging devices as potential competitors for Anger-type scintillation cameras. We modeled a 11-mm-thick germanium orthogonal strip detector (GOSD) with 2-mm pitch and compared its performance to an Anger camera with a 10-mm-thick Nal(TI) crystal. The Anger camera simulation method was validated by experimental measurements made with point and volume sources. Resolution and sensitivity were determined for air and scatter measurements. The device is intended for use in breast tumor imaging, and its expected performance was simulated in response to 5-, 7.5-, and 10-mm spherical tumors embedded in a previously reported phantom geometry used for the simulation of a CsI pixellated detector. Thorax, breast, and heart background contributions are considered in this phantom. A comparison of the results obtained indicates that the GOSD provides superior contrast than the Anger camera for every tumor to collimator distance and for every tumor dimension. Also, signal-to-noise ratio (SNR), full-width at half-maximum (FWHM), and full-width at tenth maximum (FWTM) show a better response for the germanium pixellated detector with respect to Anger camera due to better energy and spatial resolution of the germanium-based device
Attenuation compensation for breast tissue in combined CT/SPECT devices dedicated to mammography
No full textThe usefulness to generate attenuation maps from computed tomography (CT) system for compensation of photon attenuation in single photon emission computed tomography (SPECT) mammographic evaluation is discussed. The present investigation is part of a general project intended to perform co-registration of functional and morphological information. The experimental apparatus consists of a quasi-monochromatic X-ray computed tomography and a high resolution compact gamma camera. The projection data are reconstructed using filtered backprojection to form the Xray CT image, which is then properly scaled to produce an estimate of the attenuation map at the energy of the emission nuclide. The scaling is done following a two phases model of the breast tissue which takes into account the realistic average composition of the basic components. It is shown that both for simulations and measurements, the attenuation maps, generated from the CT images, estimate accurately the attenuation of breast in the reconstructed SPECT evaluations. (C) 2002 Elsevier Science B.V. All rights reserved
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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