1,721,010 research outputs found
Identification of electromagnetic coefficients with microwave imaging
No full textDans cette thèse, nous étudions, d'un point de vue théorique et numérique, l'imagerie micro-ondes. Mathématiquement, il s'agit de résoudre un problème inverse : reconstruire les coefficients diélectriques (permittivité et conductivité) à l'intérieur d'un matériau ou d'un tissu à l'aide de mesures surfaciques du champ électrique. Ce problème est modélisé par les équations de Maxwell pour le champ électrique en régime harmonique dont nous démontrons l'existence et l'unicité d'une solution dans le cas de conditions de bord mixtes. Nous nous intéressons particulièrement à la reconstruction de perturbations dans l'indice de réfraction du milieu. L'indice du milieu sain est supposé connu et, à l'aide de mesures effectuées sur l'objet d'étude, nous définissons le champ perturbé et cherchons à retrouver les perturbations. Afin de comprendre l'influence de celles-ci sur le champ électrique, nous menons une analyse de sensibilité des équations de Maxwell. L'étude numérique de cette analyse de sensibilité a conduit à des résultats utilisés pour développer un algorithme de reconstruction du support des perturbations. Nous étudions par la suite le problème de Cauchy, notamment pour démontrer un résultat d'identifiabilité avec données partielles. Nous nous intéressons également à la résolution numérique de ce problème pour répondre à la question de la complétion de données surfaciques : à partir de mesures partielles, nous en déduisons des données totales. Le problème inverse est finalement étudié sous la forme d'un problème de minimisation d'une fonctionnelle permettant de reconstruire l'amplitude des inhomogénéités recherchéesIn this thesis, we study, from a theoretical and numerical point of view, the microwave imaging. Mathematically, it is about solving an inverse problem: reconstruct the dielectric coefficients (permittivity and conductivity) inside a material or tissue from boundary measurements of the electric field. This problem is modeled by time-harmonic Maxwell's equations for the electric field for which we prove the existence and uniqueness of a solution in the case of mix boundary conditions. We are particularly interested in the reconstruction of perturbations in the refractive index of the medium. The index of the healthy medium is assumed to be known and, with the help of boundary measurements on the studied object, we define the perturbed field and try to find the perturbations. In order to understand their influence on the electric field, we lead a sensitivity analysis of Maxwell's equations. The numerical study of this sensitivity analysis led to results used to develop a reconstruction algorithm of the perturbations supports. We then study the Cauchy problem, to solve a uniqueness result with partial data. We are also interested in the numerical resolution of this problem to answer the question of the boundary data completion: from partial measurements, we deduce the total data. The inverse problem is finally studied as the minimization of a functional to reconstruct the amplitude of the searched inhomogeneitie
Multimodal analysis and inverse problems for brain imaging.
No full textDans cette thèse, nous nous intéressons au couplage de deux modalités d’imagerie cérébrale :l’électroencéphalographie (EEG) et la tomographie optique diffuse (TOD). Dans une premièrepartie, nous présentons un modèle permettant de générer des données synthétiques coupléesissues du couplage neurovasculaire. Nous justifions, à l’aide d’une analyse dimensionnelle, unmodèle de l’EEG avec des sources d’activité cérébrale électrique dépendantes du temps ainsiqu’un modèle de TOD en régime harmonique avec des paramètres optiques du cerveau va-riant au cours du temps. Nous étudions l’existence et l’unicité d’une solution pour ces deuxproblèmes. En ce qui concerne l’EEG, nous utilisons la méthode de soustraction pour nous af-franchir du problème de régularité du terme source. Nous présentons ensuite un modèle ducouplage neurovasculaire à l’échelle du neurone sous la forme d’un système d’équations diffé-rentielles ordinaires que nous utilisons pour modéliser le comportement en temps de la sourced’activité électrique ainsi que des paramètres optiques du cerveau. Enfin, nous simulons desdonnées synthétiques EEG et TOD issues d’une même activité cérébrale sur une géométrieréaliste 3D. Dans une seconde partie, nous présentons les problèmes de localisation de sourcesd’activité en EEG et d’identification des paramètres optiques en TOD. Nous étudions séparé-ment les problèmes inverses liés à l’EEG et à la TOD. Nous démontrons un résultat d’identi-fiabilité pour la reconstruction de sources mobiles d’activité neuronale en EEG. Pour ces deuxproblèmes inverses, nous présentons plusieurs méthodes de résolution numérique que nousillustrons sur une géométrie réaliste 2D. Enfin, nous proposons trois approches permettantle couplage numérique de ces deux modalités d’imagerie cérérable. La première est basée surles informations obtenues en résolvant le problème inverse de la TOD et permet de réduirel’espace de sources en EEG. Les deux autres méthodes utilisent les informations issues de larésolution du problème de localisation de sources en EEG : l’une porte sur la régularisation duproblème inverse de la TOD et l’autre sur une approche par optimisation de forme.In this thesis, we focus on the coupling of two brain imaging modalities: electroencephalography (EEG)and diffuse optical tomography (DOT). In the first part, we present a model for generating syntheticcoupled data based on neurovascular coupling. Using a dimensional analysis, we justify an EEG model with time-dependent sources of electrical brain activity and a time-harmonic DOT model withtime-varying optical brain parameters. We study the existence and uniqueness of a solution for both problems. For EEG, we use the subtraction method to overcome the problem of source termregularity. We then present a model of neurovascular coupling at the neuron scale in the form of a system of ordinary differential equations, which we use to model the time behavior of the electrical activity source and the optical parameters of the brain. Finally, we simulate synthetic EEG and DOT data from the same brain activity on a realistic 3D geometry. In the second part of this thesis, we present the problems of source localization in EEG and parameter identification in DOT. In a first time, we separately study the respective inverse problems in EEG and DOT. We demonstrate an identifiability result for thereconstruction of moving sources of neuronal activity in EEG. For both inverse problems, we present several numerical resolution methods which we illustrate on a realistic 2D geometry. Finally, we propose three approaches for the numerical coupling of these two imaging modalities. The first one is basedon the information obtained by solving the DOT inverse problem and allows to reduce the EEG sources space. The two other methods use the information from EEG source localization as a priori information for regularization of the inverse parameter problem in DOT or initial guess ofshape optimization approach
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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Some second-kind integral equations in electromagnetism
No full textWe address the derivation of new second-kind combined field integral equations for
the Krylov iterative solution of high-frequency electromagnetic scattering problems by a
perfect conductor. The proposed formulations extend the well-known Brakhage-Werner
and Combined Field Integral Equations and improve the convergence properties of their
numerical solution through a Krylov iterative method. We prove that these integral equations are well-posed for any frequency. Preliminary experiments with spherical harmonics
in the case of a spherical scatterer illustrate the good behavior of a Krylov iterative solver
used for computing the solution of these new integral equations relatively to an increase of
the frequency or/and to the presence of a large number of vectorial spherical harmonics.ou
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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