With the rapid development of digital multimedia and internet techniques in the last few years, more and more digital images are being distributed to an ever-growing number of people for sharing, studying, or other purposes. Sharing images digitally is fast and cost-efficient thus highly desirable. However, most of those digital products are exposed without any protection. Thus, without authorization, such information can be easily transferred, copied, and tampered with by using digital multimedia editing software. Watermarking is a popular resolution to the strong need of copyright protection of digital multimedia. In the image forensics scenario, a digital watermark can be used as a tool to discriminate whether original content is tampered with or not. It is embedded on digital images as an invisible message and is used to demonstrate the proof by the owner. In this thesis, we propose a novel localized geometric-distortion resilient digital watermarking scheme to embed two invisible messages to images. Our proposed scheme utilizes two complementary watermarking techniques, namely, local circular region (LCR)-based techniques and block discrete cosine transform (DCT)-based techniques, to hide two pseudo-random binary sequences in two kinds of regions and extract these two sequences from their individual embedding regions. To this end, we use the histogram and mean statistically independent of the pixel position to embed one watermark in the LCRs, whose centers are the scale invariant feature transform (SIFT) feature points themselves that are robust against various affine transformations and common image processing attacks. This watermarking technique combines the advantages of SIFT feature point extraction, local histogram computing, and blind watermark embedding and extraction in the spatial domain to resist geometric distortions. We also use Watson’s DCT-based visual model to embed the other watermark in several rich textured 80×80 regions not covered by any embedding LCR. This watermarking technique combines the advantages of Harris feature point extraction, triangle tessellation and matching, the human visual system (HVS), the spread spectrum-based blind watermark embedding and extraction. The proposed technique then uses these combined features in a DCT domain to resist common image processing attacks and to reduce the watermark synchronization problem at the same time. These two techniques complement each other and therefore can resist geometric and common image processing attacks robustly. Our proposed watermarking approach is a robust watermarking technique that is capable of resisting geometric attacks, i.e., affine transformation (rotation, scaling, and translation) attacks and other common image processing (e.g., JPEG compression and filtering operations) attacks. It demonstrates more robustness and better performance as compared with some peer systems in the literature.

Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / The tomosynthesis equipment is expensive and needs to be operated by professionals who know all their functions, to avoid damages, optimize their use in terms of image quality, radiation doses and costs. The proper operation assists in the identification of defects presented by the equipment in the initial phase of use, allowing immediate corrections and preventing major damages. Therefore, special attention should be given to quality assurance programs for these equipment and their operational processes. Distortion is the incoherent representation of the size or shape of a structure in the radiographic image. Exaggerated distortion makes radiography unacceptable for diagnosis. The objective of this study was to perform geometric distortion and artifact dispersion tests in digital breast tomosynthesis images with algorithms (computational codes) developed by our group and to compare with literature results based on the European Guidelines for Quality Assurance in Breast Cancer Screening and Diagnosis (EUREF). For the analyses, we initially obtained images with a polymethylmethacrylate simulating object composed of aluminum beads distributed at 25, 35 and 45 mm distance from the compression tray. The images were acquired in digital mammography and tomosynthesis equipment of the Hologic and GE brands installed in radiological clinics in several States of Brazil. The images were analyzed using ImageJ software. For the evaluation with the algorithms it was necessary to analyze the height of the plane in focus, the accuracy of the positioning in the plane in focus and the appearance of the aluminum balls in the adjacent planes in focus. The planes in focus represent the planes that spheres appear most clearly in the images. The images were also analyzed by code already described in the EUREF protocol. The results showed that the algorithms developed in this study are capable of evaluating image distortion by comparing the separation between the aluminum beads in the focus plane and their physical separation in the simulator object, ensuring that such algorithms can be used for analysis of images obtained in tomosynthesis equipment from different manufacturers. / Os equipamentos de tomossíntese são de alto custo e precisam ser operados por profissionais que conheçam todas as suas funções, para evitar danos e otimizar seu uso em termos de qualidade da imagem, doses de radiação ao paciente e custos. A operação adequada auxilia na identificação de defeitos apresentados pelo equipamento na fase inicial de uso, possibilitando correções imediatas e prevenindo danos maiores. Por isso, deve-se dar atenção especial aos programas de garantia de qualidade desses equipamentos e de seus processos operacionais. Distorção é a representação não exata do tamanho ou da forma de uma estrutura na imagem radiográfica. Uma distorção exagerada torna a radiografia inaceitável para o diagnóstico. O objetivo deste estudo foi avaliar a distorção geométrica e dispersão de artefatos em imagens de tomossíntese digital da mama com algoritmos (códigos computacionais) desenvolvidos por nosso grupo e comparar com resultados da literatura, baseados no protocolo europeu European Guidelines for Quality Assurance in Breast Cancer Screening and Diagnosis (EUREF). Para as análises, inicialmente obtivemos imagens com um objeto simulador de 5 mm de espessura composto de polimetilmetacrilato, ao qual estão inseridas esferas de alumínio de 1 mm de diâmetro de forma retangular. As imagens reconstruídas para a realização do teste de distorção geométrica e dispersão de artefatose foram obtidas com o objeto de teste posicionado a 25, 35 e 45 mm de distância em relação à bandeja de compressão. As imagens foram adquiridas em equipamentos de mamografia digital e tomossíntese das marcas Hologic e GE instalados em clínicas radiológicas de vários Estados do Brasil. As imagens foram analisadas utilizando o software ImageJ. Para a avaliação com os algoritmos foi necessário analisar a altura do plano em foco, a acurácia do posicionamento no plano em foco e a aparência das esferas de alumínio nos planos em foco adjacentes. Os planos em foco representam os planos que as esferas aparecem mais nítidas nas imagens. As imagens foram analisadas também por código já descrito no protocolo EUREF (NCCPM_Tools). Os resultados mostraram que os algoritmos desenvolvidos neste estudo produzem a avaliações da distorção geométrica e dispersão de artefatos semelhantes à realizada pela ferramenta computacional desenvolvida pelo protocolo europeu (EUREF), assegurando que tais algoritmos podem ser empregados para análises de imagens obtidas em equipamentos de tomossíntese de fabricantes diversos. / São Cristóvão, SE

Work deals with methods of calibration of multi-center study of magnetic resonance imaging. Magnetic resonance is the phenomenon of the substance in a magnetic field of induction B0 delivers energy in the form of RF pulse with the Larmor frequency and thus to excite particles to higher energy levels. Calibration can be performed using the distinctive and homogeneous RF phantoms. Furthermore, we can perform calibration using image registration. To calibrate the images by registering the work described in the classical linear (affine) and nonlinear. Listed below are the simulators, which are also useful for modeling various artifacts.

Les opérations géométriques appliquées aux maillages 3D introduisent des dis torsions géométriques qui peuvent être visibles pour un observateur humain. Dans cette thèse, nous étudions l’impact perceptuel de ces distorsions. Plus précisément, notre objectif est de calculer le seuil à partir duquel les distorsions géométriques locales deviennent visibles. Afin d’atteindre notre but, nous définissons tout d’abord des caractéristiques perceptuelles pour les maillages 3D. Nous avons ensuite effectué une étude expérimentale des propriétés du système visuel humain (sensibilité au contraste et effet du masquage visuel) en observant un maillage 3D. Les résultats de ces expériences sont finalement utilisés pour proposer un algorithme qui calcule le seuil de visibilité relatif à une distorsion locale. L’algorithme proposé s’adapte aux différentes conditions d’affichage (résolution et taille de l’écran), d’illumination et au type de rendu. Enfin, nous montrons l’utilité d’un tel algorithme en intégrant le seuil de visibilité dans le pipeline de plusieurs opérations géométriques (ex: simplification, subdivision adaptative) / Geometric operations applied to a 3D mesh introduce geometric distortion in the form of vertex displacement that can be visible to a human observer. In this thesis, we have studied the perceptual impact of these geometric distortions. More precisely, our goal is to compute the threshold beyond which a local geometric distortion becomes visible. In order to reach this goal, we start by evaluating perceptually relevant properties on 3D meshes. We have then performed a series of psychophysical experiments in which we measured the visibility threshold relative to various properties of the Human Visual System (contrast sensitivity and visual masking). The results of these experiments allowed us to propose an algorithm that computes the visibility threshold relative to a local geometric distortion. This algorithm is capable of adapting to the different display condition of 3D meshes (resolution, display size, illumination condition and rendering). Finally, we showcase the utility of our work by integrating the developed perceptual method in several geometric operations such as mesh simplification and adaptive subdivision