Concerns arise in radiation therapy for head and neck cancers when dental prostheses are involved. These prostheses are high-density materials that induce image artifacts in computed tomography (CT) scans used for dose calculation. Two approaches are utilized in mitigating the impact of these artifacts on the accuracy of dose calculation. First, metal artifact reduction (MAR) algorithms or dual-energy CT scans are used to recover image quality. Second, a planning protocol is adopted whereby residual artifacts are manually contoured and assigned appropriate densities. This study evaluated the current planning process using a holistic approach. In this work, an axial section of a head phantom containing dental implants at the level of the oral cavity was constructed and scanned using various protocols on two different commercial scanners; Philips and Siemens, to assess the appearance of artifacts. An MVCT image set was merged with the corresponding kVCT image to improve visualization of the dental implants for use in density overrides. Three ion chamber measurement points in the simulated mouth facilitated the determination of measured dose which was compared to calculated dose at various single beam irradiation geometries. The influence of density override values on agreement between calculation and measurement was investigated for each geometry and imaging modality. Percent error was computed, and initial results compared to results manipulated by use of; a CT density table (Head); density overrides of walls and wax; and density overrides of walls, wax, and effective density of saturation regions. The study established that normal tissue doses are not significantly affected by metal artifact reduction (MAR) algorithms, and improvements in dose calculation compared to uncorrected CT images are small. Furthermore, the inclusion of a MVCT image set improved implant visualization reducing the treatment planning time while providing more information. Evidence led to the deduction that manual overrides of effective density for clipped OMAR CT pixels reduce dose calculation errors. When the phantom was configured with amalgam and Co-Cr-Mo alloy dental implants the effective density of these implants was found to be 4.5 g/cm3. When the phantom was configured with implants containing amalgam and gold, the effective density of amalgam in the presence of gold was 5.5 g/cm3 while gold had an effective density of 6.5 g/cm3. The median and maximum range of errors for the uncorrected images were ± 0.6 % and 7.4% respectively for the phantom configured with amalgam and Co-Cr-Mo (tray one) and ± 0.5 % and 18.1 % respectively for the phantom containing amalgam and gold (tray two). The median and maximum range of errors for the corrected images after applying overrides of effective densities were ± 0.5 % and 4.7% respectively for tray one and ± 0.3 % and 7.7 % respectively for tray two. In conclusion, introduction of density overrides of walls, wax and effective density of high-density materials can reduce the errors induced by metal artifacts and improve the accuracy of dose calculations in treatment planning systems to deliver the relevant dose to a target organ. / Thesis / Master of Science (MSc)

Dans le cadre du contrôle non destructif de matériaux, les scanners à rayons X sont devenus un moyen d’assurer la validité et la qualité de pièces de productions industrielles. TomoAdour, un prestataire de services privé en digitalisation 3D et tomographie industrielle à rayons X, utilise des technologies d’acquisition basées sur ce rayonnement telles qu’un scanner médical ou tomographe industriel, de manière à observer et à analyser des produits de ses clients. La nature des objets acquis (principalement de forte densité et de grandes dimensions) fait apparaître des défauts sur les images issues de ces scanners X. Il devient dès lors difficile, voire impossible, d’analyser les images produites. On notera en particulier la présence d’artefact métallique. Ce dernier est dû à la présence de matériaux très denses dans l’objet acquis et est la conséquence de la forte atténuation des rayons dans le métal et/ou de leur distorsion dans toutes les directions. Les données reconstruites sont donc souvent difficiles à interpréter directement, car ces artefacts peuvent masquer des informations importantes. De nombreuses méthodes ont été développées durant ces dernières années et la problématique de la réduction de l’artefact métallique a largement pu être étudiée, apportant bon nombre de solutions. Pourtant, les algorithmes proposés ne s’intéressent qu’aux applications médicales et ne tiennent donc pas compte des limitations physiques propres aux échantillons industriels. De plus, les méthodes proposées basent, toutes, leur traitement sur des données brutes, c’est-à-dire des données issues du scanner avant la phase de reconstruction (ensemble de radiographies ou de projections). Dans notre cas, seules les données reconstruites (tomogrammes, i.e. volume représentant l’objet) sont disponibles et il n’existe, à l’heure actuelle, aucune approche de réduction de l’artefact métallique basée sur ces reconstructions. Le but de ces travaux de recherche est donc de proposer, dans un premier temps, un outil permettant d’identifier et de quantifier l’artefact métallique dans les données reconstruites pour la correction de ces dernières. Dans un second temps, notre travail propose un ensemble d’approches adaptées à l’analyse de matériaux dans un contexte industriel [1; 2]. / In the field of non-destructive testing of materials, computed tomography became a good way to check defects in industrial piece production. TomoAdour is a private compagny specialized in 3D digitization and x-ray computed tomography, it uses medical and industrial imaging techniques to analyze materials provided by his customers. However, tomographic analysis is difficult to achieve due to the presence of high density objects (such that metal) in most produced pieces, leading to the well-known metal artifacts in reconstructed data. In X-Ray tomography, metal artifact is characterized by a local and straight hyper-signal. This observed phenomenon is due to high attenuations of the rays in the high density materials. Many different approaches have been proposed for metal artifact reduction during the last decade. However, these methods have been developed for medical application and does not take into account physical limitations specific to industrial materials. Moreover, state-of-the-art approaches start their process from the original projection data, that is to say directly from the acquired data. In our context, only reconstructed image is available due to clinical scanner usage, and there is currently no metal artifact reduction method only based on these data. The goal of this work is first to propose a tool which permit identifying and measuring metal artifact in the reconstructed data in order to correct them. Then, our work take in interest in presenting methods developed for an industrial context[1; 2].

Diffusion Tensor Imaging (DTI) is a magnetic resonance imaging (MRI) technique used to measure in-vivo anisotropic water diffusion. This can give useful information regarding white matter integrity and has the potential to provide important biomarkers in spinal cord injury. One of the largest challenges in DTI of the spinal cord is the presence of metal which causes geometric distortions, signal pile-up, and signal voids. Because most patients with spinal cord injury have some amount of metal hardware implanted for stabilization, it is important to confront issues involving metal as DTI of the spinal cord becomes more widely examined. This study examined the characteristics of metal artifact in DTI images for several spinal surgical implants via imaging of phantoms constructed with implements suspended in agar gel to provide a homogeneous surrounding medium for analysis. A cervical spine phantom implanted with pedicle screws was also used to simulate in-vivo imaging. Optimization of the DTI sequence was also considered using different metal artifact reduction techniques including view-angle-tilting, slice thickness, and field of view size. Minor reduction in metal artifact was achieved using these techniques. The resulting image data shows that imaging near metal may be feasible in some circumstances, particularly when implantation is minimal. Also, using the cervical spine phantom it was shown that it should be possible to acquire DTI data close to the location of metal implants and thus examine DTI values of the injured spinal cord superior to the injury site. / Mechanical Engineering

Submitted by Jean Medeiros (jeanletras@uepb.edu.br) on 2017-10-23T13:03:47Z No. of bitstreams: 1 PDF - José Alberto Souza Paulino.pdf: 32306126 bytes, checksum: d8a3a4c1c537e36b743590e029308f8c (MD5) / Approved for entry into archive by Secta BC (secta.csu.bc@uepb.edu.br) on 2017-10-26T16:55:17Z (GMT) No. of bitstreams: 1 PDF - José Alberto Souza Paulino.pdf: 32306126 bytes, checksum: d8a3a4c1c537e36b743590e029308f8c (MD5) / Made available in DSpace on 2017-10-26T16:55:17Z (GMT). No. of bitstreams: 1 PDF - José Alberto Souza Paulino.pdf: 32306126 bytes, checksum: d8a3a4c1c537e36b743590e029308f8c (MD5) Previous issue date: 2017-05-08 / This research proposes to evaluate and implement a solution for metal artifact reduc- tion in computed tomography, this one aiming to meet a demand from the prototyping laboratory of the Núcleo de Tecnologias Estratégicas em Saúde (Nutes) da Univer- sidade Estadual da Paraíba, where impressions of biomodels are made for surgical planning. The CT affected by metal artifacts need to be corrected prior to the printing process, this manual intervention implies excessive delay for delivery of the biomodels. The development of the proposed solution is based on the sinogram correction method which according to Mouton et al (2013) and Gjesteby (2016) is the most utilized method for reducing metal artifacts and makes uses of linear interpolation to correction the cor- rupted data. In order to validate the preference for linear interpolation in the state of the art, others interpolative techniques were implemented and evaluated; Fist through simulations and then by a form for qualitative evaluation, upon which statistical tests were applied. The results obtained confirm the use of interpolation as the best option for the reconstruction of data corrupted by metallic artifacts. / Esta pesquisa se propõe a avaliar e implementar uma solução para redução de artefatos metálicos em tomografias computadorizadas, solução esta que visa atender uma demanda do laboratório de prototipagem do Núcleo de Tecnologias Estratégicas em Saúde (Nutes) da Universidade Estadual da Paraíba, onde são realizadas impressões de biomodelos para planejamentos cirúrgicos. As tomografias afetadas por artefatos metálicos necessitam de correção antes do processo de impressão, esta intervenção realizada de forma manual implica em demora excessiva para entrega dos biomodelos. O desenvolvimento da solução proposta baseia-se no método de correção de sinograma que, de acordo com Mouton et al (2013) e Gjesteby (2016), é o método mais difundido para redução de artefatos metálicos e faz uso da técnica de interpolação linear para correção dos dados corrompidos. Objetivando validar a preferência pelo uso da interpolação linear no estado da arte, foram implementadas outras técnicas interpolativas as quais foram submetidas a avaliação; Primeiro por meio de simulações e depois via fomulário para avaliação qualitativa, na qual foram aplicados testes estatísticos. Os resultados obtidos ratificam o uso da interpolação linear como melhor opção para reconstrução de dados corrompidos pelos artefatos metálicos.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-08-21T19:15:12Z No. of bitstreams: 1 alessianahelenamachado.pdf: 2020827 bytes, checksum: 87061a02b07d6d6e92c3f1dcee06da4c (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-08-24T12:04:37Z (GMT) No. of bitstreams: 1 alessianahelenamachado.pdf: 2020827 bytes, checksum: 87061a02b07d6d6e92c3f1dcee06da4c (MD5) / Made available in DSpace on 2017-08-24T12:04:37Z (GMT). No. of bitstreams: 1 alessianahelenamachado.pdf: 2020827 bytes, checksum: 87061a02b07d6d6e92c3f1dcee06da4c (MD5) Previous issue date: 2017-07-21 / O objetivo no presente estudo foi comparar, quantitativamente, os artefatos metálicos produzidos em imagens de tomografia computadorizada de feixe cônico (TCFC) por implantes instalados em diferentes regiões maxilomandibulares. Para isso, um total de 200 implantes, selecionados de exames de TCFC, foi dividido em quatro grupos: Grupo 1 (n = 50) - implantes localizados na região anterior da maxila; Grupo 2 (n = 50) - implantes localizados na região posterior da maxila; Grupo 3 (n = 50) - implantes localizados na região anterior da mandíbula e Grupo 4 (n = 50) - implantes localizados na região posterior da mandíbula. Os implantes ainda foram classificados em isolados ou adjacentes a outros implantes. Foram selecionados três cortes axiais de cada implante incluído na amostra (apical, médio e cervical). Nesses cortes foram mensurados os artefatos produzidos pelos implantes. Para comparar as variáveis com dois grupos foi aplicado o teste U de Mann-Whitney. Para a comparação entre os cortes axiais foram aplicados os testes de Kruskal-Wallis e Student-Newman-Keuls. A mandíbula apresentou uma quantidade de artefatos maior que a maxila (corte apical: p = 0,0024; corte médio: p < 0,0001). A região anterior produziu mais artefatos que a região posterior (corte apical: p = 0,0105; corte médio: p < 0,0316). Não houve diferença significativa na quantidade de artefatos entre implantes isolados e adjacentes e o corte cervical foi o mais acometido por artefatos. Pode-se concluir que os implantes dentários sempre produzem artefatos metálicos em imagens de TCFC, sendo esses artefatos influenciados pela localização anatômica na arcada dentária. / The objective of the present study was to compare, quantitatively, the metal artifacts produced in cone beam computed tomography (CBCT) images by dental implants installed in different maxillomandibular regions. A total of 200 implants selected from CBCT examinations were divided into four groups: Group 1 (n = 50) - implants located in the anterior maxilla; Group 2 (n = 50) - implants located in the posterior maxilla; Group 3 (n = 50) - implants located in the anterior mandible; and Group 4 (n = 50) - implants located in the posterior mandible. The implants were further classified as isolated or adjacent to other implants. Three axial slices were selected for each sampled implant (apical, middle and cervical). On each slice, the artifacts produced by the implants were counted. The Mann-Whitney U test was used to compare the variables between groups. The Kruskal-Wallis and Student-NewmanKeuls tests were used to compare the axial slices. The mandible showed a greater number of artifacts than the maxilla (apical slice: p = 0.0024; middle slice: p < 0.0001). The anterior region produced more artifacts than the posterior region (apical slice: p = 0.0105; middle slice: p < 0.0316). There was no significant difference in the number of artifacts between isolated and adjacent implants, and the cervical slice was most affected by artifacts. It can be concluded that dental implants always produce metal artifacts in CBCT images, and these artifacts are affected by the anatomical location in the dental arch.

Submitted by Geandra Rodrigues (geandrar@gmail.com) on 2018-10-24T11:31:17Z No. of bitstreams: 0 / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2018-10-24T15:48:14Z (GMT) No. of bitstreams: 0 / Made available in DSpace on 2018-10-24T15:48:14Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-08-08 / O objetivo do trabalho foi quantificar, em imagens de tomografia computadorizada de feixe cônico (TCFC) obtidas com diferentes protocolos, os artefatos metálicos produzidos por implantes de titânio instalados em diferentes regiões da mandíbula. Os implantes foram instalados em quatro diferentes regiões (incisivo, canino, pré-molar e molar) de um phamtom e submetidos a exames de TCFC com variação da posição do objeto no interior do FOV (central, anterior, posterior, direita e esquerda), variação do FOV (6 x 13 e 12 x 13 cm) e do tamanho do voxel (0,25 e 0,30 mm). Um corte axial da região cervical de cada implante foi selecionado para quantificação. Os testes de Kruskal-Wallis e Student-Newman-Keuls foram utilizados para comparação das regiões dos dentes e entre as diferentes posições do phantom dentro do FOV. O teste de Wilcoxom foi utilizado para comparar a variação de tamanho do FOV e voxel. O teste ANOVA fatorial para avaliar a interação entre as variáveis do estudo. A região de incisivo apresentou a maior quantidade de artefatos, em comparação as outras regiões (p=0,0315). Não houve diferença significativa na variação da posição do phantom dentro do FOV (p=0,7418). O FOV menor produziu mais artefatos (p<0,0001). Ao comparar as imagens produzidas com diferentes resoluções, o menor voxel produziu mais artefatos (p<0,0001). Os artefatos metálicos sofrem influência do tamanho do FOV e do voxel, além da região anatômica. A variação da localização do phantom no interior do FOV não alterou a quantidade de artefatos. / O objetivo do trabalho foi quantificar, em imagens de tomografia computadorizada de feixe cônico (TCFC) obtidas com diferentes protocolos, os artefatos metálicos produzidos por implantes de titânio instalados em diferentes regiões da mandíbula. Os implantes foram instalados em quatro diferentes regiões (incisivo, canino, pré-molar e molar) de um phamtom e submetidos a exames de TCFC com variação da posição do objeto no interior do FOV (central, anterior, posterior, direita e esquerda), variação do FOV (6 x 13 e 12 x 13 cm) e do tamanho do voxel (0,25 e 0,30 mm). Um corte axial da região cervical de cada implante foi selecionado para quantificação. Os testes de Kruskal-Wallis e Student-Newman-Keuls foram utilizados para comparação das regiões dos dentes e entre as diferentes posições do phantom dentro do FOV. O teste de Wilcoxom foi utilizado para comparar a variação de tamanho do FOV e voxel. O teste ANOVA fatorial para avaliar a interação entre as variáveis do estudo. A região de incisivo apresentou a maior quantidade de artefatos, em comparação as outras regiões (p=0,0315). Não houve diferença significativa na variação da posição do phantom dentro do FOV (p=0,7418). O FOV menor produziu mais artefatos (p<0,0001). Ao comparar as imagens produzidas com diferentes resoluções, o menor voxel produziu mais artefatos (p<0,0001). Os artefatos metálicos sofrem influência do tamanho do FOV e do voxel, além da região anatômica. A variação da localização do phantom no interior do FOV não alterou a quantidade de artefatos.

No description available.

Cette thèse est constituée de deux principaux axes de recherche portant sur l'imagerie dentaire par la tomographie à rayons X : le développement de nouvelles méthodes itératives de reconstruction tomographique afin de réduire les artefacts métalliques et la réduction de la dose délivrée au patient. Afin de réduire les artefacts métalliques, nous prendrons en compte le durcissement du spectre des faisceaux de rayons X et le rayonnement diffusé. La réduction de la dose est abordée dans cette thèse en diminuant le nombre des projections traitées. La tomographie par rayons X a pour objectif de reconstruire la cartographie des coefficients d'atténuations d'un objet inconnu de façon non destructive. Les bases mathématiques de la tomographie repose sur la transformée de Radon et son inversion. Néanmoins des artefacts métalliques apparaissent dans les images reconstruites en inversant la transformée de Radon (la méthode de rétro-projection filtrée), un certain nombre d'hypothèse faites dans cette approche ne sont pas vérifiées. En effet, la présence de métaux exacerbe les phénomènes de durcissement de spectre et l'absence de prise en compte du rayonnement diffusé. Nous nous intéressons dans cette thèse aux méthodes itératives issues d'une méthodologie Bayésienne. Afin d'obtenir des résultats de traitement compatible avec une application clinique de nos nouvelles approches, nous avons choisi un modèle direct relativement simple et classique (linéaire) associé à des approches de corrections de données. De plus, nous avons pris en compte l'incertitude liée à la correction des données en utilisant la minimisation d'un critère de moindres carrés pondérés. Nous proposons donc une nouvelle méthode de correction du durcissement du métal sans connaissances du spectre de la source et des coefficients d'atténuation des matériaux. Nous proposons également une nouvelle méthode de correction du diffusé associée sur les mesures sous certaines conditions notamment de faible dose. En imagerie médicale par tomographie à rayons X, la surexposition ou exposition non nécessaire irradiante augmente le risque de cancer radio-induit lors d'un examen du patient. Notre deuxième axe de recherche porte donc sur la réduction de la dose en diminuant le nombre de projections. Nous avons donc introduit un nouveau mode d'acquisition possédant un échantillonnage angulaire adaptatif. On utilise pour définir cette acquisition notre connaissance a priori de l'objet. Ce mode d'acquisition associé à un algorithme de reconstruction dédié, nous permet de réduire le nombre de projections tout en obtenant une qualité de reconstruction comparable au mode d'acquisition classique. Enfin, dans certains modes d’acquisition des scanners dentaires, nous avons un détecteur qui n'arrive pas à couvrir l'ensemble de l'objet. Pour s'affranchir aux problèmes liés à la tomographie locale qui se pose alors, nous utilisons des acquisitions multiples suivant des trajectoires circulaires. Nous avons adaptés les résultats développés par l’approche « super short scan » [Noo et al 2003] à cette trajectoire très particulière et au fait que le détecteur mesure uniquement des projections tronquées. Nous avons évalué nos méthodes de réduction des artefacts métalliques et de réduction de la dose en diminuant le nombre des projections sur les données réelles. Grâce à nos méthodes de réduction des artefacts métalliques, l'amélioration de qualité des images est indéniable et il n'y a pas d'introduction de nouveaux artefacts en comparant avec la méthode de l'état de l'art NMAR [Meyer et al 2010]. Par ailleurs, nous avons réussi à réduire le nombre des projections avec notre nouveau mode d'acquisition basé sur un « super short scan » appliqué à des trajectoires multiples. La qualité obtenue est comparable aux reconstructions obtenues avec les modes d'acquisition classiques ou short-scan mais avec une réduction d’au moins 20% de la dose radioactive. / This thesis contains two main themes: development of new iterative approaches for metal artifact reduction (MAR) and dose reduction in dental CT (Computed Tomography). The metal artifacts are mainly due to the beam-hardening, scatter and photon starvation in case of metal in contrast background like metallic dental implants in teeth. The first issue concerns about data correction on account of these effects. The second one involves the radiation dose reduction delivered to a patient by decreasing the number of projections. At first, the polychromatic spectra of X-ray beam and scatter can be modeled by a non-linear direct modeling in the statistical methods for the purpose of the metal artifacts reduction. However, the reconstruction by statistical methods is too much time consuming. Consequently, we proposed an iterative algorithm with a linear direct modeling based on data correction (beam-hardening and scatter). We introduced a new beam-hardening correction without knowledge of the spectra of X-ray source and the linear attenuation coefficients of the materials and a new scatter estimation method based on the measurements as well. Later, we continued to study the iterative approaches of dose reduction since the over-exposition or unnecessary exposition of irradiation during a CT scan has been increasing the patient's risk of radio-induced cancer. In practice, it may be useful that one can reconstruct an object larger than the field of view of scanner. We proposed an iterative algorithm on super-short-scans on multiple scans in this case, which contain a minimal set of the projections for an optimal dose. Furthermore, we introduced a new scanning mode of variant angular sampling to reduce the number of projections on a single scan. This was adapted to the properties and predefined interesting regions of the scanned object. It needed fewer projections than the standard scanning mode of uniform angular sampling to reconstruct the objet. All of our approaches for MAR and dose reduction have been evaluated on real data. Thanks to our MAR methods, the quality of reconstructed images was improved noticeably. Besides, it did not introduce some new artifacts compared to the MAR method of state of art NMAR [Meyer et al 2010]. We could reduce obviously the number of projections with the proposed new scanning mode and schema of super-short-scans on multiple scans in particular case.