<p>This licentiate thesis concerns the development and characterisation of X-ray imaging detector systems. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”.</p><p>With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the picture achieved. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the picture. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the picture.</p><p>The problem involving charge sharing which limits “colour” X-ray imaging is discussed in this thesis. Image quality, detector effectiveness and “colour correctness” are studied on pixellated detectors from the MEDIPIX collaboration. Characterisation measurements and simulations are compared to be able to understand the physical processes that take place in the detector. Simulations can show pointers for the future development of photon counting X-ray systems. Charge sharing can be suppressed by introducing 3D-detector structures or by developing readout systems which can correct the crosstalk between pixels.</p>

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / O incremento na incidÃncia da fluorose dentÃria ocorrido nos Ãltimos tempos, à motivo de preocupaÃÃo dos epidemiologistas em nÃvel nacional e internacional. A diferenciaÃÃo entre as alteraÃÃes do esmalte fluorÃtico e nÃo fluorÃtico leva a importantes decisÃes em epidemiologia e saÃde pÃblica bucal. O objetivo deste estudo à investigar a capacidade dos cirurgiÃes-dentistas do serviÃo pÃblico de saÃde no municÃpio de Fortaleza, em diagnosticar a fluorose dentÃria, frente Ãs diversas afecÃÃes do esmalte dentÃrio, seus graus de severidade e a conduta terapÃutica apropriada a cada caso. Utilizou-se para isso um questionÃrio com questÃes semi-estruturadas, o qual foi aplicado a 200 cirurgiÃes-dentistas, de um universo de 527 profissionais do serviÃo pÃblico de saÃde de Fortaleza. O questionÃrio foi aplicado no local de trabalho dos participantes, cujas respostas se basearam na apresentaÃÃo de 20 fotografias digitalizadas de dentes com alteraÃÃes prÃprias do esmalte. A presenÃa ou ausÃncia de fluorose dentÃria e seus graus de severidade foram determinados pelo Ãndice de Dean. NÃo foram detectadas diferenÃas significativas no nÃvel de acerto do grau de fluorose, quando os respondentes foram classificados segundo a instituiÃÃo de graduaÃÃo (p>0,05). No entanto, aqueles profissionais menos experientes apresentaram melhor desempenho (p<0,05), tanto no diagnÃstico de fluorose, quanto na conduta mas nÃo no grau de severidade da doenÃa. Os profissionais gerenciados pela Secretaria Municipal de SaÃde obtiveram desempenho superior aos gerenciados pelo Estado, em todas as percepÃÃes do teste. Quanto ao agrupamento da amostra por especialidades, nenhuma diferenÃa significativa (p>0,05) foi encontrada entre os Grupos. A mÃdia de acertos no diagnÃstico diferencial da fluorose dentÃria foi de 7,70  0,15. Valores inferiores a 30% de acertos foram obtidos no diagnÃstico do grau de severidade da fluorose, destacando-se as formas mais graves como de maior percentual de acerto. A maioria (90%) dos respondentes admitiram ter sentido dificuldades no diagnÃstico da fluorose, apesar de 75% ter recebido informaÃÃo acerca do assunto no curso de graduaÃÃo, porÃm consideraram escassa e deficiente (79,5%). A mÃdia de acertos da conduta clÃnica foi considerada muito baixa (2,71  0,76). Cerca de 70% dos inquiridos solicitaram curso de capacitaÃÃo acerca do diagnÃstico diferencial da fluorose dentÃria. A partir desses resultados, conclui-se que os cirurgiÃes-dentistas que atuam no serviÃo pÃblico de saÃde de Fortaleza apresentaram um baixo nÃvel de conhecimento e falta de preparo adequado para o diagnÃstico da fluorose dentÃria, a percepÃÃo dos seus graus de severidade e definiÃÃo da conduta clÃnica apropriada. Sugere-se que estudos mais abrangentes sejam conduzidos a fim de identificar as carÃncias do setor pÃblico de saÃde, em todos os nÃveis de atenÃÃo / Increasing levels of dental fluorosis are a major concern to public health authorities. The ability to discriminate fluorosis from other enamel modifications is an important factor to support decision makers in epidemiology and oral health. The main objective of this research is to investigate the ability of the odontologists working for the public health services in Fortaleza, to discriminate fluorosis from other enamel alterations, the severity of the disease and the correct clinical approach. Two hundred dentists out of 527 professionals of the public health services answered a semi-structured survey, based on the evaluation of 20 digitalized pictures showing enamel modifications. The presence or absence of fluorosis, as well as the severity of the disease were determined according to the Deanâs index. We were unable to detect an effect of the school of graduation (p>0.05) on the ability to diagnose fluorosis. However, the youngest, less experienced dentists performed better (p<0.05) for both fluorosis diagnosis and to determine to adequate clinical approach for each case. Also, odontologists working in the city health services were more likely to get higher scores compared to the ones working for the state services, with an average of correct answers for differential diagnosis of 7.70  0.15. Although a few subjects achieved less than 30% of correct answers, pictures displaying more severe cases of fluorosis were more likely to result in correctly answered questions. The vast majority (90%) of the subjects reported to have a poor ability do diagnose fluorosis, although 75% had received information about that disease during undergraduation. The odontologists also displayed a very poor performance (2,71  0,76) in defining the best clinical approach, and about 70% of them asked for specific training for correctly diagnose dental fluorosis. Based on the above results, we conclude that most dental surgeons working for the public health services in Fortaleza displayed a poor knowledge about dental fluorosis and lack an adequate training to diagnose and treat that disease. Similar researchs in other regions of Ceara should be performed in order to better identify the deficiencies of the public health services, at different levels

This thesis concerns the development and characterisation of X-ray imaging systems based on single photon processing. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”. With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the image. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the image. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the image. This thesis presents characterisation and simulations to provide a detailed understanding of the physical processes concerning charge sharing in detectors from the MEDIPIX collaboration. Charge summing schemes utilising pixel to pixel communications are proposed. Charge sharing can also be suppressed by introducing 3D-detector structures. In the next generation of the MEDIPIX system, Medipix3, charge summing will be implemented. This system, equipped with a 3D-silicon detector, or a thin planar high-Z detector of good quality, has the potential to become a commercial product for medical imaging. This would be beneficial to the public health within the entire European Union. / Denna avhandling berör utveckling och karaktärisering av fotonräknande röntgensystem. ”Färgröntgen” öppnar nya perspektiv för medicinsk röntgendiagnostik och även för materialröntgen inom industrin. Skillnaden i absorption av olika ”färger” kan användas för att särskilja olika material i ett objekt. Färginformationen kan till exempel användas i sjukvården för att identifiera benskörhet. Färgen på röntgenfotonen kan identifieras om detektorsystemet kan detektera varje foton individuellt. Sådana detektorsystem kallas ”fotonräknande” system. Med modern teknik är det möjligt att konstruera fotonräknande detektorsystem som kan urskilja detaljer ner till en upplösning på circa 50 µm. Med så små pixlar kommer ett problem att uppstå. I en halvledardetektor ger varje absorberad foton upphov till ett laddningsmoln som bidrar till den erhållna bilden. För höga fotonenergier är storleken på laddningsmolnet jämförbar med 50 µm och molnet kan därför fördelas över flera pixlar i bilden. Laddningsdelning är ett centralt problem delvis på grund av att bildens upplösning försämras, men framför allt för att färginformationen i bilden förstörs. Denna avhandling presenterar karaktärisering och simulering för att ge en mer detaljerad förståelse för fysikaliska processer som bidrar till laddningsdelning i detektorer från MEDIPIX-projekter. Designstrategier för summering av laddning genom kommunikation från pixel till pixel föreslås. Laddningsdelning kan också begränsas genom att introducera detektorkonstruktioner i 3D-struktur. I nästa generation av MEDIPIX-systemet, Medipix3, kommer summering av laddning att vara implementerat. Detta system, utrustat med en 3D-detektor i kisel, eller en tunn plan detektor av högabsorberande material med god kvalitet, har potentialen att kunna kommersialiseras för medicinska röntgensystem. Detta skulle bidra till bättre folkhälsa inom hela Europeiska Unionen.

This licentiate thesis concerns the development and characterisation of X-ray imaging detector systems. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”. With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the picture achieved. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the picture. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the picture. The problem involving charge sharing which limits “colour” X-ray imaging is discussed in this thesis. Image quality, detector effectiveness and “colour correctness” are studied on pixellated detectors from the MEDIPIX collaboration. Characterisation measurements and simulations are compared to be able to understand the physical processes that take place in the detector. Simulations can show pointers for the future development of photon counting X-ray systems. Charge sharing can be suppressed by introducing 3D-detector structures or by developing readout systems which can correct the crosstalk between pixels.