Orientador: Diana Rodrigues de Pina / Resumo: A tomografia computadorizada (TC) é amplamente utilizada no diagnóstico e estadiamento de patologias da região de abdômen-pelve devido a sua alta sensibilidade e especificidade. A possibilidade de adquirir maior número de imagens em menor tempo e a maior disponibilidade de equipamentos levaram a um aumento significativo dos exames de TC e consequente aumento das doses efetivas globais fornecidas por esta modalidade. Desta forma, foram desenvolvidas ferramentas que buscam reduzir as doses de radiação dos exames sem perda da qualidade da imagem. Uma destas ferramentas é a modulação automática da corrente do tubo (automatic tube current modulation – ATCM), que permite a obtenção de exames que concordam com o princípio ALARA (as low as reasonably achievable). O objetivo deste estudo foi avaliar a qualidade da imagem e as doses de radiação de diferentes protocolos de TC de abdômen-pelve com a ferramenta ATCM. Foram avaliados cinco protocolos de TC de abdômen-pelve com a ferramenta ATCM em dois tomógrafos distintos, um 16-canais e um 64 canais. Foi utilizado um fantoma antropomórfico para avaliações dosimétricas e um fantoma analítico para avaliações objetivas de qualidade da imagem. Para a análise subjetiva da qualidade da imagem, foram utilizados 15 exames retrospectivos de pacientes submetidos a TC de abdômen-pelve. Estes exames foram avaliados por um radiologista com experiência na área de tomografia de abdômen. As três análises forneceram informações que possibilitaram a escol... (Resumo completo, clicar acesso eletrônico abaixo) / Mestre

Background: It is well known that pediatric patients are different from adult patients. Not only are children of a smaller physical size, but their anatomy differs as well. They are also more vulnerable to ionizing radiation than adults are, since their larger attributable life-time risk for cancer. This entitles children as extra radiosensitive patients, and special concern should be taken regarding their radiosensitive organs. Computed tomography (CT) examinations inevitably involve exposure of all skin-deep organs although rarely being objects for the diagnostic task. For example, multiple CT head examinations increase the risk of radiation induced cataract in eye lenses. Absorbed dose to these radiosensitive skin-deep organs should therefore be decreased by available optimization strategies in accordance withthe ALARA principle -as low as reasonably achievable- which guides the process of optimization anddose reduction. Two optimization strategies to decrease absorbed dose to skin-deep organs are Bismuth (Bi) shielding and organ-based tube current modulation (organ-based TCM). Aim: The aim of this work was to assess two dose optimization strategies for decreasing absorbed dose to skin-deep organs in pediatric CT imaging: Bi shielding and organ-based TCM. The specific patient categories chosen were newborn, one year old and five year old. Materials and Methods: Three anthropomorphic phantoms representing newborn, one year old and five year old were scanned with CT protocol parameters selected in accordance with clinical routine for pediatric CT examinations at Karolinska University Hospital in Stockholm. Dose differences from introducing the optimization strategies were obtained by using thermoluminescence dosimeters (TLDs) and metal oxide semiconductor feld effect transistor dosimeters (MOSFETs). Monte Carlo estimated dose values were introduced as a comparison to further establish the validity of the obtained measured values. Results: The benefit in decreased radiation dose to anterior skin-deep organs - when applying the optimization strategies - depended on both body region and body size. Bi shielding was more advantageous the smaller and less attenuating the body was. Organ-based TCM was more advantageous, if an increased dose to posterior organs could be accepted. A less attenuating and smaller phantom did not benefit by organ-based TCM due to increased posterior irradiation. Conclusions: The general conclusion is that the optimal choice of optimization strategy will depend on both body region being scanned and age. Regarding CT head examinations, pediatric patients of ages between newborn and five year old will benefit most by application of organ-based TCM, if an increased dose to backside head can be justified. Regarding CT thorax examinations, newborn and one year old patients will benefit most by application of Bi shielding, while organ-based TCM is preferable for five year old patients.

Syfte: Syftet med detta projekt var att utvärdera kvaliteten på de digitala röntgenbilder som genereras med en On-Board imager® (OBI), monterad på en medicinsk linjäraccelerator på radioterapiavdelningen på Karolinska Universitetssjukhuset. Dessa kilovoltbilder används för att verifiera patientens position på behandlingsbordet, inför behandling, för att reducera risken att friskvävnad bestrålas i onödan. Detta utförs genom en online ortogonal 2D-2D matchningsteknik innan strålningen leveras. Material och metoder: OBI-system, utvecklad av Varian Medical System, erbjuder en protokoll med exponeringsparametrar för att hjälpa användaren att välja lämpliga inställningar för exempelvis lämplig rörström och spänning under bildtagningen de digitala kilovoltbilderna. Under detta projekt skapades flera testbilder tagna med ändrade exponeringsparametrarna i de befintliga protokollen. Dessa testbilders kontrast bedömdes sedan och jämfördes med bilder tagna med de förinställda parametrar från Varians protokoll. Resultat och slutsats: En utvärdering av de experimentella testbilderna, i jämförelse med bilder skapade med det befintliga protokollet, visade att en justering av exponeringsfaktorer inte leder till något signifikant förbättring av bildernas kontrast. / Purpose: The purpose of this project was to evaluate the quality of kilo Voltage- digital radiography images that are provided by an On-Board imager® (OBI) mounted on a medical linear accelerator at the Radiotherapy department of Karolinska University Hospital. The digital kV-images are used to verify patient position on the treatment table, in order to reduce healthy tissue irradiation, using an online orthogonal set of 2D-2D matching technique, before the treatment delivery. Material and methods: The OBI- system elaborated by Varian Medical System offers an exposure factor protocol in order to help users to choice proper exposure factors, as proper tube current and tube voltage, during acquisition of kV-images. In this project several test images were performed by changing the exposure factors. Further the test images quality regarding, quality index contrast, was assessed and compared with the image performed by the existent Varian protocol. Result and Conclusion: Evaluation of the tests images shows that changing of the exposure factors doesn’t result in any significant improvement of kV-images contrast as compared to images created with the existent protocols parameters.

Radiation from Computed Tomography (CT) is now the major contributor to population radiation dose. Despite controversy around the dose-effect relationship of radiation from CT, the linear non-threshold (LNT) theory is endorsed by many authorities, and constitutes the basis of cancer risk estimates. The purpose of this study was (1) a literature review of radiobiological theories, and methods of dose saving stategies in CT; (2) to highlight the importance of dose saving in CT, and to demonstrate how dose can be saved in a radiology department: Following a 40% reduction in reference X-ray tube current for a CT of the urinary tract, the effecitve dose and estimated lifetime attributable risk of incident cancer due to this CT in a group (n=103) were reduced by 37% and 38% in an age and sex-matched group respectively. The literature review showed that the public health implications of CT radiation exposure remain uncertain. / Health Studies / M.A. (Public Health)

Radiation from Computed Tomography (CT) is now the major contributor to population radiation dose. Despite controversy around the dose-effect relationship of radiation from CT, the linear non-threshold (LNT) theory is endorsed by many authorities, and constitutes the basis of cancer risk estimates. The purpose of this study was (1) a literature review of radiobiological theories, and methods of dose saving stategies in CT; (2) to highlight the importance of dose saving in CT, and to demonstrate how dose can be saved in a radiology department: Following a 40% reduction in reference X-ray tube current for a CT of the urinary tract, the effecitve dose and estimated lifetime attributable risk of incident cancer due to this CT in a group (n=103) were reduced by 37% and 38% in an age and sex-matched group respectively. The literature review showed that the public health implications of CT radiation exposure remain uncertain. / Health Studies / M.A. (Public Health)