A New Zealand Qualified Health Physicist (QHP) is required, under the Radiation Protection Act of 1965 and the Radiation Protection Regulations 1982, to perform auditing compliance tests on x-ray equipment at regular intervals to ensure that this equipment conforms to the Code of Safe Practice with the use of x-rays. The protocol for these tests must be approved by the National Radiation Laboratory (NRL). One of these test protocols sets out the requirements for the functionality testing of the x-ray machine Automatic Exposure Control (AEC). The current NRL protocol for AEC testing is based on the radiographic film environment (NRL C5 1994). This protocol was tested to determine its applicability to the digital computed radiography (CR) imaging systems which are replacing screen-film systems. To begin this process a comparison of the different exposure indexes for each image medium was required. This proved to be achievable using a system of exposure dose comparison. The AEC test process for both image modalities follows identical requirements but differ slightly in the test methods used to achieve these. The most significant finding throughout this stage was not the differences between protocols but was the requirement to achieve consistent exposure index values over the clinical kVp range for each image medium. This requirement, applicable to any x-ray image medium, became the focus of this thesis. The thesis has explored through experimentation, the effect of optimisation of AEC kVp compensation for the variable kVp response of an image medium, on image consistency. At Christchurch Hospital where this investigation took place the work has shown that the performance of AEC devices can be optimised to improve image consistency, indicated by a more consistent exposure index over the clinical kVp range. The optimisation process also achieves a more consistent dose response to the image plate. A dose variation of 8.3% from the average was achieved compared to 26% in the unoptimised version. No clinically significant changes to image quality were apparent in test images. Under these conditions it was found that AEC functionality could be assessed solely by the measurement of AEC dose to the image plate (IP). Use of this test method provides quantifiable time management benefits for the Medical Physicist and for the radiology departments in which they work. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1369625 / Thesis (M.Sc.(Med.Physics)) - University of Adelaide, School of Chemistry and Physics, 2009

To enhance test security of high stakes tests, it is vital to understand the way various exposure control strategies function under various IRT models. To that end the present dissertation focused on the performance of several exposure control strategies under the generalized partial credit model with an item pool of 100 and 200 items. These procedures are relatively easy to implement and have shown promise as an alternative to more complex exposure control strategies. Through unique algorithms these procedures select an item for administration from a subset of items in the item pool. The five procedures examined for efficacy were the modified within .10 logits, restricted modified within .10 logits, randomesque, restricted randomesque, and progressive restricted procedures. The modified within .10 logits, restricted modified within .10 logits, and randomesque, and restricted randomesque procedures select an item for administration from a subset of optimal items. To test the effect of the number of items available for selection in this subset, 3, 6, and 9 items were made available for selection in these procedures. Maximum information item selection was used as a base line, no exposure control, condition. The progressive restricted, restricted randomesque, and restricted modified within .10 logits procedures were found to optimally protect test security while not significantly degrading measurement precision. The restricted forms of the randomesque and modified within .10 logits procedures proved superior to their base procedures, particularly in controlling average maximum exposure rate. The incrementation of item group size in the modified within .10 logits, restricted modified within .10 logits, and randomesque, and restricted randomesque procedures demonstrated that increasing the item group size provided better test security while not significantly degrading measurement precision. Additionally, in general, the increase of the item pool size from 100 to 200 improved measurement precision and test security. Implications towards practical application are discussed and directions for future research are suggested. / text

Made available in DSpace on 2015-03-05T13:59:44Z (GMT). No. of bitstreams: 0 Previous issue date: 28 / Nenhuma / Este trabalho apresenta Flexibel Secure Service Discovery (FSSD), um protocolo para a descoberta de serviços em sistemas ubíquos. Seu projeto é centrado no compromisso entre os níveis de colaboração, segurança e privacidade que os participantes desejam na descoberta. A abordagem proposta oferece gerenciamento de confiança, além de mecanismos de controle de exposição e de acesso descentralizados. As propriedades do protocolo foram avaliadas através de simulações, variando-se os níveis de segurança e privacidade do sistema para demonstrar que a abordagem proposta lida adequadamente com o compromisso em relação à colaboração entre pares / This work presents Flexibel Secure Service Discovery (FSSD), a protocol for service discovery in ubiquitous systems. Its design is centered by the participants. The proposed approach provides trust management, in addition to descentralized mechanisms to control the exposure and access to the service information. The protocol properties were evaluated with simulation, by varying both security and privacy levels of the system in order to demonstrate that the proposed approach properly addresses the tradeoff regarding peer collaboration

Mammography screening aims to reduce the number of breast cancer deaths by early detection of the disease, which is one of the leading causes of deaths for middle aged women in the western world. The risk from the x-ray radiation in mammography is relatively low but still a factor in the benefit-risk ratio of screening. The characterization and optimization of a digital mammography system is presented in this thesis. The investigated system is shown to be highly dose efficient by employing a photon counting detector in a scanning multislit geometry. A novel automatic exposure control (AEC) is proposed and validated in clinical practise. The AEC uses the leading detector edge to measure the transmission of the breast. The exposure is modulated by altering the scan velocity during the scan. A W-Al anode-filter combination is proposed. The characterization of the photon counting detector is performed using the detective quantum efficiency. The effect of the photon counting detector and the multislit geometry on the measurement method is studied in detail. It is shown that the detector has a zero-frequency DQE of over 70\% and that it is quantum limited even at very low exposures. Efficient rejection of image-degrading secondary radiation is fundamental for a dose efficient system. The efficiency of the scatter rejection techniques currently used are quantified and compared to the multislit geometry. A system performance metric with its foundation in statistical decision theory is discussed. It is argued that a photon counting multislit system can operate at approximately half the dose compared to several other digital mammography techniques. / QC 20100825

It is not uncommon to use unidimensional item response theory (IRT) models to estimate ability in multidimensional data. Therefore it is important to understand the implications of summarizing multiple dimensions of ability into a single parameter estimate, especially if effects are confounded when applied to computerized adaptive testing (CAT). Previous studies have investigated the effects of different IRT models and ability estimators by manipulating the relationships between item and person parameters. However, in all cases, the maximum information criterion was used as the item selection method. Because maximum information is heavily influenced by the item discrimination parameter, investigating a-stratified item selection methods is tenable. The current Monte Carlo study compared maximum information, a-stratification, and a-stratification with b blocking item selection methods, alone, as well as in combination with the Sympson-Hetter exposure control strategy. The six testing conditions were conditioned on three levels of interdimensional item difficulty correlations and four levels of interdimensional examinee ability correlations. Measures of fidelity, estimation bias, error, and item usage were used to evaluate the effectiveness of the methods. Results showed either stratified item selection strategy is warranted if the goal is to obtain precise estimates of ability when using unidimensional CAT in the presence of two-dimensional data. If the goal also includes limiting bias of the estimate, Sympson-Hetter exposure control should be included. Results also confirmed that Sympson-Hetter is effective in optimizing item pool usage. Given these results, existing unidimensional CAT implementations might consider employing a stratified item selection routine plus Sympson-Hetter exposure control, rather than recalibrate the item pool under a multidimensional model.

Orientadores: Plauto Chrisptopher Aranha Watanabe, Deborah Queiroz de Freitas / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Odontologia de Piracicaba / Made available in DSpace on 2018-08-22T04:06:31Z (GMT). No. of bitstreams: 1 Vasconcelos_TaruskaVentorini_M.pdf: 4859018 bytes, checksum: c363a84d1cdfd032d7453c1b0626f663 (MD5) Previous issue date: 2013 / Resumo: Imagens por tomografia computadorizada de feixe cônico (TCFC) são frequentemente utilizadas; no entanto, seu uso envolve doses de radiação que não podem ser indiscriminadas. A redução da miliamperagem (mA) é um dos meios mais práticos para se reduzir a dose de radiação; porém, ainda não se sabe o efeito isolado de protocolos de baixa mA na qualidade das imagens, fundamental para uma correta análise dessas. Com isso, o objetivo desse estudo foi avaliar a influência da variação da miliamperagem na qualidade da imagem de TCFC de diferentes regiões ósseas de mandíbulas edêntulas. Os exames tomográficos foram realizados em oito mandíbulas secas, utilizando os diferentes valores de miliamperagem disponíveis (2, 4, 6,3, 8, 10, 12 e 15 mA), em um aparelho de tomografia computadorizada de feixe cônico (K9000, Kodak). Após a obtenção das imagens, os cortes parassagitais das regiões correspondentes às de incisivos, caninos, prémolars, 1º molares e 2º molares foram avaliados por três radiologistas, de duas maneiras. Inicialmente, foi realizada uma avaliação subjetiva da qualidade, na qual foi adotada uma escala de quatro pontos (0 a 3), de acordo com a visibilidade das estruturas; posteriormente, os avaliadores realizaram uma avaliação objetiva, através da obtenção de medidas nas imagens, as quais foram posteriormente comparadas com medidas obtidas diretamente nas mandíbulas. Vinte e cinco por cento das imagens foram reavaliadas pelos examinadores, após 30 dias, para avaliação da concordância intraexaminador. Os resultados intra e interexaminador variaram entre bom a muito bom para avaliação da qualidade da imagem e foram excelentes quando foram avaliadas as medidas. Após a aplicação do teste estatístico de Tukey, foi possível observar que existiu uma influência da corrente utilizada na qualidade da imagem; entretanto, essa influência foi limitada ao uso de mA baixas (2 e 4). Após a mA 6,3, as imagens apresentaram desempenho semelhante, demonstrando não haver associação linear entre aumento dessa e melhora da qualidade da imagem. As medidas da altura óssea não sofreram influência da mA utilizada; porém, as imagens obtidas com 6,3 mA apresentaram a menor diferença em relação às medidas reais. Assim, pôde-se concluir que a mA 6,3 apresenta um equilíbrio adequado entre dose de radiação e qualidade da imagem no aparelho avaliado, devendo assim ser utilizada, e que o uso de mA maiores deve ser evitado, uma vez que não resultaram em imagens de maior qualidade e, com isso, apresentariam maior dose de radiação sem benefícios para o paciente / Abstract: Despite cone beam computed tomography (CBCT) images are often used; this involves radiation doses that require attention. The milliamperage (mA) reduction is an easy way to reduce radiation dose; however, the isolated effect of low mA settings on image quality is not knowed. This study aimed to evaluate the milliamperage in the image quality of the CBCT in different edentulous bone regions. The examinations were done in eight dry mandibles with different milliamperage values (2, 4, 6.3, 8, 10, 12 and 15) using the KODAK 9000 CBCT unit. The cross-sectional slices of the incisive, canine, premolar, first and second molar were analyzed by three oral radiologists, in two ways. Initially, a subjective evaluation of the visibility of the anatomical structures was performed based on a four grade scale and, afterwards, an objective evaluation through bone height measurement, which was compared with the real measures obtained in the mandible. After 30 days, 25% of the sample was re-evaluated by the observers, in order to obtain the intraobserver reproducibility. The intra and interobserver values varied between good and very good for the image quality evaluation, and they were excellent when evaluating the measurements. The Tukey test showed the influence of the milliamperage in the image quality, but only in the lowest values (2 and 4). The other milliamperage values had a similar performance. The measurements of bone height were not influenced by milliamperage, but the images obtained with 6.3 mA showed the lowest difference compared to the real measurements. Thus, it was concluded that the 6.3 mA provides an appropriate balance between radiation dose and image quality, and therefore the use of higher milliamperage values should be avoided since it does not provide an improvement of the image quality and it uses higher radiation dose without any benefit for the patient / Mestrado / Radiologia Odontologica / Mestra em Radiologia Odontológica

京都大学 / 0048 / 新制・論文博士 / 博士(医学) / 乙第13270号 / 論医博第2184号 / 新制||医||1038(附属図書館) / 京都大学大学院医学研究科内科系専攻 / (主査)教授 溝脇 尚志, 教授 平井 豊博, 教授 伊達 洋至 / 学位規則第4条第2項該当 / Doctor of Medical Science / Kyoto University / DFAM

Modern image guided radiation therapy involves the use of an isocentrically mounted imaging system to take radiographs of a patient's position before the start of each treatment. Image guidance helps to minimize errors associated with a patients setup, but the radiation dose received by patients from imaging must be managed to ensure no additional risks. The Varian On-Board Imager (OBI) (Varian Medical Systems, Inc., Palo Alto, CA) does not have an automatic exposure control system and therefore requires exposure factors to be manually selected. Without patient specific exposure factors, images may become saturated and require multiple unnecessary exposures. A software based automatic exposure control system has been developed to predict optimal, patient specific exposure factors. The OBI system was modelled in terms of the x-ray tube output and detector response in order to calculate the level of detector saturation for any exposure situation. Digitally reconstructed radiographs are produced via ray-tracing through the patients' volumetric datasets that are acquired for treatment planning. The ray-trace determines the attenuation of the patient and subsequent x-ray spectra incident on the imaging detector. The resulting spectra are used in the detector response model to determine the exposure levels required to minimize detector saturation. Images calculated for various phantoms showed good agreement with the images that were acquired on the OBI. Overall, regions of detector saturation were accurately predicted and the detector response for non-saturated regions in images of an anthropomorphic phantom were calculated to generally be within 5 to 10 % of the measured values. Calculations were performed on patient data and found similar results as the phantom images, with the calculated images being able to determine detector saturation with close agreement to images that were acquired during treatment. Overall, it was shown that the system model and calculation method could potentially be used to predict patients' exposure factors before their treatment begins, thus preventing the need for multiple exposures. / Graduate / 0760 / 0574 / 0756

Automatic exposure control (AEC) is effective at reducing potentially harmful radiation doses without sacrificing image quality for many types of computed tomography (CT) scans. However, there is a need for more information regarding the use of AEC for CT head scans. This study was conducted at Johnson County Community Hospital in Mountain City, TN. Preexisting adult CT head scans (n)60 were randomly selected to form 2 stratified samples, (n)30 each. One sample used a standard protocol, and the other used a protocol with a mA-modulated AEC system, Siemens CARE Dose 4D. Causal-comparative analyses were conducted, and it was determined that AEC was effective at maintaining subjective image quality while reducing radiation doses an average of 38% for adult CT head scans. It was concluded that using AEC was an effective tool to optimize radiation doses for adult CT head scans in one particular setting, but more research on this topic is needed.

Until the developments in computer technology, linear test administrations within classical test theory framework is mostly used in testing practices. These tests contain a set of predefined items in a large range of difficulty values for collecting information from students at various ability levels. However, placing very easy and very difficult items in the same test not only cause wasting time and effort but also introduces possible extraneous variables into the measurement process such as possibility of guessing, chance of careless errors induced by boredom or frustration. Instead of administering a linear test there is another option that adapts the difficulty of test according to the ability level of examinees which is named as computerized adaptive test. Computerized adaptive tests use item response theory as a measurement framework and have algorithms responsible for item selection, ability estimation, starting rule and test termination. The present study aims to determine the applicability of computerized adaptive testing (CAT) to Turkish Pupil Monitoring System&rsquo / s (PMS) mathematics assessments. Therefore, live CAT study using only multiple choice items is designed to investigate whether to obtain comparable ability estimations. Afterwards, a Monte Carlo simulation study and a Post-hoc simulation study are designed to determine the optimum CAT algorithm for Turkish PMS mathematics assessments. In the simulation studies, both multiple-choice and open-ended items are used and different scenarios are tested regarding various starting rules, termination criterion, ability estimation methods and existence of exposure/content controls. The results of the study indicate that using Weighted Maximum Likelihood (WML) ability estimation method, easy initial item difficulty as starting rule and a fixed test reliability termination criterion (0.30 standard error as termination rule) gives the optimum CAT algorithm for Turkish PMS mathematics assessment. Additionally, item exposure and content control strategies have a positive impact on providing comparable ability estimations.