Estudos dosimétricos em radiodiagnóstico

Dissertação para a obtenção do Grau de Mestre em Engenharia BiomédicaA maior contribuição para a dose no conjunto de exposições, da população a fontes de radiação ionizantes produzidas pelo homem, provem de exames médicos de radiodiagnóstico. A frequência de exames radiológicos e a dose de radiação recebida pelos pacientes, em cada exame, tem merecido a atenção dos profissionais de saúde e das entidades de protecção e segurança radiológica. No sentido de determinar as doses associadas aos vários exames e os respectivos procedimentos, foi recentemente emitido pela Agência Internacional da Energia Atómica um código de práticas (TRS 457), útil em laboratórios de dosimetria e em hospitais. Neste contexto, a dosimetria assume uma importância central. A mensuranda é o kerma no ar, e a sua medição é realizada com recurso a instrumentos adequados tais como câmaras de ionização, entre outros, com calibração rastreável a um padrão primário. Em Portugal, o laboratório responsável pelo controlo metrológico de dosímetros é o Laboratório de Metrologia das Radiações Ionizantes (LMRI) do Instituto Tecnológico e Nuclear(ITN). O objectivo principal deste trabalho foi a realização da dosimetria dos feixes das várias qualidades de radiação do radiodiagnóstico, tais como, as RQR e RQA, aplicadas ao diagnóstico convencional, as RQT, aplicadas à tomografia computadorizada, e as qualidades definidas pelo Bureau International dês Poids et Mesures (BIPM), aplicadas à mamografia. Consequentemente foram estabelecidos os procedimentos mais adequados à calibração dos dosímetros utilizados em meio clínico. Foi estudado em particular, o melhor método para a calibração de câmaras de ionização do tipo lápis, dosímetros utilizados na tomografia computadorizada. Os resultados obtidos sugerem que a irradiação da câmara a 50% do seu comprimento sensível, é a melhor opção para a realização da calibração deste tipo de câmaras. Os resultados obtidos permitem ao LMRI estar em condições de participar numa comparação interlaboratorial das qualidades estudadas

Surface-based characteristics of the cerebellar cortex visualized with ultra-high field MRI

Although having a relatively homogeneous cytoarchitectonic organization, the cerebellar cortex is a heterogeneous region characterized by different amounts of myelin, iron and protein expression profiles. In this study, we used quantitative T1 and T2* mapping at ultra-high field (7T) MRI to investigate the tissue characteristics of the cerebellar gray matter surface and its layers. Detailed subject-specific surfaces were generated at three different cortical depths and averaged across subjects to create averaged T1 and T2* maps on the cerebellar surface. T1 surfaces showed an alternation of lower and higher T1 values when going from the median to the lateral part of the cerebellar hemispheres. In addition, longer T1 values were observed in the more superficial gray matter layers. T2* maps showed a similar longitudinal pattern, but no change related to the cortical depths. These patterns are possibly due to variations in the level of myelination, iron and zebrin protein expression

Partial‐volume modeling reveals reduced gray matter in specific thalamic nuclei early in the time course of psychosis and chronic schizophrenia

The structural complexity of the thalamus, due to its mixed composition of gray and white matter, make it challenging to disjoint and quantify each tissue contribution to the thalamic anatomy. This work promotes the use of partial-volume-based over probabilistic-based tissue segmentation approaches to better capture thalamic gray matter differences between patients at different stages of psychosis (early and chronic) and healthy controls. The study was performed on a cohort of 23 patients with schizophrenia, 41 with early psychosis and 69 age and sex-matched healthy subjects. Six tissue segmentation approaches were employed to obtain the gray matter concentration/probability images. The statistical tests were applied at three different anatomical scales: whole thalamus, thalamic subregions and voxel-wise. The results suggest that the partial volume model estimation of gray matter is more sensitive to detect atrophies within the thalamus of patients with psychosis. However all the methods detected gray matter deficit in the pulvinar, particularly in early stages of psychosis. This study demonstrates also that the gray matter decrease varies nonlinearly with age and between nuclei. While a gray matter loss was found in the pulvinar of patients in both stages of psychosis, reduced gray matter in the mediodorsal was only observed in early psychosis subjects. Finally, our analyses point to alterations in a sub-region comprising the lateral posterior and ventral posterior nuclei. The obtained results reinforce the hypothesis that thalamic gray matter assessment is more reliable when the tissues segmentation method takes into account the partial volume effect

The Combined Quantification and Interpretation of Multiple Quantitative Magnetic Resonance Imaging Metrics Enlightens Longitudinal Changes Compatible with Brain Repair in Relapsing-Remitting Multiple Sclerosis Patients

ObjectiveQuantitative and semi-quantitative MRI (qMRI) metrics provide complementary specificity and differential sensitivity to pathological brain changes compatible with brain inflammation, degeneration, and repair. Moreover, advanced magnetic resonance imaging (MRI) metrics with overlapping elements amplify the true tissue-related information and limit measurement noise. In this work, we combined multiple advanced MRI parameters to assess focal and diffuse brain changes over 2 years in a group of early-stage relapsing-remitting MS patients.MethodsThirty relapsing-remitting MS patients with less than 5 years disease duration and nine healthy subjects underwent 3T MRI at baseline and after 2 years including T1, T2, T2* relaxometry, and magnetization transfer imaging. To assess longitudinal changes in normal-appearing (NA) tissue and lesions, we used analyses of variance and Bonferroni correction for multiple comparisons. Multivariate linear regression was used to assess the correlation between clinical outcome and multiparametric MRI changes in lesions and NA tissue.ResultsIn patients, we measured a significant longitudinal decrease of mean T2 relaxation times in NA white matter (p = 0.005) and a decrease of T1 relaxation times in the pallidum (p < 0.05), which are compatible with edema reabsorption and/or iron deposition. No longitudinal changes in qMRI metrics were observed in controls. In MS lesions, we measured a decrease in T1 relaxation time (p-value < 2.2e−16) and a significant increase in MTR (p-value < 1e−6), suggesting repair mechanisms, such as remyelination, increased axonal density, and/or a gliosis. Last, the evolution of advanced MRI metrics—and not changes in lesions or brain volume—were correlated to motor and cognitive tests scores evolution (Adj-R2 > 0.4, p < 0.05). In summary, the combination of multiple advanced MRI provided evidence of changes compatible with focal and diffuse brain repair at early MS stages as suggested by histopathological studies

Portuguese Study Of Mean Glandular Dose In Mammography And Comparison With European References

To characterise the mean glandular dose (MGD) in a sample of healthcare providers for digital mammography in Portugal. To compare the achieved values with European references. The MGD was measured on a poly-methyl-methacrylate phantom (45 mm) for each system using dosimeters. In addition, MGD was estimated using exposure settings collected from mammography exams in clinical context. Data were collected from 25 computed-radiography systems (CR) and 13 integrated digital (DR). For both measurements (phantom and clinical exposures), the average MGD for CR was higher compared to the DR. For CR the mean MGD was 1.85 mGy (CC projection) and 2.10 mGy (MLO projection). For DR systems the corresponding values were 1.54 mGy (CC) and 1.68 mGy (MLO). The average MGD obtained using both methods and for both technologies is within the acceptable reference range proposed by European guidelines (<2.5 mGy). Dose Reference Levels implementation should be the next step to optimise mammography practice in Portugal