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Authors
Abstract(s)
The human brain undergoes a complex process of development during childhood and adolescence. During this maturation process, microstructural changes can be examined in detail using neuroimaging techniques. In particular, diffusion magnetic resonance imaging (MRI) uses the diffusion of water molecules to generate contrast in magnetic resonance (MR) images. Although this non-invasive medical tool currently shows great promise, allowing different reconstruction methods to quantify the neural environment, their limitations do not allow it to be fully characterized. With this said, it is relevant to incorporate in one analysis different reconstruction algorithms in order to cross different informations and draw more clarifying conclusions. Therefore, the aim of this project is to investigate tissue microstructure during the maturation process in the Vervet monkey brains from childhood to adulthood. For that, DTI(Diffusion Tensor Imaging) and NODDI(Neurite Orientation Dispersion and Density Imaging) models were studied in exvivo data from 25 healthy monkeys. The first model uses the orientation of white matter (WM) fibres in order to estimate diffusion related parameters as fractional anisotropy (FA),mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD).The latter tries to fit a MR signal to biological structures, estimating the morphology and volume of neurites. Regions of interest (ROIs) were manually drawn and tractography was performed in order to visualize and quantify maturation changes in specific pathways through the analysis of these parameters. Besides collecting valuable information on the microstructure features that changed during maturation, information about myelination was possible to acquire due to volume analysis. Volume increase is shown in all fibres and global WM, demonstrating extended myelin production until early adulthood.On the other hand, volume increase in grey matter(GM)was not so abrupt, coherent with its lack of myelin sheets. Regarding changes in the brain’smicrostructure, the WM fibres get a preferred orientation,reflected on the increasing anisotropy. At the same time, fibres’structures get denser,which might lead to a decreased rate of diffusivity. Both the intracellular and the isotropic volumes increase. Using the general linear model (GLM)fromSPSS, it was shown that these microstructure features appear to vary more during the first 12months of age (5yearsinhumans). It was also demonstrated that anisotropy can be defined by intracellular volume and dispersion, concluding that NODDI and DTI should be used together as they complement each other. Lastly, it was discussed that myelination and microstructure processes might occur in the same timescale as their trends are similar. These findings enrich the understanding of the maturation on the brain and its changing features from childhood to adulthood in monkeys.
Description
Tese de mestrado integrado, Engenharia Biomédica e Biofísica (Biofísica Médica e Fisiologia de Sistemas) Universidade de Lisboa, Faculdade de Ciências, 2018
Keywords
Maturação IRM de difusão DTI NODDI Tratografia Teses de mestrado - 2018