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Evaluation par IRM multimodale des modifications cérébrales chez des patients Alzheimer à un stade prodromique : optimisation de la relaxométrie T2* par IRM

Abstract : One of the main goals of modern neuroimaging is the identification of new markers that can help in the diagnosis and monitoring of neurological pathologies. Multimodal magnetic resonance imaging (MRIm), is an approach allowing the evaluation of several complementary biomarkers within one MRI. This approach has already demonstrated its efficiency in several recent studies, and in particular in Parkinson's disease. We added a new biomarker to the MRIm approach previously used i.e. shape changes of subcortical structures based on T1 images. This marker is now a part of our MRIm approach along with: (i) volumetry from T1 images, (ii) microstructural integrity and orientation from diffusion images and (iii) metal deposits from T2* relaxometry. We applied this multimodal MRI approach to an other neurodegenerative disease, the Alzheimer's disease at a prodromal stage. Results of this preliminary study gave us the opportunity to suggest the existence of two different physiopathological processes at the prodromal phase of the Alzheimer's disease. In fact we observed atrophy with modification of the microstructural integrity for the hippocampus and the amygdala, while only atrophy has been observed for the thalamus and the putamen. Those results also confirmed the necessity of studying neurodegenerative diseases in a multimodal way. Among MRIm markers, the T2* relaxometry for the quantification of intracerebral iron is one of the methods which has been developed lately at the Inserm U825. Dysregulation of iron metabolism and its accumulation are involved in the physiopathology of several neurodegenerative diseases like Parkinson's disease. The experience gained through the different clinical validation of this method in recent years has led us to improve it. Our work was to improve T2* relaxometry by optimizing the acquisition of the images on one hand, and the processing of the images on the other hand. We compared several resolutions, acquisition antennas, number of acquisition by echo time, to determine which parameters gave the higher signal to noise ratio. For the part about the process of the images, we compared the method used as a reference, the least square method using a Levenberg-Marquard algorithm, to an other method, the singular value decomposition to obtain the best estimation of the relaxation rate R2*. Then we were able to develop an optimized T2* relaxometry sequence, which we compared to the one used in the first study, but in the physiological ageing model. Finally in addition to allowing discrimination between elderly and young people, the results obtained with this new sequence were found to be much less sensitive to noise.
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Pierre Eustache. Evaluation par IRM multimodale des modifications cérébrales chez des patients Alzheimer à un stade prodromique : optimisation de la relaxométrie T2* par IRM. Sciences agricoles. Université Paul Sabatier - Toulouse III, 2015. Français. ⟨NNT : 2015TOU30222⟩. ⟨tel-01361184⟩

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