La prévalence de changements cérébraux pathologiques dans la population âgée est très élevée, même chez des individus en bonne santé et pleinement autonomes. L’accumulation anormale de la protéine bêta-amyloïde (A), un biomarqueur-clé de la maladie d’Alzheimer (MA), et les hypersignaux de la substance blanche (HSB), qui sont des lésions des petits vaisseaux cérébraux de la substance blanche, sont parmi les pathologies cérébrales liées à l’âge les plus répandues. Un ensemble de preuves scientifiques s’appuyant sur des données neuropathologiques, neuropsychologiques et d’imagerie cérébrale suggère que les personnes âgées cognitivement normales ayant une charge lésionnelle cérébrale importante présentent un déclin cognitif accentué. Ainsi, les changements neuropathologiques chez les aînés en bonne santé sont particulièrement intéressants, car ils constituent des cibles thérapeutiques prometteuses et pourraient contribuer au dépistage précoce de la MA. Cette thèse avait pour but d’examiner les associations entre la charge A, les HSB et l’épaisseur corticale à travers différents domaines de la cognition dans une cohorte de 104 personnes âgées cognitivement normales, en tenant compte de l’âge, du sexe et du niveau d’éducation. Tous les participants ont été soumis à une évaluation neuropsychologique détaillée, ainsi qu’à des examens en résonance magnétique (IRM) structurelle et en tomographie par émission de positons (TEP) avec le Pittsburgh Compound B (PIB). La sévérité des HSB a été quantifiée avec l’échelle Age-Related White Matter Changes (ARWMC). Mis ensemble, les résultats des deux articles empiriques composant cette thèse font ressortir des effets indépendants et additifs de la charge A et du volume de l’hippocampe droit sur la mémoire épisodique, de sorte qu’une pathologie A élevée et un volume plus faible prédisaient des performances moindres. La charge A et les HSB contribuaient de façon additive à une mémoire de travail diminuée et exerçaient des effets délétères synergiques sur la flexibilité mentale et l’attention. Les HSB étaient négativement associés au langage, dont ils étaient le plus important prédicteur. À l’inverse, ni les dépôts A ni les HSB n’étaient corrélés à la mémoire sémantique, la vitesse de traitement de l’information et les fonctions visuospatiales. Dans l’article 1, nous avons démontré que le déclin lié à l’âge au niveau de la mémoire épisodique, la mémoire de travail, la flexibilité mentale et le langage était entièrement médié par le degré de pathologie cérébrale. Dans l’article 2, nous avons démontré qu’il n’y avait pas d’association positive entre les cartes d’épaisseur corticale et les fonctions cognitives, à l’exception de la mémoire sémantique. En outre, la charge A, les HSB et les mesures d’IRM structurelle étaient indépendants les uns des autres, appuyant plutôt la notion de trajectoires physiopathologiques distinctes. Un résultat important de l’article 2 était la relation positive entre la mémoire sémantique et l’épaisseur corticale dans le lobe temporal antérieur (LTA), une région connue pour son rôle unique dans l’intégration des connaissances sémantiques à un niveau transmodal. Les résultats de cette thèse mettent en lumière le rôle médiateur de pathologies cérébrales prévalentes, soit la charge A et les HSB, dans le vieillissement cognitif, et suggèrent que celles-ci induisent des changements cognitifs par le biais de mécanismes physiopathologiques autres que l’atrophie cérébrale. Sur le plan clinique, nos travaux soulignent la pertinence de diversifier les outils d’évaluation utilisés pour le dépistage des troubles cognitifs chez la personne âgée, avec un accent particulier sur la mémoire sémantique. / Compelling evidence shows that pathological changes are highly prevalent in the aging brain, even in otherwise healthy individuals who remain fully functional. Among the most common age-related brain lesions are the abnormal deposition of beta-amyloid (A) peptide, a well-known hallmark of Alzheimer’s disease (AD), and white matter hyperintensities (WMH), which are regarded as a radiological marker of cerebral small vessel disease. Converging evidence from neuropathological, neuropsychological and neuroimaging data suggests that normal older adults harboring high levels of brain pathology exhibit exacerbated cognitive decline. Thus, neuropathological changes in healthy elderly people deserve particular attention as they are potential targets for early intervention and might contribute to the early identification of AD. This thesis sought to examine the associations between Aβ burden, WMH, cortical thickness and cognitive performances across multiple domains in a cohort of 104 cognitively normal older adults, while accounting for age, sex and years of formal education. All participants underwent an extensive neuropsychological assessment along with structural magnetic resonance imaging (MRI) and positron emission tomography (PET) with Pittsburgh Compound B (PIB). WMH severity was assessed using the age-related white matter changes (ARWMC) scale. Combined findings from the two empirical articles making up this thesis demonstrate independent, additive effects of A burden and right hippocampal volume on episodic memory whereby increased A deposition and reduced volume predict decreased performance. A burden and WMH contributed additively to poorer working memory and exerted deleterious synergistic effects on mental flexibility and attention. WMH was the most important predictor of linguistic abilities with higher lesion severity being associated with worse performances on language tasks. Conversely, neither A deposition nor WMH were correlated with semantic memory, processing speed and visuospatial abilities. In article 1, we demonstrated that age-dependent decline in episodic memory, working memory, mental flexibility and language was fully mediated by the extent of brain pathology. In article 2, cognition was not found to be positively associated with cortical thickness in the vertex-wise analyses, except for the domain of semantic memory. Furthermore, A burden, WMH and structural MRI measures were independent of one another, supporting the notion of distinct pathophysiological pathways. A notable finding of article 2 was that thinner cortical thickness in the left anterior temporal lobe (ATL) predicted poorer semantic memory, which is coherent with the role of the ATL in heteromodal semantic processing. The results presented in this thesis shed light on the role of prevalent brain pathologies, namely A burden and WMH, in driving age-related cognitive changes, and suggest that these changes can occur through pathways that are distinct from brain atrophy. Clinically speaking, this work lends support for the inclusion of a wider array of measures to routine screening for cognitive impairment in older adults, with an emphasis on semantic memory.

Introdução: A tomografia por emissão de pósitrons (PET) permite a avaliação in vivo de alvos moleculares em doenças neurodegenerativas, como a doença de Alzheimer (DA). A deposição de placa Beta-amiloide pode ser avaliada por PET 11C-PIB, enquanto o PET 18F-FDG é utilizado para avaliar o metabolismo da glicose cerebral, que pode ser um indicador de lesão neuronal e disfunção sináptica. Além disso, a captação cerebral precoce de radiofármacos de PETamiloide pode determinar o fluxo sanguíneo cerebral regional. Mais estudos correlacionando a fase inicial de perfusão do 11C-PIB (11C-pPIB) e 18F-FDG ainda são necessários, considerando que o fluxo sanguíneo e o metabolismo da glicose cerebral são geralmente acoplados em repouso e durante as ativações neuronais. Objetivo: Avaliar se existe concordância diagnóstica e/ou topográfica entre a imagem na fase de perfusão do 11C-PIB (11C-pPIB), obtida entre 0 e 10 minutos, e a imagem metabólica de PET 18F-FDG através da quantificação por SPM (Statistical Parametric Mapping) e por análise visual, em sujeitos com DA e CCLa comparados aos controles idosos saudáveis. Métodos: CAPEPesq: Nº1.454.598. Noventa e três sujeitos foram alocados em três grupos de acordo com o diagnóstico clínico: doença de Alzheimer (DA - n = 27); Comprometimento Cognitivo Leve amnéstico (CCLa - n = 39); Controle idosos saudáveis (n = 27), estes foram submetidos a exames de imagens de RM ponderada em T1 e de PET/CT. A PET/CT 18F-FDG foi realizada 30 minutos após a injeção do radiofármaco e a PET/CT 11C-pPIB foi adquirida imediatamente após a injeção do radiofármaco, e os primeiros 10 minutos da aquisição foram considerados na análise. Imagens de PET foram corrigidas para efeito de volume parcial e as imagens foram espacialmente normalizadas utilizando um modelo anatômico personalizado da própria amostra (template), para análise por Mapa Estatístico Paramétrico (SPM8). A análise visual e individual foi realizada por dois médicos nucleares com experiência na área, cegos em relação à identificação das imagens, seus respectivos radiofármacos e diagnóstico clínico. Eles foram solicitados a fornecer um diagnóstico e indicar uma classificação com base na inspeção visual das imagens de 18F-FDG e 11CpPIB, e também na avaliação individual dos mapas-t de SPM (análise baseada em voxel comparando um único sujeito do grupo DA com um grupo de controle cognitivamente normal). Resultados e Discussão: Na análise por SPM, o 11CpPIB mostrou menor captação difusa cortical do que 18F-FDG. Na análise entre grupos, há uma diferença na captação de 11C-pPIB e 18F-FDG, o que é esperado, uma vez que a biodistribuição é uma propriedade particular de cada biomarcador de PET. Na comparação do grupo DA em relação ao grupo controle, os indivíduos com DA apresentaram diminuição da captação de 11C-pPIB nas regiões temporo-límbicas: amígdala e hipocampo (E = esquerdo) P = 0,006, amígdala e hipocampo (D = direito) P = 0,023; giro parahipocampal (E) P = 0,008 (D) P = 0,015; temporal superior (E) P = 0,012 (D) P = 0,015. No 18F-FDG, houve diminuição da captação no grupo DA comparado ao grupo controle nas seguintes regiões: córtex do cíngulo posterior (E) P = 0,028; pré-cuneus (E) P= 0,029; giro temporal médio (E) P = 0,039; giro temporal inferior (E) P = 0,044. Na comparação do grupo CCLa em relação ao grupo controle, os indivíduos com CCLa apresentaram diminuição da captação de 11C-pPIB na região do giro parahipocampal (E) P = 0,012. Na identificação visual, 100% das imagens PET 18F-FDG e 99% das imagens PET 11C-pPIB foram corretamente identificadas. Na análise visual e individual, foram observadas reduções na captação de 11CpPIB envolvendo a região temporal medial nos indivíduos com DA que não foi detectada pelo 18F-FDG. Isso poderia significar algum tipo de dissociação entre a perfusão e o metabolismo. Conclusão: Nossos achados sugerem que não há concordância diagnóstica e topográfica perfeita entre a imagem do metabolismo de glicose por PET com 18F-FDG e o padrão de perfusão cerebral usando o marcador PET 11C-PIB em certas estruturas cerebrais em idosos saudáveis, CCLa e pacientes com DA, na quantificação por SPM e na análise visual. Como um biomarcador duplo, a PET 11C-pPIB pode fornecer informações complementares sobre alterações fisiológicas no envelhecimento, e ajudar a elucidar e entender melhor a patologia das doenças relacionadas a memória / Introduction: Positron emission tomography (PET) allows in vivo evaluation of molecular targets in neurodegenerative diseases, such as Alzheimer´s Disease (AD). Beta-amyloid plaque deposition can be assessed by 11C-PIB PET while 18FFDG PET is used to assess cerebral glucose metabolism, which can be an indicator for neuronal injury and synaptic dysfunction. In addition, early cerebral uptake of PET-amyloid radiopharmaceuticals can determine regional cerebral blood flow. More studies correlating early-phase 11C-PIB (11C-pPIB) and 18F-FDG are still needed considering that blood flow and cerebral glucose metabolism are usually coupled at rest and during neuronal activations. The aim of this study is to evaluate topographic similarities and differences between cerebral perfusion images obtained with early 11C-PIB PET images and the metabolic images obtained with 18F-FDG PET. Methods: CAPEPesq: Nº1.454.598. Ninety-three subjects were allocated into three groups according to clinical diagnosis: Alzheimer\'s disease (AD, n=27); Mild Cognitive Impairment amnestic (aMCI, n=39); Elderly healthy control (n=27), they underwent T1-weighted MRI and PET/CT imaging. 18F-FDG PET/CT acquisition was performed 30 minutes after tracer injection and 11C-pPIB PET/CT was acquired immediately after the tracer injection and the first 10 minutes of the acquisition was considered in the analysis. PET images were corrected for partial volume effect and the images were spatially normalized using a custom anatomical template of the sample itself, for analysis by Statistical Parametric Mapping (SPM8). Visual and individual analysis were performed by two experient nuclear medicine physicians, blind in relation to the identification of the images, their respective radiopharmaceuticals and clinical diagnosis. They were asked to provide a diagnosis and to indicate their level of confidence on the basis of visual inspection of 18F-FDG and 11C-pPIB images, and also individual assessment of SPM t-maps (voxel-based analysis comparing a single subject of AD group to a cognitively normal control group). Results and Discussion: In the analysis by SPM, the 11C-pPIB showed lower cortical diffuse uptake than 18F-FDG. In the analysis between groups, there is a difference in 11CpPIB and 18F-FDG uptake, what is expected since biodistribution is a particular propriety of each PET tracer. The control group versus the AD group, individuals with AD presented a decreased 11C-pPIB uptake in the temporo-limbic regions: amygdala and hippocampal (L = left) P = 0.006; amygdala and hippocampal (R = right) P = 0.023; parahippocampal gyrus (L) P = 0.008 (R) P = 0.015; and superior temporal (L) P = 0.012 (R) P = 0.015. In the 18F-FDG, there was a decreased uptake in the AD group compared to the control group in the following regions: posterior cingulate cortex (L) P = 0.028; precuneus (L) P= 0.029; medial temporal gyrus (L) P = 0.039; and inferior temporal gyrus (L) P = 0.044. In the comparison of aMCI group versus the control group, individuals with aMCI presented a decreased 11C-pPIB uptake in the region: parahippocampal gyrus (L) P = 0.012. In the visual identification, 100% of 18F-FDG PET images and 99% of 11C-pPIB PET images were correctly recognized. In the visual and individual analysis, it was observed reductions in 11C-pPIB uptake involving medial temporal region in the AD subjects that was not detected by 18F-FDG.This could mean some kind of decoupling between perfusion and metabolism. Conclusion: Our findings suggest that there is no perfect diagnostic and topographical concordance between the imaging of 18F-FDG PET glucose and the cerebral perfusion pattern using the 11C-PIB PET marker in certain brain structures in healthy elderly, aMCI and patients suggestive of AD, quantification by SPM and visual analysis. As a double biomarker, 11C-PIB can provide complementary information on pathological aging of the brain, and it could help elucidate and better understand the pathology of memory-related diseases