Alzheimer's disease FDG-PET Positron emission tomography Alzheimer's dementia Alzheimer’s disease Amyloid-PET Brain connectivity Dementia diagnosis Early-onset Alzheimer's disease Medial temporal lobe dysfunction Microglia activation Mild Cognitive Impairment Alzheimer's disease Mild cognitive impairment PET PET imaging [11C]-(R)-PK11195 PET [18F]-FDG PET [18F]FDG amyloid burden bilingualism brain metabolic connectivity brain reserve cerebral glucose metabolism fluorine-18-fluorodeoxyglucose PET graph theory logopenic variant of primary progressive aphasia neurotransmission non fluent variant of primary progressive aphasia positron emission tomography primary progressive aphasia radiotracers semantic variant of primary progressive aphasia sparse inverse covariance estimation synucleinopathy