Early Diagnosis of Alzheimer-like Dementia: Benefit of MRI and PET Imaging

The physio-pathology of Alzheimer's disease (AD) remains unknown and there is no cure. Thus, the search for objective markers of preclinical first signs of cognitive impairment, is currently a major public health issue. Early detection of the disease is a major challenge to hope to slow or even stop the neurodegenerative process before the stage of dementia. In AD the investigators observe: * A reduction in the volume of brain hippocampi associated with an alteration of the diffusion of water molecules in the white matter. * A structural brain degeneration coupled with a decrease in cerebral glucose metabolism. Recent publications show that cerebrospinal fluid (CSF)flow is also altered, probably due to dysfunction of the choroid plexus. Hence the potential interest to study is, in addition to conventional imaging, the imaging of CSF dynamics and choroid plexus metabolism. In that aim,the investigators use two imaging modalities: * Magnetic resonance imaging (MRI) is used to assess blood and CSF flow in the brain * Positron emission tomography (PET) is used to assess glucose metabolism in grey/white matter and also in choroid plexus. The investigators expect that, because of choroid plexus atrophy in AD, CSF flow would be altered as well as glucose metabolism dynamic in choroid plexus.