The present study aims at combining biochemical methods with various types of imaging techniques to identify the pathophysiology of Alzheimer's disease (AD). The main interest is to find markers associated with the very early steps in the pathology of this disease. The investigators shall thus screen for i) molecules in cerebrospinal fluid (CSF) and plasma specific for AD, and ii) brain imaging markers (e.g. MRI and PET) that correlate to detailed clinical assessments.