The purpose of this study is to assess the level of neuroinflammation in Alzheimer Disease subject (mild to moderate) estimated with Binding Potential (BP) of [18F]DPA-714, and its relationship with the kinetics of cognitive decline over a 24-month follow-up period (as assessed by Alzheimer's Disease Assessment Scale-cognitive subscale (ADAS-Cog) and Mini-Mental State Examination (MMSE) scores).

The purpose of this study is to assess the level of neuroinflammation in Alzheimer Disease subject (mild to moderate) estimated with Binding Potential (BP) of [18F]DPA-714 , and its relationship with the kinetics of cognitive decline over a 24-month follow-up period (as assessed by ADAS-Cog and MMSE scores).

(DPA-714 : N,N-diethyl-2-[4-(2-fluoroethoxy)phenyl]-5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-acetamide)

Inclusion Criteria:

• Informed consent
• Age more than 50 years (included)
• necessary knowledge of French (write and oral) to do neuropsychological tests
• Study level upper (or equal) than 7 years (considering first year of grammar-school as start)
• People with Alzheimer Disease defined as National Institute of Neurological and Communicative Disorders and Stroke (NINCDS) and the Alzheimer's Disease and Related Disorders Association (ADRDA) standards : Light to mild AD defined by Mini-Mental State Examination (MMSE) score between 15 and 25 (included)
• Social security affiliation.

Exclusion Criteria:

• MMSE score lower than 15 and upper or equal to 26
• Evolutive disease which could possibly had consequences on central nervous system
• Inflammatory disease or evolutive neoplasia and/or C reactive protein (CRP) upper than 10mg/L
• Chronic use of alchohol and/or drug
• Serious depression defined by Montgomery Asberg Depression Rating Scale (MADRS) score higher than 18
• Surgical or medical condition in the last 3 months
• Long term treatment which could possibly interfere with inflammatory process (especially the month before PET [18F]DPA-714 imaging).
• Treatment by N-Methyl-D-Aspartate antagonist
• Treatment by Minocycline
• Treatment by benzodiazepine (especially the month before PET [18F]DPA-714 imaging) (Zolpidem, zopiclone and loprazolam excepted)
• Anomaly at neurological examination which is not a classical symptom
• Contraindication to magnetic resonance imaging (RMI)
• Florbetapir[18F] hypersensibility
• Participation to an other experimental protocol with drug.
• people under guardianship