The first objective is to asses influence of age on amyloid load measured by PET imaging using Pittsburgh B compound (PiB) radio-tracer, in Alzheimer's disease(AD). This will allow the determination of brains age-specific deterioration factors by comparing Early onset AD (EOAD), Late onset AD (LOAD)and atypical focal cortical AD (PCA and LPA). The amount of brain lesions in AD patients is estimated by:

1. measuring the rate of cortical brain atrophy,
2. FDG imaging of glucose metabolism reflecting neuronal activity, and
3. for patients who benefited from a lumbar puncture; Cortical-spinal fluid (CSF) amounts of amyloïd and tau proteins are measured.

Literature data suggests there are different types of AD depending on their age of onset, called EOAD and LOAD. These two categories are distinguished by the localization of brain atrophy : severe and 'posterior' in EOAD and more 'anterior' in LOAD. Neuro-pathologic data suggests some atypical focal cortical atrophy, characterized by a respect of episodic memory, may be classified within EOAD.

PiB-based PET imaging allows the in-vivo visualization and quantification of amyloïd load.

We want to answer the question whether the amount of amyloïd protein may be lower in LOAD than EOAD in patients showing the same level of dementia, and thus identify ageing-specific cognitive disorders and understand witch factors influence etio-pathology of typical and atypical Alzheimer's disease.

Inclusion Criteria:

• AD patients: clinical dementia rating between 0.5 and 2 free and cued recall test (Grober and Buschke) cued-recall < 18/48 and total recall < 40/48
• atypical AD : i visual-spatial disorder and respect of episodic memory progressive evolution, Balint syndrome ii progressive language disorder constituted of logopenic aphasia respect of episodic memory
• controls: age over 30 MMSE over or equal to 27 normal neuropsychiatric state for age and education level

Exclusion Criteria:

• for every patients : psychiatric disorders age under18 absence of social security counter indication to MRI supposed or actual alcoholism or drug addiction pregnancy

patients have to fulfil criteria for (1)AD with an amnesic syndrome of the media-temporal type, EOAD have lower frequency and (2) and for atypical Ad respecting episodic memory, inaugural language disorder or visual-spatial disorder