The diagnosis of Alzheimer's disease (AD) is based on clinical and neuropathological criteria.

Some patients have a contributive CSF biology to determinate a high level risk of AD (Tau + phospho-tau ratio increased and Ab42 / decreased Aβ40), but others have an intermediate CSF biology (Tau and/or phospho-tau increased but Ab42 ratio / normal Aβ40) and are unclassifiable.

18F-Florbetaben (Neuraceq®), a radioisotope in positron emission tomography (PET), selectively binds to amyloid plaques, with high detection sensitivity (98%).

Detection of amyloid plaques by PET imaging separate patients according to the criteria of Dubois, as with AD and allow them to benefit a cholinesterase inhibitor treatment.

If negative, the diagnosis of AD can be excluded with a high level of confidence to prevent initiating unnecessary treatment, expensive for the community.

This is the first imaging study of amyloid plaques targeting population whose diagnosis of AD is uncertain according to the CSF biology.

The aim of this study is to describe the results of amyloid PET in case of intermediate CSF biology and to separate patients as AD or not according to the criteria of Dubois

Inclusion Criteria:

• Major subjects who have given their consent to participate in the study.
• Patients with suspected AD and whose CSF markers give intermediate results (CSF punction less than 6 months): meaning tau and/or phospho-tau increased and Ab42 / Aβ40 normal (Ab42 / Aβ40 > 0.07).
• Non-indication to perform a PET 18F-Florbetaben.
• Affiliation to the french social security .

Exclusion Criteria:

• Patients under guardianship.
• Inability to perform a PET 18F-Florbetaben (agitated patient, confused, ..).
• Pregnancy, lactation.