Can brain scans unlock the secret to Alzheimer's resilience?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: 1. For all subjects: * Affiliation to a social security insurance or beneficiary * Informed consent form signed by the participant or his / her legal representative * Participants aged 40 to 80 years. 2. Selection of AD-Y group \- In vivo proof of Alzheimer's pathology: * Determination of specific proteins on the cerebrospinal fluid (CSF, a routine care procedure). The values considered pathological (AD) are Aβ1-42 peptide \<500 (μg / ml), and / or tau protein\> 450 and phosphorylated tau protein\> 60, IATI index \<1, tau / Aβ protein ratios \> 1.23 as well as phosphorylated tau protein / Aβ1-42\> 0.211. * And / or a positive PET-amyloid imaging test. * Early-onset episodic memory deficit (\<65 years), progressive onset with evidence of hippocampal amnesic syndrome at neuropsychological assessment. In memory tests, the amnesic hippocampal syndrome is defined by: a deficit of the free recall despite a reinforced encoding, an effectiveness of the indexing or an impairment of the recognition capabilities, the presence of intrusions. The presence during the tests of false memories spontaneous (intrusions) or provoked (false recognitions) is also very contributive to the definition of amnesic syndrome of the hippocampal type. 3. PCA group selection Patients with a clinical and cognitive profile suggestive of PCA, characterized by: * an in vivo proof of the Alzheimer pathology (see selection of the AD-Y group) * a specific impairment of neuro-visual abilities, in the absence of major disorders of episodic memory (hippocampal) and executive functions. Two possible variants: * occipito-temporal variant: visuo-perceptive deficit in the foreground, early onset and progressive worsening; lack of visual identification of objects, symbols, words or faces; * biparietal variant: visuospatial deficit in the foreground, early settlement and progressive worsening; Gerstmann syndrome; Balint syndrome; gestural apraxia; visual-spatial neglect. 4. Selection of the control subjects group * Normal neurological and neuropsychological examinations. * Control subjects will be matched in age to patients. Non-inclusion Criteria: 1. General non-inclusion criteria: * Medical history of torsade de pointe or risk of torsade de pointes * Patient treated with drugs known to lengthen QT (see www.crediblemeds.org) * Contraindication to radiopharmaceutical injection: For precautions of safety of use of the radiopharmaceutical, a blood sample allowing to check the renal and hepatic functions will be realized before imagery. The delay between the sampling and the neuroimaging visit is left to the investigator's discretion based on the patient's biological results. In particular, the glomerular filtration rate will be calculated from the results obtained. In the event of renal insufficiency (GFR 30mL / min / 1.73m2), hepatic insufficiency or any other biological anomaly of grade 3 or higher detected during these analyzes, the participant will not be able to carry out PET imaging. In this case, the results of the analyzes will be sent to the doctor indicated by the participant. This evaluation, which involves a determination of serum creatinine, is part of the standard routine biological assessment performed in the context of cognitive disorders Inability to provide informed consent by participant or legal representative: * Patient deprived of liberty by decision of justice or not benefiting from social cover. * Person in the process of participating in another therapeutic research or in a period of exclusion from another research. * Participants with a contraindication to MRI: pacemaker or cardiac defibrillator, implanted equipment activated by an electrical, magnetic or mechanical system, haemostatic clips of intracerebral aneurysms or carotid arteries , carriers of orthopedic implants. * Contraindication to radiopharmaceutical injection: known hypersensitivity to the active substance or to any of the excipients, renal impairment (GFR 30mL / min / 1.73m2), hepatic insufficiency or any other biological abnormality of grade 3 or higher * Person suffering from claustrophobia. * Pregnancy (for women of childbearing age, a urine pregnancy test will be performed on the day of the inclusion visit and the PET-MRI examination). * Any symptoms or biological values suggestive of a systemic disorder (renal, hepatic, cardiovascular, pulmonary) or any other medical conditions that could interfere with the interpretation of test results or compromise the health of patients. * Person subject to a legal safeguard. 2. Specific non-inclusion criteria for AD-Y and PCA patients: * Sudden appearance of cognitive deficits. * Gait disturbances, convulsions, major behavior modification. * Focal alterations to neurological examination, extrapyramidal signs, hallucinations, fluctuations. cognitive. * Psychiatric, cerebrovascular, metabolic, inflammatory pathology. 3. Specific non-inclusion criteria for control subjects: * Pathological neurological examination * History of neurological disease (in particular ischemic stroke or neurodegenerative disease) or psychiatric illness (particularly severe depression, psychosis, or bipolar illness still requiring drug treatment at the time of inclusion) * Physical affection that is serious or can interfere with cognitive functions.