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: