At the screening visit, each subject (or caregiver in the case of AD subjects) will be asked to provide written informed consent or assent. During the screening phase (maximum duration - 60 days) subject medical, neurological and surgical history, clinical assessments and a neuro-psychiatric evaluation will be performed on all eligible subjects. Subjects will be allowed to leave the center after all evaluations have been completed. During this period an MRI of the brain will be performed. During the PBR-111 PET imaging day, all subjects will receive a single IV injection of PBR-111 and scanning will be performed over a 3.5 hour period. Each subject will have a telephone follow-up 7 days (+/- 3 days) thereafter to assess for adverse events.