This is a phase 1, open-label, single-center, non-randomized single dose study to assess the kinetics, clearance and cerebral distribution of PBR-111 positron emission tomography (PET) imaging in detecting microglial activation in the brain in patients with probable Alzheimer disease (AD) compared to healthy volunteers (HVs). All aspects related to image acquisition, processing and visual as well as quantitative evaluation will be developed, optimized and validated (where required).