Participants will have a medical history and examination. This will include questions about thinking and memory. Participants will have blood drawn for routine lab tests. Blood will also be tested for a special genetic test to make sure the 11C-PBR28 scan will work. A special genetic blood test will also be done to see if participants have genes that are associated with dementia with Lewy bodies or other causes of dementia. Participants will not receive the results of these special genetic tests. Participants will also have a magnetic resonance imaging (MRI) scan of the brain.