The goal of this clinical trial is to evaluate an investigational ultralow dose PET imaging technique for Alzheimer's disease, mild cognitive impairment, other forms of dementia detection and monitoring. The main question it aims to answer is: Can the investigators optimize the timing, scan duration, and image reconstruction to reduce the radiation dose 10-100 fold of the current clinical standard? Participants will be injected with a radioactive tracer called 18F-Florbetapir or 18F-Flutemetamol and be imaged on a new type of high-sensitivity PET scanner for up to 3 hours