 The purpose of this 2-part open-label study is to investigate the safety and efficacy of 18FMK-6240 as a Positron Emission Tomography PET imaging agent for quantifying brain burden of neurofibrillary tangle NFT pathology. Brain NFT deposition is a pathology finding in Alzheimers Disease AD, and density of brain NFTs has been shown to correlate with severity of cognitive impairment in AD. The objectives of the study include performing the following with respect to 18FMK-6240 administered as a PET imaging agent: assess safety and tolerability; determine radiation safety profile; determine optimal imaging protocol parameters for quantification of brain NFTs in AD; compare tracer binding in brain PET scans from participants with AD of varying severity, participants with amnestic mild cognitive impairment MCI and healthy elderly participants; and evaluate intra-subject test-retest T-RT variability of tracer uptake in brain regions of interest. The primary efficacy hypothesis is that the intra-subject T-RT variability in brain regions associated with NFT deposition in AD is 10. 