This study is being done to determine the relationship between tau tangles and cognitive impairment in elderly subjects with different clinical and biomarker profiles of Alzheimer's disease (AD). Subjects will undergo screen that includes neuropsychological testing and brain MRI. This study uses a special type of scan called a PET scan to take pictures of the brain. During the PET scan, a special radioactive dye called 18F-MK-6240 is injected into the body. 18F-MK-6240 sticks to abnormal tangles made of the protein tau. Subjects will have the option to have lumbar puncture performed to measure CSF concentrations of biomarkers.