• Change in [18F]MK-6240 uptake [ Time Frame: Baseline to 12 months ]
  Composite and regional SUVR, voxel based statistics or change in tau distribution
• Correlate the changes in [18F]MK-6240 uptake and changes in clinical cognitive assessments by Mini-Mental Status Exam (MMSE) [ Time Frame: Baseline to 12 months ]
  The MMSE is a sensitive, valid and reliable 30-point questionnaire that is used extensively in clinical and research settings to measure cognitive impairment. It is commonly used as screening tool for dementia. It is also used to estimate the severity and progression of cognitive impairment and to follow the course of cognitive changes in an individual over time; thus, making it an effective way to document an individual's response to treatment.
• Correlate the changes in [18F]MK-6240 uptake and changes in clinical cognitive assessments by Clinical Dementia Rating Scale (CDR). [ Time Frame: Baseline to 12 months ]
  CDR was developed primarily for use in persons with dementia of the Alzheimer type. The six domains of CDR are: Memory, Orientation, Judgment and Problem-solving, Community Affairs, Home and Hobbies, and Personal Care. Each domain is rated on a 5-point scale of functioning as follows: 0, no impairment; 0.5, questionable impairment; 1, mild impairment; 2, moderate impairment; and 3, severe impairment (personal care is scored on a 4-point scale without a 0.5 rating available). Each domain is rated on a 5-point scale of functioning as follows: 0, no impairment; 0.5, questionable impairment; 1, mild impairment; 2, moderate impairment; and 3, severe impairment (personal care is scored on a 4-point scale without a 0.5 rating available).
• Correlate the changes in [18F]MK-6240 uptake and changes in clinical cognitive assessments by Alzheimer's Disease Assessment Scale - Cognitive Subscale (ADAS-cog) [ Time Frame: Baseline to 12 months ]
  The ADAS-Cog is one of the most frequently used tests to measure cognition in clinical trials in AD. The ADAS-Cog is a more thorough battery than the Mini Mental State Exam, and primarily measures language and verbal episodic memory. The ADAS-Cog consists of 13 items and takes approximately 25 minutes to administer. The ADAS was developed as a two-part scale: one that measured cognitive functions and one that measured non-cognitive functions such as mood and behavior. Most current research, including this study, uses the ADAS-Cog, which is the sub-scale that measures cognitive ability.

• Change in [18F]MK-6240 uptake [ Time Frame: Baseline to 24 months ]
  Descriptive statistics will be used to describe the tau deposition (e.g. composite and regional SUVR, voxel based statistics or change in tau distribution) using [18F]MK-6240 PET.
• Change in the Cross-sectional comparison of [18F]MK-6240 uptake [ Time Frame: Baseline, 6 months, 12 months, and 24 months. ]
  Change in the cross-sectional comparison of [18F]MK-6240 uptake in Alzheimer's disease, Mild Cognitive Impairment, and healthy volunteers will be studied

• Alzheimer Disease
• Mild Cognitive Impairment

Inclusion Criteria for all subjects:

• Signed and dated written informed consent must be obtained from the subject to enter the study and before any assessment is performed.
• Pregnancy: Participant is not pregnant at the time of the PET and MRI imaging exams. Urine pregnancy tests will be conducted as needed with pre-menopausal women who are of child-bearing potential.
• Willing and able to undergo study procedures and study schedule
• Availability of a study partner who has frequent and sufficient contact with the subject and is able to provide accurate information regarding the subject's cognitive and functional abilities for the CDR, agrees to accompany the subject and provide information at visits or is available by phone. The study partner must have sufficient cognitive capacity, in the judgment of the investigator, to accurately report upon the subject's behavior and cognitive and functional abilities.
• Healthy with no clinically relevant finding on physical examination at screening and upon reporting for the Baseline [18F]MK-6240 imaging visit.

Inclusion Criteria for Healthy Volunteers

• Normal Cognition based on cognitive results at screening.
• Healthy with no clinically relevant finding on physical examination at screening and upon reporting for the Baseline [18F]MK-6240 imaging visit.
• CDR global score =0

Inclusion Criteria for Subjects with a Diagnosis of MCI or Dementia Due to AD

• Have screening [11C]PiB PET imaging demonstrating amyloid binding based on qualitative read or DVR index value >1.20.
• MMSE score 26-30 (inclusive), CDR global score 0.5 for subjects with MCI
• MMSE score 22-26 (inclusive), CDR global score 0.5 or 1 for subjects with mild dementia due to AD
• MMSE score 16-21 (inclusive), CDR global score 1-2 for subjects with moderate dementia due to AD
• Subjects with MCI must meet 2018 research criteria for MCI (Jack et al., 2018).
• Subjects with dementia must meet 2018 research criteria for dementia (Jack et al., 2018).
• A structural brain MRI with no evidence of non-AD disease to account for dementia or MRI exclusion criteria.

Exclusion Criteria for all subjects

• Lack of capacity to provide informed consent at study entry.
• Subject has received an investigational drug or device within 30 days of screening. Other experimental PET radiotracer drugs are not excluded.
• For women, pregnant, lactating or breastfeeding or intention to become pregnant.
• Evidence of unstable or untreated clinically significant gastrointestinal, cardiovascular, hepatic, renal, hematological, neoplastic, endocrine, alternative neurological, immunodeficiency, pulmonary, or other disorder or disease. Stable, treated chronic medical conditions like hypertension, hypercholesterolemia, diabetes mellitus, non-metastatic dermatologic or prostatic cancer, etc. are acceptable as long as they do not, in the study investigator's opinion, contribute to cognitive dysfunction or limit participation in study procedures.
• Any illness or other consideration that makes it unlikely that the subject will be able to complete the 26-month study.
• Current or prior history (within past 5 years) of significant alcohol or substance abuse as determined by the investigator.
• Psychiatric disorders that may interfere with the study including current major Axis I DSM-V disorders including but not limited to severe Major depression, current or history of bipolar I disorder, or schizophrenia.
• Non-English speakers or subjects who are unable to comprehend study materials are excluded at entry
• MRI exclusion criteria include: Findings that may be responsible for neurologic status of the subject such as significant evidence of cerebrovascular disease with multiple infarcts, infectious disease, space-occupying lesion, normal pressure hydrocephalus, CNS trauma, or any other structural abnormality that may impact cognition or image analysis, as judged by the investigator.
• MRI-incompatible implants or devices such as certain cardiac pacemakers or defibrillators, insulin pumps, cochlear implants, metallic ocular foreign body, implanted neural stimulators, CNS aneurysm clips and other medical implants that have not been certified for MRI, or history of claustrophobia in MRI that prevents completion of MRI protocol.
• Treatment with any therapeutic molecule that targets Aβ or tau within 12 months prior to screening.