Longitudinal Evaluation of [18F]GTP1 as a PET Radioligand for Imaging Tau in the Brain of Participants With Alzheimer's Disease Compared to Healthy Participants

Inclusion Criteria: For All Participants: \- Availability of a study partner who, in the investigator's judgment, has frequent and sufficient contact with the participant and is able to provide accurate information regarding the participant's cognitive and functional abilities, agrees to accompany the participant and provide information at visits For Healthy Participants: * Healthy with no clinically relevant finding on physical examination at screening and upon reporting for the Baseline \[18F\]GTP1 imaging visit * Have no cognitive complaint * Have a Clinical Dementia Rating Scale (CDR) global score = 0 * Have a Mini-Mental State Examination (MMSE) score of 28-30 For Participants With a Diagnosis of AD: * Participants with mild or moderate AD must meet National Institute on Aging - Alzheimer's Association (NIA-AA) core clinical criteria for probable AD dementia, with an amnestic presentation * Participants with prodromal AD must meet NIA-AA core clinical criteria for mild cognitive impairment (MCI) * Have screening \[18F\]florbetapir PET imaging demonstrating amyloid binding based on qualitative visual read * A brain MRI consistent with a diagnosis of AD, with no evidence of non-AD disease to account for dementia or MRI exclusion criteria * Medications taken for symptomatic treatment of AD must remain stable for at least 30 days prior to screening visit * Satisfy one of the following subgroups: Approximately 20 prodromal AD (MMSE 24-30, CDR = 0.5); Approximately 20 mild AD (MMSE 22-30, CDR = 0.5 or 1); Approximately 20 moderate AD (MMSE 16-21, CDR = 0.5 or 1 or 2) Exclusion Criteria: * Current or prior history of any drug or alcohol abuse * Participants with any significant psychiatric, neurological, or unstable medical disorder expected to interfere with the study * Participants unable to undergo MRI and PET scan * For participants contributing CSF samples, any contraindication to lumbar puncture * Prior participation in other research protocols or clinical care in the last year such a radiation exposure combined with that from the present study exceeds an effective dose of 50 millisievert (mSV), the allowable annual limit for research participants as stipulated by the Food and Drug Administration (FDA)