Approximately 50 participants enrolled in the LEARN study (NCT02488720) will be enrolled in the LEARN-Tau study. The study designed to evaluate the imaging characteristics of 18F-AV-1451 in participants enrolled in the LEARN study and to expand the safety database of 18F-AV-1451.

The study will run in parallel to the LEARN study. In this study, participants will undergo up to four (4) 18F-AV-1451 PET scans over a 4.5 year period. Imaging visits will occur at the throughout the participant's participation in the LEARN study (corresponding to LEARN Visit 1, between Visit 4 and 6, Visit 8, and Visit 11).

The LEARN-Tau study will (1) look at change in the amount of tau protein in the brain over time, measured by the 18F-AV-1451 PET scan, (2) see if tau protein in the brain of older individuals is associated with memory problems and (3) evaluate the safety of 18F-AV-1451 and any side effects that might be associated with it.

Site investigators, participants, and study partners will not be informed of the results of the 18F-AV-1451 PET scan results as they relate to the study; however, any findings that may be of potential medical concern will be provided for appropriate follow-up.

Participants should meet inclusion and exclusion criteria for the LEARN study, and in addition:

Inclusion Criteria:

• Male or female that have consented and are currently enrolled in the LEARN protocol;
• Participants who sign an IRB approved informed consent form prior to any study procedure; and
• Participants who in the opinion of the investigator can tolerate the PET scan procedures.

Exclusion Criteria:

• Has any condition that, in the investigator's opinion, could increase risk to the participant, limit the participant's ability to tolerate the experimental procedures, or interfere with analysis of the data. (For example, participants with chronic back pain might not be able to lie still during the scanning procedures.);
• Has abnormal findings on physical examination or laboratory screening tests that suggest the participant might have a condition that could, in the opinion of the investigator, affect his or her response to the radiopharmaceutical and related testing procedures;
• Is deemed likely to be unable to perform all of the imaging procedures for any reason;
• Has a history of risk factors for torsades de pointes, including clinically significant findings on ECG, or is taking medications known to prolong QT interval such as citalopram ≥ 40 mg/day, disopyramide, dofetilide, ibutilide, procainamide, quinidine, sotalol, or bepridil; A list of restricted medications will be provided.
• Are females of childbearing potential (extremely unlikely in the LEARN eligible population ages 65 to 85) who are not surgically sterile, not refraining from sexual activity or not using reliable methods of contraception. Females of childbearing potential must not be pregnant (negative serum or urine β-hCG at the time of screening and negative serum or urine β-hCG on imaging day) or breastfeeding at screening. Females must agree to avoid becoming pregnant, and both females and males must agree to refrain from sexual activity or to use reliable contraceptive methods for 24 hours following administration of 18F-AV-1451 Injection; Males with female partners who are pregnant or of childbearing potential must agree to refrain from sexual activity for 24 hours following administration of 18F-AV-1451 Injection. Additionally, males must agree not to donate sperm for 24 hours following administration of 18F-AV-1451 Injection;
• Has hypersensitivity to 18F-AV-1451 or any of its excipients.