To collect Tau PET/CT imaging in older adults diagnosed with Mild Cognitive Impairment (MCI) or Alzheimer's Disease (AD) in the Mismatch Prospective Cohort Study (MPC-Tau) study to determine relationship to clinical, cognitive, and other biomarker data. Findings from this study will likely provide insight into the phenotypic variability of Alzheimer's Disease and other related pathologies.

This is a cross-sectional study using the radiotracer AV-1451 to determine the relationship of tau pathology to other biomarker data of MPC participants. All subjects will already have been enrolled in the MPC Study. For the current protocol, participants will provide informed consent before beginning any study procedures. After screening assessments, participants will undergo PET/CT scan imaging with AV-1451.

Inclusion Criteria:

1. Enrolled in MPC study (Protocol 850160 at UPenn; Protocol 2022P002447 at MGH).
2. Reliable study partner to accompany participant to the PET/CT scan
3. A brain MRI must be performed within 6 months prior to their study AV-1451 TAU PET/CT scan date and be deemed of adequate quality that the scan may be used for study analysis, including 3T high-resolution imaging of medial temporal lobe structures.

Exclusion Criteria:

1. Have any medical or psychiatric conditions that, in the opinion of the investigator, would compromise the subject's safety or successful participation in the study.
2. Have evidence of structural abnormalities such as major stroke or mass on MRI that is likely to interfere with analysis of the PET/CT scan.
3. Inability to tolerate or contraindication to imaging procedures in the opinion of an investigator or treating physician.
4. Have a history of significant ongoing alcohol or substance abuse based on self- report.
5. Female participants of child-bearing age will not be able to participate in this study, determined by self-report.

Population includes individuals with a clinical diagnosis of MCI or possible/probably AD