[18F]PI-2620 Phase 3 Histopathological Study

Inclusion Criteria: Only subjects who meet all of the following criteria will be eligible for enrollment into the study: 1. Males and females aged 50 years and over 2. Have a projected life expectancy of ≤ 1 year as determined by the investigator (terminal medical condition including but not limited to end-stage dementia, end-stage congestive heart failure, end-stage chronic obstructive pulmonary disease (COPD), or end-stage cancer) 3. Written informed consent obtained from the subject and/or the subject's legally authorized representative (LAR), as applicable, to consent for study procedures and brain donation (consent consistent with the legal requirements of the State in which the subject dies) 4. Can tolerate study procedures including lying down in PET scanner. The investigator will carefully assess each subject and use medical judgment to determine whether the subject can tolerate the imaging procedure Exclusion Criteria: Subjects will be excluded from the enrollment if they: 1. Are receiving aggressive treatment with life sustaining measures (e.g. receiving chemotherapy; palliative chemotherapy is allowed) 2. Are known to have a structural brain lesion that would interfere either with PET imaging or pathological assessment (e.g. lesions are typically \> 2 cm at their greatest extent and may include stroke, primary or metastatic neoplasm, other tumors or cystic lesions. Subjects with a history of major stroke or traumatic brain injury or other structural lesion as well as cases with a history of primary Central Nervous System (CNS) neoplasm or known metastatic cancer must be discussed with the study sponsor prior to enrollment) 3. Have suspected encephalopathy due to alcoholism or end-stage liver disease 4. Are known to have a Glomerular Filtration Rate below \< 15 mL/min 5. Have received an investigational or approved therapy directly targeting amyloid or tau 6. Are females of childbearing potential who are pregnant, lactating or breastfeeding, or who are not using adequate contraception 7. Have implants such as implanted 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 (in case an MRI is planned to be performed)