The purpose of this research study is to determine the safety of a radiotracer 18F-Fluselenamyl using positron emission tomography (PET) imaging.

The investigators will first complete whole-body PET dosimetry studies in healthy adult normal volunteers to calculate the actual radiation dose of each human organ and determine the allowable dose for a human subject when receiving a single dose for a PET scan.

Second, imaging of the brain and neck will be completed in a wide range of ages of healthy adult normal control participants and participants with mild cognitive impairment, both male and females to characterize 18F-Fluselenamyl uptake in the brain, its binding to beta-amyloid plaques, and radiolabeled metabolite will be completed. Amyloid is a protein related to dementia of Alzheimer's disease. 11C-PIB PET imaging along with MRI will also be completed in the same participants and the data will be compared with 18F-Fluselenmayl. 11C-PIB and 18F-Fluselenamyl both bind to beta-amyloid plaques.

Finally, a comparison of the normal control participants to patients with Alzheimer's disease will be completed.

Inclusion Criteria:

• Male or Female, any race
• Age ≥ 18 years
• Healthy volunteers or volunteers with Alzheimer's disease

Exclusion Criteria:

• Has hypersensitivity to 18F-Fluselenamyl or any of its excipients ;
• Has hypersensitivity to 11C-PIB or any of its excipients ;
• Incapable of providing written informed consent or lacking a legally authorized representative (LAR) to provide informed consent ;
• Unwilling or unable to undergo PET scans tracer injections ;
• Unwilling or unable to undergo MRI (Aim 2 and Aim 3);
• Any condition that, in the Investigator's opinion, could increase risk to the participant, limit the participant's ability to tolerate the research procedures, or interfere with the collection/analysis of the data (e.g., renal or liver failure, advanced cancer);
• Women who are currently pregnant or breast-feeding;
• Current or recent (within 12 months prior to screening) participation in research studies involving radioactive agents such that the total research-related radiation dose to the participant in any given year would exceed the limits set forth in the U.S. Code of Federal Regulations (CFR) Title 21 Section 361.1.

Aim 1: Dosimetry - 8 healthy adult normal volunteers (4 Males, 4 Females) will undergo whole-body PET/CT imaging for assessing the safety, dosimetry, and metabolism of 18F-Fluselenamyl.

Aim 2: Proof of Concept- 36 participants (18 healthy and 18 participants with mild cognitive decline)will undergo 18F-Fluselenamyl imaging of the brain and neck, 11C-PIB imaging of the brain and neck, MRI, and Cognitive testing.

Aim 3: Performance group- Aim 2 participants will be invited for additional imaging.

Aim 3A- 10 participants from Aim 2 will undergo repeat 18F-Fluselenamyl imaging ~ 1 month after baseline imaging Aim 3B- 18 participants from Aim 2 will have a longitudinal follow-up visit ~ 18 months after the initial study. They undergo repeat 18F-Fluselenamyl, 11C-PIB, MRI, and Cognitive testing.