Eligibility Criteria:
Inclusion Criteria:
* Alzheimer's disease patients will be recruited for this study. The following criteria will be met for inclusion of AD subjects in this study:
* The participant is 50 years or older.
* Written informed consent is obtained.
* Participants have a clinical diagnosis of probable Alzheimer's disease based on National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS/ADRDA) criteria.
* Clinical Dementia Rating Scale score ≤ 2.
* Modified Hachinski Ischemia Scale score of ≤ 4.
* For females, non-child bearing potential or a negative urine or blood pregnancy test on day of \[18F\]-FEPPA injection.
Exclusion Criteria:
* Alzheimer's subjects will be excluded from participation for the following reasons:
* The subject has a history of significant cerebrovascular disease.
* The subject has a clinically significant abnormal laboratory value and/or clinically significant unstable medical or psychiatric illness
* The subject has evidence of clinically significant gastrointestinal, cardiovascular, hepatic, renal, hematological, neoplastic, endocrine, neurological, immunodeficiency, pulmonary, or other disorder or disease.
* Pregnancy
Inclusion Criteria:
* The following criteria will be met for inclusion of PD subjects in this study:
* The participant is 30 years or older.
* Written informed consent is obtained.
* Participants have a clinical diagnosis of Parkinson disease (at least two of the three cardinal symptoms: resting tremor, rigidity, bradykinesia).
* Hoehn and Yahr ≤4.
* For females, non-child bearing potential or a negative urine or blood pregnancy test on day of \[18F\]-FEPPA injection.
Exclusion Criteria:
* Parkinson's subjects will be excluded from participation for the following reasons:
* The subject has a clinically significant abnormal laboratory value and/or clinically significant unstable medical or psychiatric illness
* The subject has evidence of clinically significant gastrointestinal, cardiovascular, hepatic, renal, hematological, neoplastic, endocrine, neurological, immunodeficiency, pulmonary, or other disorder or disease.
* Pregnancy
Inclusion Criteria:
* Multiple Sclerosis Subject Selection. Subjects who have a clinical diagnosis of Multiple Sclerosis (MS) will be recruited for this study. The following criteria will be met for inclusion of MS subjects in this study:
* The participant is 18 years or older.
* Written informed consent is obtained.
* Participants have a clinical diagnosis of Multiple Sclerosis (per the 2005 Revised McDonald Criteria; Polman, et al., 2005).
* Kurtzke Expanded Disability Status Scale (EDSS) ≤ 7.5.
* For females, non-child bearing potential or a negative urine or blood pregnancy test on day of \[18F\]-FEPPA injection.
Exclusion Criteria:
* MS subjects will be excluded from participation for the following reasons:
* The subject has a clinically significant abnormal laboratory value and/or clinically significant unstable medical or psychiatric illness
* The subject has evidence of clinically significant gastrointestinal, cardiovascular, hepatic, renal, hematological, neoplastic, endocrine, neurological, immunodeficiency, pulmonary, or other disorder or disease.
* Pregnancy