The specific aims of the study are: Primary: To determine the presence and regional distribution of microglial activation, as assessed by Fluorine-18 (18F) labeled "Peripheral Benzodiazepine Receptor 06" (PBR06) -PET, in subjects with active Relapsing Remitting Multiple Sclerosis (RRMS), Secondary Progressive Multiple Sclerosis (SPMS), and Alzheimer's Disease (AD) as compared to healthy controls Secondary: 1. To assess the relationship between microglial activation and clinical variables including disease severity and comorbidities (such as pain, fatigue and/or depression), as well as clinical MRI findings (such as lesions and atrophy) 2. A pilot substudy aims to establish the non-inferiority of \[F-18\]PBR06 as compared with Carbon-11 \[C-11\] labeled "Peripheral Benzodiazepine Receptor 28" (PBR28) PET in patients with RRMS. Hypothesis: The working hypothesis is that there is microglial activation in multiple sclerosis and Alzheimer's disease as compared to healthy controls and that the pattern/ regional distribution of microglial activation is different in Multiple Sclerosis (MS) versus AD and correlates with disease severity and comorbidities. In addition, the investigators hypothesize that \[F-18\]PBR06-PET scans will be at least as good as \[C-11\]PBR28-PET scans, the current gold standard.
Inclusion Criteria: 1. Male and female subjects age 18 to 70 years. For Alzheimer's Disease patients and healthy controls, the age range is going to be 18 to 85 years. This is because Alzheimer's disease subjects are usually older and it'd be difficult to recruit Alzheimer's patients who are younger than 70 years. Multiple Sclerosis, on the other hand, is a disease of a younger population. 2. For RRMS, it needs to be active, which is defined as at least one relapse in the past 12 months, at least one gadolinium enhancing lesion on MRI within 12 months of enrollment or at least one new FLAIR bright lesion on MRI within 6 months of enrollment. 3. For SPMS, deterioration in EDSS score in the last year is required. 4. For the subjects in the Ocrelizumab arm, only the subjects who have already been prescribed Ocrelizumab by their treating MS neurologist but have not yet started the first infusion, will be included. 5. For the Foralumab arm, patients enrolled in approved IRB protocols (2021P000105, 2022P000005, 2022P001075) will be enrolled. 6. For the steroids arm, only subjects who have already been prescribed steroids by their treating MS neurologist, but have not yet initiated treatment, will be included. 7. AD subjects with MMSE score of 20-26. 8. Subjects willing to undergo PET and MRI imaging 9. Subjects willing and able to give informed consent Exclusion Criteria: 1. Individuals with a known alternate neurologic disorder, previous head injury, or substance abuse. 2. Individuals with bipolar disease and schizophrenia 3. Concurrent medical conditions that contraindicate study procedures. 4. Women who are pregnant or nursing. Also, any woman who is seeking to become pregnant or suspects she is pregnant will be excluded from enrollment. 5. Claustrophobia 6. Non-MRI compatible implanted devices 7. For the Ocrelizumab arm, subjects already on Ocrelizumab will be excluded. 8. For the steroids arm, subjects who have already started steroid treatment will be excluded. 9\. For the Ocrelizumab arm, steroids arm, and longitudinal arm, subjects with abnormal serum creatinine will be excluded.