If a participant still meet the inclusion and exclusion criteria after these procedures are done, then the participant will have one PET scan using 11C-PBR28. This scan will take place at the Columbia University Medical Center Kreitchman PET Center. On the day of the PET scan you will have an intravenous (IV) catheter (tube) placed in the participant's arm. The participant will also have an arterial catheter placed in an artery of your wrist. Participant will be asked to lay down on a PET scanner. A CT scan will be taken of the participant's head. A small amount of 11C-PBR28 will be injected into the IV. A member of the study team will draw blood from the arterial catheter. The PET scan will last 90 minutes. The participant will be asked to lay still during the scan.