Imaging Study for Multiple Sclerosis with 18F 3F4AP

Inclusion Criteria: * Subjects must be ≥18 and \<65 years of age; * Able to understand and provide informed consent prior to study procedures Exclusion Criteria: * Subjects with known structural brain disease (e.g. brain tumor or stroke); * Any contraindication to MRI and/or PET, including: * Subjects with life vest; * Subjects with implanted heart device (e.g. ICD, Pacemaker); * Subjects with metallic fragment or foreign body; * Subjects with other form of devices or prosthesis that are not MRI compatible, such as insulin pump, joint replacement, hearing aid, cochlear implant, permanent contraceptive devices, etc.; * Subjects with severe claustrophobia * Relative or absolute contraindication to Dotarem contrast: * history of renal disease including acute or chronic severe renal insufficiency (glomerular filtration rate \<60 mL/min/1.73m2); * history of diabetes mellitus, systemic lupus, multiple myeloma, nephrogenic systemic fibrosis, and other co-morbidities; * History of hypersensitive reactions to Dotarem and/or gadolinium contrast agent; * Radiation exposure exceeds current Radiology Department guidelines (i.e., 50 mSv in the prior 12 months); * Female subjects only: Positive serum pregnancy test, or lactating, or possibility of pregnancy cannot be ruled out prior to dosing; * Inability to provide written informed consent; * Any clinically significant acute or unstable physical or psychiatric condition, judged by the investigators based on medical history or screening physical examination, to be incompatible with the study; * Any physical or psychiatric condition judged by the investigators to be incompatible with the study, based on medical history or screening physical examination; * Abnormal results on blood tests judged by the investigators to be incompatible with the study.