Our overall objective is to obtain an initial assessment of the potential value of using \[18F\]3F4AP for imaging demyelinating diseases such as multiple sclerosis: * Aim 1) Assess the safety of \[18F\]3F4AP in healthy volunteers and subjects with multiple sclerosis (MS). Hypothesis 1: Administration of \[18F\]3F4AP will result in no changes in vitals or other adverse events. * Aim 2) Assess the pharmacokinetics of a bolus infusion of \[18F\]3F4AP in humans including healthy volunteers and MS patients. Hypothesis 2: the pharmacokinetics of \[18F\]3F4AP at the whole brain level will be similar in controls and MS subjects. The kinetics in demyelinated lesions will be slower than in healthy control areas. * Aim 3) Assess the reproducibility of \[18F\]3F4AP in humans. Hypothesis 3: the test/retest variability of \[18F\]3F4AP within the same subject will be lower than 10%. * Aim 4) Correlate MR brain images with \[18F\]3F4AP PET brain images. Hypothesis 4A: all the lesions seen on the MRI will show increased signal (VT or SUV) on the PET images. Hypothesis 4B: some of the lesions on the MRI will show increased signal (VT or SUV) on the PET but not all. * Aim 5) Correlate \[18F\]3F4AP PET signal with neuropsychological testing in people with MS. Hypothesis 5: increased PET signal (VT or SUV) will correlate with impaired Single Digit Modality Test (SDMT) scores. * Aim 6) Correlate \[18F\]3F4AP PET signal with EDSS score in people with MS. Hypothesis 6: increased PET signal (VT or SUV) will correlate with higher EDSS scores.