Inclusion Criteria: For Parts A & B: - Body mass index (BMI) of 18 to 34 kg/m2, inclusive, and total body weight ≥ 50 kg - Documentation of normal Allen's test result at Screening and on PET scanning days in the arm that will be used for arterial line placement For Part A only: • Healthy male and female participants without clinically significant deviation from normal in medical history, physical examination (PE), electrocardiograms (ECGs), and clinical laboratory determinations For Part B only: - Male or female participant diagnosed with MS according to the 2017 revisions of the McDonald diagnostic criteria - Expanded Disability Status Scale (EDSS) score between 0 to 6.5, inclusive, at Screening Exclusion Criteria: For Parts A & B: - Benign MS defined as a baseline EDSS of 2.0 with MS diagnosis ≥ 10 years prior to Day 1. Spinal MS without clinical or radiological evidence of brain lesions. Any other combination of clinical and radiological data suggestive of an absence of inflammatory brain lesions. - Any major surgery within 4 weeks of study treatment administration and/or any minor surgery within 2 weeks of tracer administration For Part A only: • Any significant acute or chronic medical illness For Part B only: - Any significant acute or chronic medical illness (other than MS) posing a risk to the participant's safety or negatively affecting the ability to detect CNS PET signal - MS relapse within 14 days prior to Day 1. Participants with a MS relapse within 30 days prior to Day 1 must agree to have their second PET scan scheduled on Day 1 or Day 2 Other protocol-defined inclusion/exclusion criteria apply