Assess Safety, Bio-distribution, Radiation Dosimetry and Optimize the Imaging Protocol of GEH120714 (18F) Injection in Healthy Volunteers and Participants With Relapsing and Remitting Multiple Sclerosis (rrMS).

Inclusion Criteria: Inclusion Criteria for all Participants: * The subject has a clinically normal or acceptable medical history and physical examination at screening. * The subject has an acceptable kidney function (eGFR \>60 mL/min/1.73m2) for administration of Gadolinium. * The subject has a trans-locator protein (TSPO) platelet assay conducted with documented results. Inclusion Criteria for all healthy volunteers: * The subject has no clinical history or signs of neurological impairment. * The subject has a normal MRI without central white white matter lesions. Inclusion Criteria specific for participants with relapsing and remitting results sclerosis (rrMS): * Subject with a previous diagnosis of rrMS, presenting with (a) clinically active disease at time of screening or, or preferentially, (b) clinically active disease at time of screening and having at least one gadolinium--enhanced lesion on MRI. * The subject at screening has a MRI scan of sufficient quality for Volume of Interest (VOI) definition and co-alignment with Positron Emission Tomography (PET). Exclusion Criteria: General Exclusion Criteria for all participants: * The subject has a contraindication for Magnetic Resonance Imaging (MRI). * The subject has known allergies to Gadolinium contrast agent. * The subject has received significant ionising radiation exposure from clinical trials or medical examinations in the last 12 months. Exclusion Criteria specific for healthy volunteers: * The subject has family history of multiple sclerosis (MS). * The subject is undergoing monitoring of occupational ionising radiation exposure. Exclusion Criteria specific for participants with remitting multiple sclerosis (rrMS): * The subject has a past history of cerebrovascular disease or vasculitis. * The subject has a history of head injury with prolonged coma.