This study will involve both healthy participants and participants with AN. Study participation for healthy participants will include two PET scans and one magnetic resonance imaging (MRI) scan, which, if the participants prefer, can all be completed in one study visit. Study participation for participants with AN will include three PET and two MRI scans. The first PET and MRI scans will be performed upon entry into the hospital as an inpatient. The remaining scans will be conducted 2 to 4 weeks after participants have accomplished weight restoration.