The HDGECs will perform 2 study visits: Visit 1 (screening) and Visit 2 (PET analysis utilizing the radioligand [18F]MNI-659). There will also be 2 telephone follow ups; one after Visit 1 (telephone follow-up after screening) and one after Visit 2, (telephone follow-up after PET). Study visits and telephone follow-ups will take place during a maximum of 97 days.