Reliability of the Human Brain Connectome

Background: \- Magnetic resonance imaging (MRI) is used to investigate brain function. Researchers want to use MRI to better understand the function patterns and connections between brain regions in healthy people. This might help people with brain diseases in the future. Objectives: * To evaluate MRI methods performed twice on the same day. * To evaluate brain function using positron emission tomography (PET). Eligibility: \- Healthy volunteers at least 18 years old. Design: * Visit 1: * Participants will be screened with medical history, physical exam, and interview about drug and alcohol use and psychiatric history. * They will give blood and urine samples. Their breath will be tested for alcohol and smoking. * Visit 2: * Participants will have urine collected. They will have MRI scans, some while resting, some while doing tasks on a computer. * The MRI scanner is a metal cylinder in a strong magnetic field. Participants will lie on a table that slides in and out of the cylinder, with a coil over their head. Participants will get earplugs for loud noises. * Visit 3: * Participants will have urine collected. * A needle will guide a thin plastic tube (catheter) into each arm. The needle will be removed, leaving the catheter in the vein. * Participants will then have a PET scan. They will get the chemical 18FDG in the catheter. They will lie on a bed that slides in and out of the PET scanner, with a cap on their head. * Participants may have tests of memory, attention, concentration, and thinking. They may complete interviews, questionnaires, tests on paper or computer, and simple actions. * Participants will wear a device for 1 week between visits to measure activity and sleep.