Brain Inflammation in Major Depressive Disorder Background

Background: \- Studies have shown that inflammation plays an important role in depression. Brain inflammation may contribute to depression, and may make it more difficult to treat some kinds of depression with current therapies. Researchers want to use magnetic resonance imaging (MRI) and positron emission tomography (PET) scanning to study inflammation in the brain. To do so, they will use a contrast agent, which is a chemical that can show inflammation during an imaging study. Objectives: \- To see if people with major depressive disorder have increased inflammation in the brain. Eligibility: \- Individuals at least 18 years of age who have major depressive disorder. Design: * Participants will be screened with a physical exam and medical history. They will provide blood samples before the scanning sessions. * Participants will have a PET scan after the screening visit. They will have a dose of the contrast agent before the study. This scan will look for possible brain inflammation. * Participants will also have an MRI scan. This scan will take pictures of the brain for comparison studies. * Treatment will not be provided as part of this study.