The purpose of this study is to (a) measure how brain structure, metabolism (how the brain uses energy), and function predict response to electroconvulsive therapy (ECT) for treatment of depression; and (b) measure how ECT changes brain metabolism and function. We will ask you to undergo magnetic resonance imaging (MRI) at three sessions: (1) prior to your first ECT therapy session, (2) after having 4-6 ECT therapy sessions, and (3) approximately two months following your first ECT therapy sessions.