Understanding and Treating Suicidal Ideation With Ketamine

The goal of this clinical trial is to treat active suicidal ideation in individuals with Major Depressive Disorder (MDD) using ketamine and to understand suicidal ideation by examining biological mechanisms using magnetic resonance imaging (MRI), and psychological mechanisms through validated clinical scales and qualitative interviews. The main questions it aims to answer are: 1. Will ketamine will reduce suicidal ideation in a significant proportion of study participants? 2. Will reduction in suicidal ideation will be accompanied by rapid changes in neuroimaging biomarkers? Participants will receive four intravenous (IV) ketamine infusions administered twice weekly for two weeks. Participants will undergo two identical MRI scans: 1) within 48 hours prior to starting ketamine treatment, and 2) 24 hours after the fourth ketamine infusion. Suicidal ideation and depressive symptoms will be assessed prior to each imaging session alongside additional self-report measures. Qualitative interviews will occur within 72 hours after the fourth ketamine treatment.