A subset of 20 patients with TRD will receive a 100-hour (+/- 4-hours)ketamine infusion with head MRIs pre (2) and post (1) infusion. Little research has been done on the mechanism of ketamine's putative antidepressant action. There is now a consensus that, in early stages of the novel treatment development for depression, clinical studies should be paired with mechanistic studies (neuroimaging) to understand the underlying mechanism and validate this as a treatment target. Ketamine is thought to have an antidepressant effect by increasing synaptic connections and therefore increasing connectivity in critical cognitive/emotional circuits.