A subset of participants in the study "Randomized, Double-Blind, Placebo-Controlled Phase 2 Pilot Study of MDMA-Assisted Psychotherapy for Anxiety Associated with a Life-Threatening Illness" (NCT02427568) without any contraindicating factors for brain imaging will undergo three fMRI scans as part of this observational study. The first scan will occur prior to experimental sessions and a second scan will occur after two experimental sessions of MDMA or placebo-assisted psychotherapy. Participants will then be scanned a third time after they have completed a total of three sessions of MDMA-assisted psychotherapy (this includes both subjects that are originally in the MDMA-assisted psychotherapy group, as well as the subjects in the placebo group who will cross over and complete three active MDMA-assisted psychotherapy sessions subsequent to their placebo-assisted psychotherapy. The primary endpoint will be the scan after the second session of MDMA-assisted psychotherapy.