This study uses positron emission tomography (PET) imaging to measure kappa opioid receptors (KOR) in the brains of individuals with and without post-traumatic stress disorder (PTSD). The investigators propose to recruit 45 drug-naïve individuals, N=15 patients with PTSD, N=15 trauma-exposed, but asymptomatic healthy control subjects (TC) and N=15 non-trauma exposed healthy control subjects (HC) to participate in one magnetic resonance imaging (MRI) and one PET study. The investigators will also carefully document trauma history, and collect behavioral and neuroendocrine measures to provide a more integrative view on the neurobiology of PTSD and its phenotype. The investigators predict PTSD will show greater carbon - 11 (11C)[11C]LY2795050 volume of distribution (VT) (i.e. KOR binding) values than control populations in an a priori defined PTSD circuit.