Post-traumatic stress disorder (PTSD) is an anxiety disorder that can develop after exposure to a traumatic event. PTSD symptoms may include emotional numbness, loss of interest in activities that were once enjoyable, irritability, and sleep problems. Medication therapy, behavioral therapy, and a combination of both therapies are among the available treatment options for people with PTSD. Cognitive behavioral therapy (CBT), a type of talking therapy that has been shown to be effective in treating PTSD, teaches patients how to alter their thinking to, in turn, improve how they feel. A selective serotonin reuptake inhibitor (SSRI) is a type of medication that has also been effective in treating PTSD. Information about the comparative physiological effects of each of these treatments on people with PTSD is needed. This study will evaluate which parts of the brain are affected by CBT treatment versus SSRI treatment in people with PTSD.