The purpose of this study is to develop a new test to help diagnose mild traumatic brain injury (mTBI) and post-traumatic stress disorder (PTSD) in soldiers. Chemicals in the brain will be measured using a Magnetic Resonance Spectroscopy (MRS) scan. In the study, the investigators will compare the information they obtain from scans of participants with mTBI, PTSD or both, to scans from healthy volunteers to understand the differences between these groups. If the results can tell the difference between participants with mTBI and PTSD, the investigators should be able to help safely diagnose patients in the future.