Participants have two MRI scans within 1 week (inclusive of the one performed for screening). MRI uses a magnetic field and radio waves to obtain images of body tissues and organs. The scanner is a metal cylinder surrounded by a strong magnetic field. During the MRI, the subject lies on a table that can slide in and out of the cylinder. Participants will be tested with magnet strengths of 1.5 and 3 Tesla; the higher the Tesla, the greater the ability to see brain changes. Each scan may last up to 90 minutes. In addition to the MRI scans, participants undergo cognitive testing that measures memory and thought processes and complete forms that test and quantify fatigue level, stress, anxiety and depression