Individuals will be asked to participate one time, for 1-year, or over a 2-year period. During the development phase of the study, each imaging series will consist of 3 or more MRI scans. At the initial imaging visit a pre-ferumoxytol scan will be done, followed by ferumoxytol injection, and then an immediate post-injection scan. The subsequent scans will be concluded within 96 hours of ferumoxytol injection (typically at 48 hours). Those who participate for 1-year will have repeat imaging at approximate times 0, 6 months, and 12 months. Those who participate for 2-years will have repeat imaging at approximate times 0, 3, 6, 12, 18, and 24 months after enrollment.