This study proposes that more advanced MRI techniques currently used in clinical medicine can be applied to enable response to therapy to be determined earlier than currently is possible for patients with myelofibrosis. In particular, this study is designed to determine to what extent abnormalities in diffusion of water molecules (diffusion MRI) and/or fat content in bone marrow (T1-weighted imaging) define extent of initial disease and serve as early predictors of response to therapy.