The purpose of this study is to see whether fast imaging with MRI and the usual contrast material used for MRI, predicts which patients will do well with treatment. Some studies suggest that MRIs done right before surgery may be able to tell how much of the cancer was killed by the chemotherapy. This study will see if this is true in patients with osteogenic sarcoma (OS) and Ewing's sarcoma (ES). This study will also see if MRIs done early in treatment can tell if the chemotherapy is working.