This study will examine the use of a variation of standard magnetic resonance imaging (MRI) called diffusion tensor MRI (DT-MRI) for distinguishing injured brain tissue due to radiation therapy (radiation necrosis) from the return of a brain tumor that was previously removed (tumor recurrence). DT-MRI differs from standard MRI in the way that computers process the images; there is no difference in the experience of having the procedure done. Both radiation necrosis and tumor recurrence can occur within weeks to months following brain radiation treatment. Because the treatment and management options for the two conditions differ significantly, distinguishing the two is of critical importance. Currently, surgical biopsy is required to make this differentiation.