A Comparison of MRI Perfusion and FDG PET/CT to Distinguish Between Radiation Injury and Tumor Progression

This study will examine if MRI perfusion and PET/CT can tell growing tumor and radiation injury apart. MRI perfusion looks at the blood vessels in the tumor. PET/CT looks if the tumor cells are actively growing. The investigators will do these two tests and see which one is better. Patients will remain on study until the completion of either the MRI perfusion or PET/CT that are within 12 weeks of each other. After one of these scans, the patient will have no active interventions and will be off study. Optional: Restriction Spectrum Imaging (RSI) Sequence RSI sequence is an advanced way of looking at your brain. The scan allows doctors to see how water is moving within brain tumors or within brain cells. The extra sequence takes additional 4-5 minutes in the scanner. The RSI sequence is optional. The patient will only be asked to participate if the doctor believes that it will be helpful. Off study: Patients will remain on study until the completion of either the MRI perfusion or PET/CT that are within 12 weeks of each other. After one of these scans, the patient will have no active interventions and will be off study. Patients will obtain a standard of care brain MRI scan about every 2-3 months. These MRI scans will be used to track disease progression.