The purpose of this study is to get information regarding the usefulness and accuracy of this new MRI technique - termed arterial spin labeling (ASL) - in the diagnosis of pediatric brain tumors. This technique can non-invasively measure the blood flow to the brain during MRI examination. Because aggressive brain tumors require new blood vessels and a high level of blood supply to support its growth, we believe the tumor blood flow measured using ASL is a reliable indicator of tumor grade. This information may be useful for doctors to make treatment decisions and to monitor patients' response to treatment.