The purpose of this study is to look at the Sodium Magnetic Resonance Imaging (MRI) findings in children with brain tumors and healthy children. An MRI is a machine which takes pictures of the brain. The type of MRI image in this study is a new type that is not yet FDA approved and the study is looking to see if the new type of MRI image is useful in looking at sodium in the brain and brain tumors. Sodium MRI (also known as 23Na-MRI) is a useful non-invasive imaging technique to assess biochemical and physiologic changes in tissues, e.g. integrity of cells and tissue viability. Sodium MRI has been beneficial in the evaluation of several neurological disorders including acute ischemic strokes, multiple sclerosis, amyotrophic lateral sclerosis, migraines, and multiple tumor types, including tumors outside of the central nervous system.