Skull Base and Low Grade Glioma Neurocognitive Magnetic Resonance Imaging (MRI) Study

The purpose of this study is to: * estimate the degree of memory loss, if any following radiotherapy to the base of skull or brain as measured by standard neurocognitive battery testing. * describe radiotherapy dose-related changes in vascular perfusion, in spectroscopic parameters of neuronal injury and changes in the degree and directionality of tissue water diffusivity (diffusion tensor imaging) as a measure of white axonal injury. * to relate these imaging characteristics to the degree of memory loss.