This study is being performed to validate a new technique for measuring kidney function. Patients are being asked to volunteer for this study because they require serial contrast enhanced MR scans to monitor their response to chemotherapy. Because some chemo-therapeutic agents can be toxic to the kidney the patient's kidney function will also be evaluated using conventional methods, and the results of these tests can be compared to those obtained using MRI. We plan to study 50 children in this study. The additional procedure for measuring renal function will add 10 minutes to the duration of the MRI exam and will have no effect on the routine brain study. If validated the proposed MRI technique would allow renal function to be evaluated at the time of a routine, contrast enhanced MRI exam and would avoid additional testing using radioactive tracers or urine collection over 24 hours.