Clinical measurement of renal function is generally performed using either laboratory tests or nuclear medicine techniques, however, both of these techniques suffer from some limitations. Notably the lab tests only assess global, rather than individual kidney, function and the nuclear medicine tests are demanding to perform well. Children with brain tumors are often treated with chemotherapeutic in order to try and kill the cancer. Amongst the known side effect of some of the drugs used in the chemotherapy is the fact that they may damage the kidneys. For this reason the function of the kidneys is assessed using a laboratory test at 3 or 6 months intervals during the treatment. In addition, to the problem mentioned above the tests also require a separate visit to the hospital. Children with brain tumors who are undergoing chemotherapy also routinely have contrast-enhanced MRI of the brain performed at intervals of three months in order to evaluate the response of the tumor to the chemotherapy. Recently, MRI techniques have been developed which can evaluate single kidney renal function. The aim of this study is to establish if a single MRI exam can be used to assess both the effect of the chemotherapy on both the tumor and the renal function. The results of the MRI measurement of the single kidney renal function would be combined to provide a measure of global renal function and this would be compared with that obtained from the laboratory test. The MRI exam will require only require an extra 10 minutes of scanning time and will not affect the rest of the MRI exam in any way.