 Brain tumours often have low oxygen levels, and that makes them more resistant to radiation therapy. If patients breathe the right mixture of oxygen during treatment, radiation may work better. In this study, patients with brain tumour will undergo a special MRI test while they breathe different mixtures of oxygen and carbon dioxide to find out whether oxygen levels improve in the tumor. Patients will also be asked to repeat this MRI test during the second week of radiation therapy, as well as 3 months, 6 months and 1 year after RT. The MRI test after RT will help us understand how the blood vessels in the normal brain are affected by radiation. 