Radiotherapy - Cerebrovascular Reactivity (RT-CVR Study)

Inclusion Criteria: * Patients with primary gliomas visible on MRI who are planning to receive radiotherapy or * Patients with brain metastases\>1cm who are planning to receive radiotherapy * Karnofsky Performance Status \>60 * Age 18 years * Patients must be willing to visit Toronto Western Hospital for at least one MRI scan prior radiotherapy Exclusion Criteria: * Seizures not controlled with medications, or non compliance with prescribed anti-seizure medication * Prior radiation therapy to the brain * Pregnancy * Unwilling or unable to co-operate with breathing maneuvers * Respiratory or cardiac limitations to breathing at 20 L/min * Medical contra-indications to limited hypercapnia or hypocapnia (known increased intracerebral pressure, metabolic acidosis or alkalosis) * Contraindication to MRI (patients weighing\>136 kgs-weight limit for the scanner tables; allergy to MR contrast agent; patients with pacemakers, cerebral aneurysm clips, shrapnel injury or implantable electronic devices not compatible with MRI) * Other medical conditions deemed by the PI or associates to make the patients ineligible for protocol procedures