Gamma knife radiosurgery uses gamma rays to treat brain tumours and other brain abnormalities without the need for invasive surgery. A stereotactic frame is fixed to the head, and an MRI scan and a cerebral angiogram (x-ray to view blood vessels in the brain) are done before the surgery. These scans allow the surgeon to calculate the dose of radiation needed to treat the diseased brain tissues, while protecting the surrounding normal tissue from radiation damage. The patient is then positioned in the gamma knife machine, and focused beams of radiation are concentrated onto the diseased brain tissues. Gamma knife surgery is a non-invasive procedure. It is very safe and precise, and usually performed as an outpatient procedure.