The Radiosurgery Procedure On the day of the treatment, a lightweight frame is attached to the patient's head. With the frame in place, the patient has an MRI, CT, or angiography to precisely locate the diseased area to be treated. A series of images is taken and transferred to the computerized treatment planning system. The system localizes the target, determines its coordinates, and ensures that the radiation field conforms precisely to the target. The Gamma Knife is then used to direct multiple beams of ionizing gamma radiation from 201 separate cobalt sources to the targeted area within the brain. Intersecting at the target, the concentrated rays eradicate the tumor or vascular abnormalities, while surrounding tissues receive minimal radiation exposure. Treatment sessions typically last 40 to 60 minutes. Patients usually return to normal activities the same day or the following day.