Skull decompression. I had to have a cage or something like that screwed to my skull for a gamma knife operation to kill a brain tumour. I had to keep my head completely still for over an hour, so they screwed the cage to my skull, and then to the table itself. The skull gets a bit compressed and I really felt the pressure, but it didn't hurt that much and I got used to it pretty fast. The worst part is when they take it off and your skull decompresses. Worse than a migraine.