My 3 yr old has had 2 MRI's this year, both with light anaesthetic (at The Evalina), one when he was 2.8 one 3 and 1 day!
The first time we were day case, no other tests. 2nd we went in Mon, had a load of non invasive tests - heart ultrasounds, chest Xrays etc stayed overnight and then had MRI 9.30am tuesday.
Some honest advice coming up....
INSIST (oh yes in CAPS) that they come and get you BEFORE he is brought round, you will not be able to enter the room until they have taken his throat tube out but you need to be on hand in that split second after - learnt this too my cost on #1 as they didn't come and get me 'till he'd been awake for 15 mins took me 45 to get him to stop screaming. I threatened to sit on the floor outside the door this time and they took me very seriously! Were very dilligent in getting me in time for #2.
I would rather go for the gas everytime, so much less stress (imho) if they put all the needles in AFTER they have knocked them out. DS just lay in my arms watching Bob the Builder DVD and the anaesthatist put the tube through her fingers and held her hand near his mouth (note to you, don't breathe too deeply at this point)off he drifted and that was all he knew - although it can make them struggle a little, but didn't happen either time.
If it is only a brain MRI he will probably only be out for 50 mins.
Befriend all the nurses, get them to work with you, explain your fears (out of earshot)- I even called them up beforehand to ask questions.
If you need any info, do not hesitate to ask, I know how worringing it can be.