Okeydokey, Helena has had 2.
I expect you've seen an MRI scanner on the news or something. They vary slightly by brand and whether you are private or NHS but basically it is a short tunnel (usually 3 or 4 foot long) which doesn't have a lot of room in it. I had one when 34 weeks pregnant and only just had clearance. It can get quite hot in there (though they are ventilated) and very loud. The noise is the electromagnets which generate the fields they need to take the picture banging around as they go on and off, quite rapidly. They're big heavy metal things so you can imagine. Generally speaking they give you headphones, so it's not deafening. It's more like having roadworks outside your window but with double glazing.
If it's a child who won't keep still, they usually give a general anaesthetic. It's weaker than the sort they give for surgery and pretty low risk. Dependant on age and hospital preference it may be an injection or (more likely) gas through a mask.
If you're having the anaesthetic, they will say no eating or drinking from midnight the night before or similar. Rules vary. This is because the anaesthetic makes you a bit nauseaous (sp?) and there is a small risk you may barf and inhale. Unlikely but obviously more of a worry if your head is inside a tunnel where no-one can see you.
You need to dress in clothes with absolutely no metal parts at all, or they will undress you and put you in a hospital gown. No metal zips or eyelets etc. It sometimes takes a bit of thought to work out what to wear on the day!
You turn up about 1/2 hour early and fill in forms. They are one-size-fits-all so you have to confirm that eg your child is not pregnant or breastfeeding. (I had to write in that I was pg although I was the size of a house!) Obviously they also want to know about implants etc. which may make it impossible to do the procedure.
If you're not having anaesthetic, they help you up onto the bench thing, put your headphones on and slide you in to the tunnel. You have to stay still for about 20 mins. Absolutely still if the pics are going to work.
If you're having the anaesthetic, we got to meet the anaesthetist first and discuss with her. Then they took us through to an anteroom and put her under. DH sat her on his knee and I held her hand as they put the mask on. It's the hardest part of the whole thing because seeing your child pass out is surprisingly emotional. I've held DD1 through bouncing-across-the-floor seizures and she's had days where I don't think she recognised my face and still I cried both times they put her under GA. I don't know why!
They didn't let us go in with her and shooed us off pretty quickly. When you are gone they put a tube down the throat (to keep the airway clear in case of vomiting) and sellotape the eyelids shut. (If the eyes open while anaesthetised, you don't blink so your eyeballs dry out and are quite uncomfortable!)
They send you away for about 1/2 hour. Go to the cafe and get a cuppa to pass the time.
We then went back and waited and they called us into the "recovery" room where DD1 was coming round. She screamed and screamed the second time, and we were quite worried - till we realised she was STARVING hungry. Take something to eat and drink with you because often they don't let you home till they have eaten and drunk and kept it down, about an hour after coming out. I think the queasy feeling can sneak up on you afterwards and you don't want a barfing child in a car seat.
As usual with scans, the technicians can't tell you anything, and you are waiting for a consultant to read the pix and tell you what it all means. So time for results dependant on that - but about 10 days is reasonable.
hth!