Arabica, I've CATed you.
The MRI machine is very bangy, but because they are concentrating on your middle rather than your head, your head is actually right at the far end of the machine (so you can see the room, not just the machine) which, I imagine, makes it less claustrophobic than an MRI of your own brain.
They need to take a number of images and they may have a few false starts whilst they determine the position of the baby's head. Obviously if the baby is very wriggly that will make things take longer.
The radiologist actually talked us through the images within half an hour of taking them, and gave them to us to take to the obs consultant. He phoned her to discuss them whilst we drove across town to the obs hospital, and we saw the obs consultant later that day. It was a shattering day but I was glad to get it all over with.
We had quite a long wait for the MRI as there was an emergency. The waiting was stressful but even more upsetting was seeing the poor boy who was the emergency case, and his stricken family 
.
It was emotionally very draining. I'd recommend taking with you a trashy novel, sweets, drinks and someone else if at all possible. I wouldn't recommend driving but that may be because I am a very wussy driver. Despite what I've said above, I did find the machine a little claustrophobic so it might be worth thinking of a few techniques that would help you (deep breathing, reciting, counting, whatever)
As I'm sure you know, with chromosomal abnormalities ruled out the prospects are much brighter for your baby. And there are brain abnormalities that don't necessarily affect the baby's development.
I would strongly recommend enquiring about counselling to help you get through the rest of the pregnancy. It made a huge difference to me as I am able, to an extent, to "park" my worries in the knowledge that I could sob my heart out once a week with the counsellor.
xxx