MABS, I haven't had a chance to read all replies, so sorry if I'm repeating anything.
When ds1 was 4 moths he had to have all over x-rays, a MRI and other tests where her had to lay still. For the x-rays the nurse held him still, and I would never let that happen again, even now, almost 4 years later I can hear him crying, and the whole scene.
But for the MRI he was just sedated, and that was enough, maybe you could ask for this, especially as you dd is 8.
I hope it all goes well, our one went well and he had only the one. Since then he has quite a few scans on his kidneys where again he has to lay still, and he gets on fine, has been known to go to sleep!
Hope it all goes well MABS, just remember, if you are not happy with what the hospital do, say so, we never did, and it is something I still regret to this day.