I've had both a CAT scan and an MRI scan. The CAT scan was shorter and I didn't have to go all the way inside the big machine. The CAT was only 5 minutes and the MRI was 20 minutes long. With both scans you lie down flat, the bed is slid into a tomblike machine which takes photos of your head. There is a lot of whirring sounds - even with my profound hearing loss I could feel it.
Take off all your jewellery and hair pins/scrunchies beforehand.
You basically have to lie there and it's quite boring actually. Just close your eyes and imagine yourself on a desert island beach swaying on a hammock!