It's quite painless but at the same time daunting. The hardest part is the boredom/keeping still, and depending on what you have done, the possible feeling of claustraphobia. I only had me knee done, so I wasn't "inserted" into the tube head first.
You will be asked to gown up. When they are ready, you will go in, and the nurse will sort you out position wise. There will be a lot of noise.
As said, the hardest part will be the boredom and keeping absolutely still, which you have to to get a good scan.
Once over, get dressed, go home, and wait for your doctor to get the results.