If prolapsed disc is the problem, and your pain signals are telling you to avoid certain positions, then chances are those are the positions that will aggravate the situation, ie cause the disc to bulge. Simply taking military grade pain relief and adopting that position at the precise moment the imaging is done will show the bulges at their worst, and may make the situation appear more serious than it really is.
Add to all of that - 'perhaps'. However, many a health problem has been aggravated by hospital investigations to find out what is going on.
What to do then? Well, while you are waiting for the scan, check in with a GOOD osteopath and see what else is going on to feed this problem. There may be many useful things that can be done to relieve strain from the affected disc (if that's what it is). This may reduce the pain, the likelyhood of further damage, the chances of needing surgery, the discomfort of the scan.
[Incidentally, some medics will try to put you off by saying that manipulation of the area is not a good idea. They may well be right in that recomendation, but would be wrong in thinking that would be all that an osteopath would consider doing to help. To the medical/surgical specialist, this is a local problem, to a good osteopath it is a body-wide problem: the disc might be the last thing they would work on. Furthermore, manipulation is just one available method, just as not every visit to the doctor involves an injection, right? But I would speak to the osteopath beforehand and discuss the case to make sure you are happy with whom you have chosen. Get a personal recommendation also.]