I think if it was me, I'd be tempted to hold off the CT until after the MRI was done and the results available. The reason for that would be that I'd always go for the lowest impact, least risk test first, and only then if that didn't show anything, go for the next. As you say, CT does carry a load of radiation, and radiation is cumulative - the more you have when you are younger, the more risk you face later, especially if you end up absolutely having to have CTs for some other reason.
But, it sounds hopeful for you with the new rheumie. If nothing shows on the MRI, perhaps you could ask about a nuclear bone scan - to be done when you feel you have active inflammation. That would be quite useful in that it covers the whole body, so you don't have to decide for yourself which areas are likely to show up something, or which is worst.
