CT scans are supposedly the best, and I have heard that MRI's actually sometimes "over diagnose" sinsusitis which is why some doctors prefer the CT scan.
I had a CT scan, that supposedly wasn't that bad. The ENT was only going to operate on an enlarged turbinate because the CT didn't show much sinus inflamation. Only at the insistance of myself and my allergist did he open up the my maxilary sinus to "just enlarge the opening and take a look" He ended up cleaning a lot of gunk out. The nurse even commented to me that I would be feeling a whole lot better now, because I had a lot of infection and it took him longer than usual to clean it all out.
Anyway, I just want to point out that in my case, the CT scan wasn't very accurate at all. I wish I could've have gotten an MRI also. I have always felt that my left side should've been operated on also.