I don't understand the difference. Is it where the camera positions itself? Since you're lying in the same position in the chamber no matter what, I know it's hard to tell where the rays are being shot through you to pick up an image. But, did they give you an explanation? The SI is a part of the hip/pelvis region, so why wouldn't a hip/pelvis MRI pick it up? Mine was done so long ago that I don't know what area they were shooting, other than hip.
As for doing more, there is a doc in NC who removes tailbones for those who need it. I can't remember his name, but he was apparently the only one in the country doing it. It seems he'd be qualified and willing to remove bone spurs, if anyone is. By now, he may have retired, as I'd heard he was considering retirement a few years ago when I first heard about him. However, there was a younger doc training under him, so there should still be someone doing it.
.................Oh, okay. I just did a search. It's Dr. Dickerson in Charlotte. Do a search for him or for coccyx removal surgery.