I've had a bunch of MRI's since my surgery, 18 months ago. One immediately after, three months, 6 months, and another at 6 months. It appears I MAY have a recurrent turmor near my trigeminal nerve and it's around 1.2 cm. However, there is also a theory that an outer capsule of the AN may have been left after surgery, and now it's collapsing and forming what appears to be a tumor. In my case, all MRIs are done at Hopkins and I have a consistent history to compare. However, this is NOT an exact science. Over the next couple MRIs, it should become more apparent.
I would say one of the MOST important criteria is to at least have all MRIs read at the same facility. This normally requires having the MRI done at the same location. I know from my Dad's cancer, the MRIs do have different tolerances, but it's the consistent interpretation of the MRIs that is critical. I fly 1,000 miles to get my MRIs at the same facility as my surgery.