Sacroiliitis is the hallmark of Ankylosing Spondylitis, now termed as Axial Spondyloarthritis. It is therefore important to know what makes for the diagnosis, so that patients can avail themselves to the emerging and effective treatments.
Merely having radiographic (XRay) evidence of sacroiliitis may not necessarily justify treatment, be it with NSAIDs/coxibs or biologics (anti-TNF/IL17). The sacroiliitis should also be determined to be "active", so as not to escalate immunosuppression unnecessarily. This is where MRI comes in, with subchondral Bone Marrow Edema (BME) being a defining feature.
Differentials, such as Osteoarthritis, Gout and Infections (eg TB), also need to be excluded, so that the right therapy is given.