Fig. 4.

Established sacroiliitis not fulfilling the ASAS definition of a positive MRI. Despite radiography that only shows some capsule calcification (arrow), MRI-T1 clearly displays bilateral ankylosis in the midpart of the joint. The ankylosis is not amenable to detection by radiography because this part of the joint is not in parallel orientation to the X-ray beam. MRI-STIR shows no osteitis. This patient with established axSpA fulfills neither the radiographic standard (mNYC is negative) nor the ASAS definition for a positive MRI (no bone marrow edema) and represents another example of ‘false-negative’ imaging if the ASAS definitions are used for clinical diagnosis