One main reason for this is that spinal changes on both MRI and radiography are considered to occur later in the course of the disease.

5 On the other hand, imaging of the spine, although more frequently performed in daily practice for identification of any cause of back pain, has not yet been included in these criteria. 4 Imaging of the SIJ has been included in the classification criteria of axSpA as a key objective criterion, next to human leucocyte antigen B27. 3 With the introduction of MRI, the inflammatory nature of the disease was visualised and recognised as an objective manifestation of active disease. 1 2 Radiographs have been used for decades for imaging of structural changes of both the SIJ and spine, which occur due to long-standing disease. Imaging of the sacroiliac joints (SIJ) and the spine are important tools for the correct recognition of axSpA. Professor Walter P Maksymowych, Medicine, Alberta Health, Edmonton, T6R 2G8, Canada

14 Department of Medicine, University of Alberta, Edmonton, Canada CARE Arthritis, Edmonton, Alberta, Canada.

13 Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands, and Atrium Medical Center, Heerlen, The Netherlands.

12 Leiden University Medical Center, Leiden, The Netherlands.11 Klinikum Bielefeld, University of Bielefeld, Bielefeld, Germany.10 Charité - Universitätsmedizin Berlin and German Rheumatism Research Centre, Berlin, Germany.9 VIB Inflammation Research Center, Ghent University, Ghent, Belgium and Rheumatology Department, Ghent University Hospital, Gent, Belgium.8 Practice Buchsbaum, Rheumatology, Schaffhausen, Switzerland.7 Department of Rheumatology, Northwick Park Hospital, London North West University Healthcare NHS Trust, London, UK.6 Centre for Rheumatology and MRC Centre for Neuromuscular Diseases, University College London, London, UK.5 Department of Rheumatology, University College London, London, UK.4 Department of Diagnostic Imaging, Sheba Medical Center, affiliated to the Sackler school of medicine, Tel-Aviv University, Tel Aviv, Israel.3 Department of Radiology and Diagnostic Imaging, University of Alberta, Edmonton, Canada and Medical Imaging Consultants, Edmonton, Alberta, Canada.2 Copenhagen Center for Arthritis Research, Center for Rheumatology and Spine Diseases, Rigshospitalet, Glostrup and Department of Clinical Medicine, University of Copenhagen, Copenhagen, Denmark.

1 Rheumazentrum Ruhrgebiet Herne, Ruhr-University Bochum, Bochum, Germany.

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