Imaging in Axial Spondyloarthritis.

BACKGROUND: Axial spondyloarthritis (axSpA) covers the stage of non-radiographic axial spondyloarthritis (nr-axSpA) and classic ankylosing spondylitis. The pathognomonic findings of axSpA are mainly inflammatory and osteoproliferative changes in the sacroiliac joints (SIJ) and the spine. Various imaging techniques are being used in daily practice for assessment of disease-specific changes, such as periarticular bone marrow edema, erosions, sclerosis, fat metaplasia and ankylosis in the SIJ or spondylitis, spondylodiscitis, facet joint involvement, or syndesmophytes in the spine of patients with axSpA. Conventional radiographs are still considered the gold standard for assessment of structural changes, while the method of for detection of inflammatory changes is magnetic resonance imaging (MRI). A result for an MRI in the SIJ is considered positive for axSpA when more than one lesion is present on one MRI slice, If there is one lesion only, this should be present on at least two consecutive slices. For the spine, inflammatory lesions should preferably be located in the corner of the vertebral bodies, while occurrence of spondylitis in three or more vertebral corners is considered highly suggestive of axSpA. This review gives a detailed overview about the benefits and limitations of all available imaging techniques in patients with axSpA, explains the usage of imaging techniques in the context of diagnosis and differential diagnosis of the disease, and reports on the potential future trends in the area of imaging of the axial skeleton in patients who are suspicious for this diagnosis.