Conventional radiography has for many years been used to diagnose sacroiliitis and the findings by radiography are still part of the accepted international diagnostic criteria for ankylosing spondylitis (radiographic axSpA). However, radiography can only show permanent (structural) damage of the joints and/or bones. The diagnosis can therefore be difficult to obtain and may be delayed for many years unless radiography early in the disease is replaced or supplemented by cross sectional imaging (CT or MRI). In later stages of the disease detection of both sacroiliitis and spinal changes can often be obtained by radiography.
Computer tomography (CT) is more sensitive than radiography for detecting structural changes in the form of erosion of joint facets, joint space alteration and increased bone density (sclerosis). However, CT cannot visualise early active inflammation in the subchondral bone unless using modern Dual-energy CT-technique. Nevertheless, CT can be of help in the diagnosis of minor erosive SIJ changes, which can be difficult to detect by MRI, and in the diagnosis of anatomical variation at the SIJ such as accessory joints. In later stages of the disease, CT can be valuable for the detection of spinal fractures etc.
Magnetic resonance imaging (MRI) has been proven superior to CT and radiography for detecting early sacroiliitis because MRI can show active osseous and joint space inflammation in addition to structural damage. In later stages, MR can provide valuable information about disease activity corresponding to the SIJ as well as the spine. MRI is therefore increasingly used in the diagnosis and follow-up of axSpA being appropriate for measuring the disease activity and thereby assess the effect of therapy.