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. However, radiography can only show permanent (structural) damage of the joints and/or bone. 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 early sacroiliitis. Erosion of the joint facets, joint space alteration and increased bone density (sclerosis) are considerably better visualised, but CT cannot visualise early active inflammation unless there is concomitant structural damage. In later stages of the disease CT can be valuable for the detection of fractures etc.
Magnetic resonance imaging (MRI) is superior to CT and radiography for detecting early sacroiliitis because MRI can show active inflammation in addition to structural damage. In later stages MR can provide valuable information about disease activity corresponding to the sacroiliac joints as well as the spine. MRI is therefore appropriate for measuring the disease activity and can be used to monitor the disease with regard to indication for treatment and for assessing the effect of therapy.