Magnetic resonance scanning (MR-scanning) or 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.