MRI is a sensitive method for detecting active inflammatory changes in the spine presenting as edema in the bone marrow and/or joints. In early stages there is often inflammation corresponding to vertebral corners and there may be signs of inflammation at the costovertebral joints. Later on there may also be active inflammatory changes at the costotransverse and apophyseal joints as well as at the paravertebral ligamentous structures.
The inflammation at vertebral corner often results in osseous destruction leading to squaring of the vertebral bodies, fat depositions and/or ankylosis across the intervertebral space. New bone formation in the form of syndesmophytes or parasyndesmophytes are, however, often difficult to detect by MRI unless being edematous or containing fat deposition.