Magnetic resonance imaging (MRI) can often give valuable information in the early stage of spinal involvement. It can also provide information to be used for monitoring therapy. MRI can provide evidence of active inflammation, which cannot be seen in other ways, in addition to chronic structural changes.

Typical MR changes in ankylosing spondylitis depend on the disease stage.

Early stage: Signs of disease activity in the form of oedema and/or contrast enhancement usually localised to the vertebral corners at the transition between the thoracic and lumbar spine or at the joints between thoracic vertebral bodies and the ribs (costovertebral joints).

Middle stage: Often a mixture of signs indicating activity as in early stages and chronic changes in the form of fatty deposition in the bone marrow beneath the vertebral plates, sometimes accompanied by new bone formation between vertebral bodies.

Late stage: More pronounced, chronic changes with partial or complete fusion of the vertebral bodies. During active phases there are concomitant regions showing signs of disease activity.