Conventional radiography is a well-established method for detecting structural spinal changes such as new bone formation between the vertebral bodies and vertebral fusion. The slim syndesmophytes characteristic of AS changes are detectable by radiography, but rarely seen by MRI unless being edematous or containing fat deposition.
For the diagnosis of spinal changes a frontal and lateral radiograph (two perpendicular planes) are needed. However, despite using two planes the thoracic spine can be difficult to evaluate on radiographs due to overlaying structures.
Assessment of the severity of spinal abnormalities (grading) in axSpA is usually performed in consistence with the internationally recommended SASS or modified SASS (mSASSS) method https://doi.org/10.1093/rheumatology/key128.