Imaging Tests to Diagnose Axial Spondyloarthritis
Reviewed by: HU Medical Review Board | Last reviewed: May 2020 | Last updated: September 2020
Diagnosing AxSpA spectrum conditions can be challenging and the process of doing so is multi-faceted. A variety of symptoms may be present, with or without visible changes on imaging tests. It’s important to have a thorough diagnostic process that includes a variety of evaluations, including a medical history, physical examination, and if need be, various tests.
Tests can be helpful in confirming a diagnosis or providing more information about a condition. It is possible to have inconclusive results from tests and still be diagnosed with an AxSpA spectrum condition based on a medical history and physical examination. While tests can provide more information, they are not the only thing a diagnosis should be based upon.
Imaging tests can be used to help diagnose a condition on the AxSpA spectrum, but it’s important to remember that a person can still have an AxSpA spectrum condition but not have any visible changes on an imaging test. A diagnosis can be made based on a medical history and physical exam.
What kinds of imaging tests are generally used in the diagnostic process, and how?
While imaging can be used in the diagnostic process, it can also be helpful to manage AxSpA spectrum conditions and monitor treatment response and disease progression. Commonly used imaging tests can include conventional radiography (X-rays), magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound (US).
Conventional radiography, which is X-ray, is usually the first-line approach to imaging for the AxSpA spectrum. It is well-established and inexpensive, and is the gold standard to visualize structural changes in the spine and sacroiliac (SI) joints.1 What is seen on x-ray is usually the result of chronic inflammation, causing either destruction of bone or new bony growths.1 In the SI joints, this can include erosion, syndesmophytes (bony growths in a ligament), sclerosis (narrowing), and calcifications, among other findings.1
Magnetic resonance imaging
Magnetic resonance imaging (MRI) is able to show not only the structure of the anatomy, but also physiological processes occurring in the body. It is the only imaging test that is able to detect both active inflammatory and structural lesions, and how they are distributed in the body.1,2 This makes it especially useful for early diagnosis of AxSpA spectrum conditions when changes aren’t seen on X-ray.1,2 MRI can also detect things like inflammation of the SI joints, synovitis (inflammation of the synovial membrane, which lines certain joints), small erosions not visible on x-rays, and sacroiliitis (inflammation of one or both SI joints).1
Computed tomography (CT) takes x-ray measurements from different angles and produces cross-sections of the body, giving health care providers pictures of “slices” of the body.3 CT has been found to be more effective than MRI in finding structural lesions in the SI joints for AxSpA spectrum conditions, due to the shape of the joints and the ability to see different angles of these joints from the imaging.1 This makes CT especially valuable in diagnosing sacroiliitis.1 CT has also been found to be more effective than MRI in detecting new bone production, sclerosis, and chronic bone changes.1 One drawback, however, is the increased amount of radiation a person is exposed to with CT, compared to other imaging tests. CT should not be used as the default imaging test.
Spectral CT is a newer kind of CT that is able to show images of chronic sacroiliitis and detect active sacroiliitis in those living with AxSpA spectrum conditions.1 It can also show bone sclerosis and erosions.1
Ultrasound (US) can be helpful in assessing soft tissue lesions in those with inflammatory conditions.1 It was more sensitive than MRI in detecting enthesitis (inflammation of entheses, where tendons or ligaments insert into bone) in those with AxSpA spectrum conditions.1 Color Doppler US has been shown to find SI joint and spine inflammation and is sensitive to treatment response changes.1 US may be helpful in locating disease activity or monitoring known disease activity and response to treatment; it is less helpful in the diagnostic process itself.1
While imaging tests can be helpful, they are just one part of the diagnostic process. A person can have negative imaging findings and still be diagnosed with a condition on the AxSpA spectrum.