Imaging Tests

There is no single test that can diagnose ankylosing spondylitis (AS), and doctors use a series of tests to make the diagnosis. In addition to a physical exam and patient history, physicians use imaging scans to view the inflammation and/or possible joint damage that may be occurring in someone with AS. In addition to being used during the diagnostic process, imaging is also helpful in monitoring a person’s disease, how well a treatment is working, determining the extent of the disease, and possible progression.¹

There are several imaging techniques that are used to view the joints and bones in people with suspected or diagnosed AS:

• X-ray
• Magnetic resonance imaging (MRI)
• Computed tomography (CT) scan
• Ultrasound
• Dual-energy x-ray absorptiometry (DXA) scan, or a bone mineral density test

X-ray

Using a small amount of electromagnetic radiation, an x-ray can show erosive changes in the joints, which occur in AS. A classic first sign of AS is sacroiliitis, inflammation in the sacroiliac joints, which are located between the base of the spine and the pelvis. X-rays, also called radiographs, are the most important imaging tool for diagnosing and monitoring AS, and a diagnosis of AS can be made definitively if typical features of AS are visible on x-rays, such as radiographic evidence of sacroiliac joint involvement.^1,2

AS can cause erosion of the affected joints and ankylosis, stiffened or fused joints. These changes can be visible on x-rays. Syndesmophytes, the calcification of the ligaments that attach to the joints, is also characteristic of AS and can be seen on x-rays.¹ However, x-rays may not detect AS in the earliest stages of the disease.³ X-rays cannot show inflammation or small areas of damage.

MRI

MRI uses powerful magnets and radio waves to create pictures of the interior structures of the body. MRI may be used to view inflammation or joint damage in the spine and the peripheral joints. MRI can view the soft tissues and detect inflammatory changes in the joint and entheses, the locations where ligaments and tendons attach to the bones. While MRI are more sensitive than x-rays and more likely to detect the early stages of AS, they are expensive, not easily accessible in all areas, and are not always used.^1,3

CT scan

CT scans uses special x-ray technology to create cross-sectional images of the body. While CT scan is not generally used in diagnosis of AS, it can provide more detail than conventional x-rays on sacroiliac joint involvement. CT scans are not helpful in soft-tissue changes, such as the inflammation of entheses (enthesitis).¹

Ultrasound

Ultrasound uses sound waves to produce an image and is useful in differentiating inflammation in the synovial capsule of the joint (synovitis) from the attachment points of the tendons or ligaments (enthesitis). While ultrasound is not useful for the spine or sacroiliac joints, it may be used to assess peripheral joints (those in the arms or legs).¹

DXA scan

DXA scans measure a person’s bone mineral density and are commonly called bone mineral density tests. DXA scans are used to diagnose osteoporosis. People with AS are at an increased risk of developing osteoporosis, a condition in which the bones are weakened and brittle and at an increased risk of fracture. DXA scans may be used to screen people who are not having symptoms or in people who have already had a fracture to assess their bone density.^4,5