Musculoskeletal Imaging

For Suspected Axial Spondyloarthritis, What Is the Best Initial Imaging Study?

·
For Suspected Axial Spondyloarthritis, What Is the Best Initial Imaging Study?

A 34-year-old software developer presents to your clinic with a six-month history of insidious low back pain and stiffness. The pain is worst upon waking, gradually improves with movement throughout the day, and sometimes wakes him from sleep in the early morning. He has no history of trauma. Your clinical suspicion is high for an inflammatory process, specifically axial spondyloarthritis (AxSpA). You know that imaging is the next step to support this diagnosis, but the optimal initial study isn’t always clear. This article provides a detailed clinical workflow for this exact scenario: initial imaging in a patient with suspected AxSpA. Based on the American College of Radiology (ACR) Appropriateness Criteria, the recommended first study, `Radiography sacroiliac joints`, is rated `Usually Appropriate`.

Who Fits This Clinical Scenario?

This guidance applies to adult patients presenting for their initial diagnostic workup of suspected axial spondyloarthritis. The key inclusion criteria are clinical features suggestive of inflammatory back pain, which typically include:

  • Insidious onset of back pain
  • Age of onset less than 45 years
  • Pain and stiffness that improve with exercise but not with rest
  • Nocturnal pain, particularly in the second half of the night
  • Morning stiffness lasting at least 30 minutes

This workflow is specifically for the initial imaging evaluation. It is crucial to distinguish this from related but distinct clinical situations. This guidance does not apply if:

  • The patient has already had radiographs. If initial radiographs are negative but clinical suspicion remains high, the next step is different. That situation is covered in the ACR variant: Inflammatory back pain. Suspected axial spondyloarthritis. Additional imaging following radiographs.
  • The patient has a known diagnosis of AxSpA. Imaging to monitor treatment response or disease progression follows a separate pathway, addressed in the variant: Known axial spondyloarthritis. Follow-up for treatment response or disease progression.
  • The primary concern is an acute fracture in a patient with established disease. A patient with known ankylosis who sustains trauma requires a different imaging strategy, as detailed in: Axial spondyloarthritis with spine ankylosis. Suspected fracture.

What Diagnoses Are You Working Up in This Scenario?

When ordering initial imaging for suspected inflammatory back pain, you are primarily investigating for signs of sacroiliitis, the hallmark of axial spondyloarthritis. However, the differential diagnosis remains broad at this stage.

Axial Spondyloarthritis (AxSpA): This is the principal diagnosis under consideration. AxSpA is a spectrum of chronic inflammatory disease affecting the axial skeleton. The workup aims to identify features of sacroiliitis, which can classify the patient as having either non-radiographic AxSpA (nr-axSpA, with inflammation visible on MRI but not structural damage on X-ray) or radiographic AxSpA, also known as ankylosing spondylitis (AS), where clear structural changes are visible on radiographs.

Degenerative Disease: Osteoarthritis of the sacroiliac (SI) joints or lumbar spine is a common mimic. While typically presenting as mechanical back pain (worse with activity), degenerative changes like osteophytes, subchondral sclerosis, and joint space narrowing can sometimes be confused with inflammatory changes on imaging. Radiographs are effective at identifying these features.

Infectious Sacroiliitis: Though less common, septic arthritis of the SI joint is a critical diagnosis not to miss. It usually presents more acutely with fever, severe unilateral pain, and leukocytosis. Imaging may show unilateral joint space widening, erosions, and adjacent soft tissue changes.

Other Inflammatory Conditions: Sacroiliitis can also be a feature of other related spondyloarthropathies, such as psoriatic arthritis, reactive arthritis, or arthritis associated with inflammatory bowel disease. The imaging findings of sacroiliitis are often similar, but the broader clinical context helps differentiate the underlying cause.

Why Is Radiography of the Sacroiliac Joints the Recommended Initial Study?

For the initial evaluation of suspected axial spondyloarthritis, the ACR rates `Radiography sacroiliac joints` as `Usually Appropriate`. This recommendation is grounded in a diagnostic strategy that prioritizes the identification of established, structural damage as defined by long-standing classification criteria.

The primary rationale is the central role of radiographic sacroiliitis in the modified New York criteria for ankylosing spondylitis. Plain radiographs are highly effective, widely available, and inexpensive for detecting the chronic structural changes of sacroiliitis, including sclerosis (increased bone density), erosions (loss of cortical bone), joint space narrowing, and, in advanced cases, ankylosis (bony fusion). A definitive finding on a pelvic radiograph can be sufficient to establish a diagnosis of radiographic AxSpA (ankylosing spondylitis), allowing for prompt referral and treatment initiation.

Why are more advanced modalities rated lower for initial imaging?

  • MRI Sacroiliac Joints without IV Contrast: This study is rated `Usually not appropriate` for the initial step. While MRI is superior for detecting early inflammatory changes like bone marrow edema (osteitis), the established diagnostic pathway begins with an assessment for structural damage. MRI is the logical next step if radiographs are negative or equivocal but clinical suspicion remains high, as it can diagnose non-radiographic AxSpA. Starting with MRI for every patient would increase costs and imaging utilization without changing management for those with clear radiographic findings.
  • CT Sacroiliac Joints without IV Contrast: Also rated `Usually not appropriate`, CT is excellent for delineating bony anatomy and can be useful for clarifying equivocal radiographic findings. However, it involves significantly more ionizing radiation than radiography and does not visualize active inflammation (bone marrow edema) like MRI. Its role is typically reserved for problem-solving rather than initial screening.

From a safety and practical standpoint, a dedicated anteroposterior (AP) view of the pelvis or specific SI joint views (like the Ferguson view) provides the necessary information with a low radiation dose (adult RRL=☢☢, 0.1-1 mSv). This is a key consideration, as patients with AxSpA may require periodic imaging over their lifetime.

What’s Next After Radiography? Downstream Workflow

The results of the initial sacroiliac joint radiographs will guide your subsequent management and potential need for further imaging. The decision tree branches based on whether the findings are positive, negative, or equivocal.

If Radiographs Are Positive: Clear evidence of sacroiliitis (e.g., bilateral grade ≥2 or unilateral grade ≥3) in a patient with clinical symptoms of inflammatory back pain is highly suggestive of radiographic axial spondyloarthritis (ankylosing spondylitis). The next step is typically a referral to a rheumatologist to confirm the diagnosis, assess for extra-articular manifestations, and initiate disease-modifying therapy. No further diagnostic imaging of the SI joints is usually needed at this stage.

If Radiographs Are Negative or Equivocal: A normal radiograph does not rule out AxSpA. Structural changes can take years to become visible on X-ray. If your clinical suspicion remains high based on symptoms, inflammatory markers (CRP, ESR), and/or HLA-B27 status, the workup should continue. This is the point where you transition to the next clinical scenario: Inflammatory back pain. Suspected axial spondyloarthritis. Additional imaging following radiographs. In that context, an MRI of the sacroiliac joints without contrast becomes the recommended study to look for active inflammation (bone marrow edema) consistent with non-radiographic AxSpA.

If Findings Are Indeterminate: Sometimes, radiographs may show subtle or nonspecific changes, such as mild sclerosis, that could represent early inflammatory disease or simply be degenerative. In these cases, correlation with clinical factors is paramount. If suspicion for AxSpA persists, proceeding to an MRI of the SI joints is the most appropriate next step to clarify the diagnosis.

Pitfalls to Avoid (and When to Get Help)

Navigating the initial workup for suspected AxSpA requires careful interpretation of both clinical and imaging findings. Here are a few common pitfalls to avoid:

  • Stopping the Workup Prematurely: Do not dismiss the possibility of AxSpA based solely on negative initial radiographs, especially in patients with a compelling clinical history. This is the most common reason for a delayed diagnosis of non-radiographic AxSpA.
  • Misinterpreting Degenerative Changes: Osteoarthritis of the SI joints can cause sclerosis, which can be mistaken for inflammatory sacroiliitis. Look for other degenerative signs like osteophytes and a vacuum phenomenon, which are not typical of AxSpA.
  • Ordering the Wrong Views: A standard lumbar spine series may not adequately visualize the SI joints. Be specific in your order: request an “AP pelvis” or dedicated “SI joint views” (e.g., Ferguson view) to ensure the joints are properly evaluated.
  • Over-relying on HLA-B27: While the HLA-B27 gene is strongly associated with AxSpA, its absence does not exclude the diagnosis, nor does its presence confirm it. It is one piece of a larger puzzle.

If the clinical picture and imaging results are discordant or confusing, or if the patient presents with red flag symptoms like acute neurologic deficits or suspected infection, an urgent referral to rheumatology or spine surgery is warranted.

Related ACR Topics and Tools

This article focuses on a single, common clinical scenario. For a comprehensive overview of all related presentations and imaging pathways, as well as tools to help with ordering and patient communication, please refer to the following resources.

Frequently Asked Questions

Why shouldn’t I order an MRI of the sacroiliac joints as the very first test?

While MRI is more sensitive for early inflammation, the American College of Radiology (ACR) recommends starting with radiographs because they are cost-effective, low-radiation, and directly assess for the structural damage (e.g., erosions, fusion) used in the formal classification criteria for ankylosing spondylitis. A positive radiograph can secure the diagnosis. MRI is the appropriate next step if radiographs are negative but clinical suspicion for axial spondyloarthritis remains high.

What specific radiographic views should I order for suspected sacroiliitis?

An anteroposterior (AP) view of the pelvis is often sufficient as it allows for symmetric comparison of both sacroiliac joints and can also reveal pathology in the hips. Some institutions prefer dedicated SI joint views, such as an angled (Ferguson) view, which projects the joints with less overlap from other pelvic structures. Check with your local radiology department, but ordering an ‘AP Pelvis for SI joints’ is a standard starting point.

Does a normal radiograph of the SI joints definitively rule out axial spondyloarthritis?

No, a normal radiograph does not rule out the condition. It can take 5 to 10 years for the characteristic structural changes of sacroiliitis to become visible on X-ray. A patient with a normal radiograph but strong clinical features may have non-radiographic axial spondyloarthritis (nr-axSpA), which requires an MRI to diagnose.

If I also suspect involvement of the spine, should I order spine radiographs at the same time?

The ACR rates ‘Radiography sacroiliac joints and spine area of interest’ as ‘Usually Appropriate.’ If the patient has significant symptoms localized to the thoracic or lumbar spine in addition to buttock pain, obtaining spine radiographs concurrently can be reasonable to look for syndesmophytes, vertebral body squaring, or other signs of spondylitis. However, the key initial finding for diagnosis is typically sacroiliitis.

How does the patient’s HLA-B27 status affect my decision to order imaging?

A positive HLA-B27 test significantly increases the pre-test probability of axial spondyloarthritis in a patient with inflammatory back pain. This would strengthen your resolve to proceed with imaging and to pursue an MRI if initial radiographs are negative. Conversely, a negative HLA-B27 test lowers the probability but does not exclude the diagnosis, as a percentage of patients with AxSpA are HLA-B27 negative. The decision to image should be based primarily on clinical symptoms.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 26, 2026