Musculoskeletal Imaging

What Imaging Is Next for Suspected Axial Spondyloarthritis with Negative SI Joint Scans?

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What Imaging Is Next for Suspected Axial Spondyloarthritis with Negative SI Joint Scans?

A 34-year-old software engineer presents with six months of insidious-onset low back pain and stiffness, worst in the morning and improving with a walk. His inflammatory markers are elevated, and he is HLA-B27 positive. You correctly started the workup with radiographs and then an MRI of the sacroiliac (SI) joints, but both were negative for sacroiliitis. The patient’s symptoms persist, and your suspicion for axial spondyloarthritis (axSpA) remains high. This clinical crossroads—where the classic initial findings are absent—requires a deliberate next step. This article details the American College of Radiology (ACR) guided workflow for this specific scenario, where the next recommended study, ‘Radiography spine area of interest’, is rated Usually Appropriate.

Who Fits This Clinical Scenario for Suspected Axial Spondyloarthritis?

This guidance applies to a specific patient population: those with a moderate to high clinical suspicion for axial spondyloarthritis who have already undergone an initial imaging workup of the sacroiliac joints that returned negative. The key inclusion criteria are:

  • Symptoms consistent with inflammatory back pain (e.g., onset before age 40, insidious onset, improvement with exercise, nocturnal pain).
  • High-suspicion features may be present, such as a positive HLA-B27 test, elevated inflammatory markers (ESR/CRP), or a family history of spondyloarthropathy.
  • Crucially, both initial radiographs of the SI joints AND a subsequent MRI of the SI joints have been performed and show no definitive evidence of sacroiliitis.

It is critical to distinguish this scenario from similar but distinct clinical situations. This workflow does not apply if:

  • The patient is undergoing initial imaging. For a patient with suspected axSpA who has had no prior imaging, the workup starts differently. This is covered in the initial imaging variant of this topic.
  • SI joint imaging was positive. If either radiographs or MRI confirmed sacroiliitis, the diagnosis is more straightforward, and the clinical question shifts from diagnosis to management or assessing spinal involvement.
  • The patient has known axSpA. Imaging to monitor for treatment response or disease progression follows a separate pathway.
  • There is suspicion of a fracture. A patient with known ankylosis and acute pain after trauma requires an urgent and different imaging protocol to rule out a spinal fracture.

What Diagnoses Are You Working Up When SI Joint Imaging Is Negative?

With sacroiliitis provisionally ruled out by high-sensitivity imaging, the diagnostic focus shifts to the spine itself or to conditions that can mimic axSpA. The differential diagnosis at this stage is targeted.

Axial Spondyloarthritis with Primary Spinal Involvement
This remains the primary diagnosis to confirm or exclude. In a subset of patients, the inflammatory and structural changes of axSpA may manifest in the thoracic or lumbar spine before, or in the absence of, significant sacroiliac joint disease. The goal is to identify spinal findings like Romanus lesions (inflammatory changes at the corners of vertebral bodies), vertebral squaring, or syndesmophytes (bony bridges).

Diffuse Idiopathic Skeletal Hyperostosis (DISH)
Often seen in older patients, DISH is a non-inflammatory condition that can cause spinal stiffness and pain. It is characterized by “flowing” ossification along the front and sides of the vertebral bodies, which can be mistaken for the ankylosis of axSpA. Radiographs are particularly effective at distinguishing the thick, flowing osteophytes of DISH from the thin, vertical syndesmophytes of axSpA.

Multilevel Degenerative Disc Disease
Severe spondylosis is a common cause of chronic back pain and can mimic the stiffness of inflammatory disease. Imaging helps differentiate the characteristic features of degeneration—disc space narrowing, marginal osteophytes, and endplate sclerosis—from the inflammatory erosions, bone marrow edema, and syndesmophyte formation of axSpA.

Scheuermann’s Disease
In younger adults and adolescents, this condition of abnormal vertebral growth can lead to thoracic kyphosis, back pain, and stiffness. Radiographs are diagnostic, revealing characteristic vertebral body wedging and endplate irregularities (Schmorl’s nodes).

Why Is Spine Radiography the Next Step After Negative SI Joint Imaging?

After a negative high-sensitivity MRI of the SI joints, the ACR Appropriateness Criteria panel rates ‘Radiography spine area of interest’ as Usually Appropriate. This recommendation is based on a logical progression from assessing active inflammation in the pelvis to searching for chronic, structural changes in the spine.

The primary rationale for starting with spine radiographs is their ability to effectively and efficiently detect the hallmark structural changes of established axSpA. These include:

  • Vertebral Body Squaring: Loss of the normal anterior concavity of the vertebral bodies due to inflammation and erosion.
  • Syndesmophytes: Thin, vertically oriented bony growths that bridge the intervertebral disc space, distinct from the horizontal osteophytes of degenerative disease.
  • Ankylosis: The fusion of vertebral segments, classically described as a “bamboo spine” in advanced disease.

Radiography is widely available, low-cost, and provides an excellent baseline for monitoring structural progression over time. It is also the best initial modality for differentiating axSpA from mimics like DISH.

It is important to note that several other studies are also considered Usually Appropriate in this scenario, and the choice among them depends on the specific clinical question:

  • MRI spine area of interest (without or with contrast): This is the most sensitive study for detecting active inflammation (bone marrow edema or osteitis) in the spine. If the clinical suspicion for ongoing, active disease is very high despite the negative SI joint MRI, proceeding directly to a spine MRI is a valid and powerful strategy. It carries no ionizing radiation (O 0 mSv).
  • CT spine area of interest (without contrast): CT provides superior anatomic detail of bony structures compared to radiographs. It is excellent for detecting subtle erosions, fractures, or early syndesmophyte formation when radiographs are inconclusive. However, it involves a higher radiation dose (Varies).

Conversely, some studies are deemed Usually not appropriate. A Bone scan with SPECT/CT is non-specific for the type of inflammation seen in axSpA and carries a significant radiation burden (☢☢☢ 1-10 mSv). Similarly, US spine area of interest has no role in evaluating the vertebral column for this indication.

What’s Next After Spine Radiography? Downstream Clinical Workflow

The results of the spine radiographs will guide your subsequent management and diagnostic steps. The workflow branches based on the findings.

If Radiographs Are Positive
The presence of characteristic findings like syndesmophytes or vertebral squaring on radiographs can help confirm a diagnosis of ankylosing spondylitis (the radiographic subset of axSpA). This finding solidifies the diagnosis and allows for the initiation of targeted therapies and referral to a rheumatologist for long-term management.

If Radiographs Are Negative
A negative spine radiograph does not end the investigation. Because radiographs only show chronic structural damage, active inflammation may still be present. The next logical step, as supported by the ACR criteria, is to obtain an MRI of the spine. This is performed specifically to look for bone marrow edema in the vertebral corners (Romanus lesions) or facet joints, which would support a diagnosis of non-radiographic axial spondyloarthritis (nr-axSpA).

If Radiographs Are Indeterminate
If the findings are subtle or equivocal, the choice of the next study depends on the clinical question. To better characterize subtle bony changes, a CT of the spine may be useful. If the primary question is about active inflammation, an MRI of the spine is the superior choice.

If a comprehensive imaging workup—including SI joint radiographs, SI joint MRI, spine radiographs, and spine MRI—is entirely negative, the diagnosis of axSpA becomes much less likely. At this stage, it is essential to revisit the differential diagnosis, consider non-spondyloarthritic causes of back pain, and consult with a rheumatologist.

Common Pitfalls to Avoid in This AxSpA Workup

Navigating this diagnostic pathway requires avoiding several common missteps that can delay diagnosis or lead to unnecessary testing.

  • Stopping the workup prematurely: The most significant pitfall is concluding that axSpA is ruled out based solely on negative SI joint imaging. The disease can be spine-dominant, and evaluation of the spine is mandatory when clinical suspicion persists.
  • Misinterpreting degenerative vs. inflammatory changes: Differentiating the horizontal osteophytes of osteoarthritis from the vertical syndesmophytes of axSpA is a critical radiological skill. An incorrect interpretation can lead to a missed diagnosis or misdiagnosis.
  • Ordering the wrong MRI sequences: When ordering a spine MRI to assess for active inflammation, ensure the requested protocol includes fluid-sensitive, fat-suppressed sequences like STIR (Short Tau Inversion Recovery). These are essential for visualizing bone marrow edema.

If a patient with suspected or known axSpA presents with a sudden increase in pain, new neurologic deficits, or a history of even minor trauma, you must escalate care immediately to rule out an occult spinal fracture, which can be unstable in an ankylosed spine.

Related ACR Topics and Tools

For a comprehensive overview of all clinical variants related to this topic, please consult our parent guide. Additional GigHz tools can help you navigate adjacent scenarios, understand imaging protocols, and discuss radiation safety with your patients.

Frequently Asked Questions

Why not just order a whole spine MRI instead of radiographs first?

While a whole spine MRI is also rated ‘Usually Appropriate’ and is more sensitive for active inflammation, starting with radiographs is often a practical first step. Radiographs are faster, less expensive, and excellent for detecting the chronic structural changes that define radiographic axSpA (ankylosing spondylitis). They also effectively rule out mimics like DISH. If radiographs are negative, an MRI is the logical next step to assess for active, non-radiographic disease.

If the SI joint MRI was negative, is it possible it was a false negative?

While MRI of the SI joints is highly sensitive for active sacroiliitis, false negatives are possible, though uncommon. This could be due to technical factors (e.g., suboptimal sequences), very early disease, or intermittent inflammation. However, if a technically adequate SI joint MRI is negative, the diagnostic yield of repeating it is low. The higher-yield next step is to evaluate the spine, where the disease may be manifesting.

Should I order imaging of the cervical, thoracic, or lumbar spine?

The ‘area of interest’ should be guided by the patient’s symptoms. However, axSpA commonly affects the thoracolumbar junction. If symptoms are non-specific, imaging of the thoracic and lumbar spine is a reasonable starting point. A whole spine protocol may be considered if there is clinical concern for disease at multiple levels, though this increases scan time and complexity.

Does a positive HLA-B27 test change this imaging recommendation?

A positive HLA-B27 test significantly increases the pre-test probability of axSpA, strengthening the indication for further imaging after a negative initial workup. It does not, however, change the recommended sequence of studies. The workflow—spine radiographs followed by spine MRI if needed—remains the most logical approach to identify either structural or inflammatory evidence of disease.

What if all imaging, including spine radiographs and spine MRI, is negative?

If a comprehensive imaging evaluation of both the sacroiliac joints and the spine is negative, the diagnosis of axial spondyloarthritis becomes unlikely, though not impossible. At this point, it is crucial to reconsider alternative diagnoses for chronic back pain, such as mechanical causes, fibromyalgia, or other systemic conditions. A referral to a rheumatologist for a comprehensive clinical re-evaluation is strongly recommended.

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