MRI for Sacroiliac Joint Pain

What Is the Sacroiliac Joint?

The sacroiliac (SI) joints are two large joints where the sacrum (the triangular bone at the base of the spine) meets the iliac bones of the pelvis. They sit either side of the lower spine, just below the familiar "dimples" on the lower back. Despite being surrounded by some of the strongest ligaments in the body, the SI joints allow a small degree of movement — typically 2-4mm of rotation and translation — that's important for shock absorption during walking and load transfer between the spine and legs.

SI joint pain (sacroiliac joint dysfunction or sacroiliitis) is a surprisingly common cause of lower back and buttock pain. Studies suggest that the SI joint is the pain source in 15-30% of people with chronic lower back pain — yet it's frequently overlooked and misdiagnosed as lumbar disc disease, hip pathology, or piriformis syndrome.

SI joint problems can arise from mechanical causes (pregnancy, leg length discrepancy, trauma, previous lumbar fusion), inflammatory conditions (ankylosing spondylitis and other spondyloarthropathies), infection (septic sacroiliitis), or degeneration (osteoarthritis). The inflammatory causes are particularly important to identify because they require specific treatment with biological therapies.

Why MRI Is Used for SI Joint Assessment

X-rays and CT show the bony structure of the SI joints but miss the most clinically important information — early inflammation and soft tissue changes. MRI fills this gap:

• Detecting early sacroiliitis: MRI can detect sacroiliitis years before X-ray changes appear. This is critical for diagnosing axial spondyloarthritis (including ankylosing spondylitis) at an early, treatable stage. The ASAS (Assessment of SpondyloArthritis international Society) classification criteria now include MRI-proven sacroiliitis as a pathway to diagnosis.
• Distinguishing inflammatory from mechanical causes: The pattern and distribution of MRI findings help differentiate inflammatory sacroiliitis (typically bilateral and symmetrical, involving the subchondral bone) from mechanical SI joint dysfunction (often unilateral, with periarticular oedema).
• Monitoring treatment response: In patients with axial spondyloarthritis on biologic therapy (anti-TNF or anti-IL-17 agents), follow-up MRI can show resolution of inflammation — confirming that treatment is working.
• Infection detection: Septic sacroiliitis (rare but serious) shows dramatic bone marrow oedema, joint fluid, and often soft tissue abscess — findings that are clearly visible on MRI but may be missed on X-ray.
• Pre-injection planning: Before therapeutic SI joint injections (steroid or local anaesthetic), MRI confirms the joint as the pain source and excludes other pathology.

MRI Sequences Used

• T1-weighted coronal oblique (angled along the SI joints): Shows structural changes — erosions (small bites out of the joint surface), sclerosis (dense bone appearing dark on T1), and fatty marrow deposition (bright areas adjacent to the joint surfaces, indicating previous inflammation that has healed).
• STIR coronal oblique: The most important sequence for detecting active inflammation. Bone marrow oedema (BMO) — fluid within the bone adjacent to the SI joint surfaces — appears as bright areas on STIR. This is the defining MRI feature of active sacroiliitis and the key criterion in the ASAS definition.
• T1-weighted axial: Supplements the coronal oblique views, showing erosions and structural changes in a different plane.
• STIR axial: Detects oedema in the axial plane and identifies any soft tissue changes, including gluteal or piriformis muscle oedema.
• T2-weighted with fat saturation: An alternative to STIR for detecting oedema, with slightly different contrast characteristics.
• Post-gadolinium T1 fat-saturated (if used): Enhancement of the synovium (joint lining), capsule, and surrounding soft tissues indicates active inflammation. Not routinely used but can add value in ambiguous cases or when assessing for infection.

What Radiologists Look For

• Bone marrow oedema (BMO): The hallmark of active sacroiliitis. Bright areas on STIR in the subchondral bone (within 1cm of the joint surface) on the iliac and/or sacral side. The ASAS definition requires BMO "clearly present" in at least two consecutive slices, or in at least two locations on a single slice, to meet the imaging criterion for sacroiliitis.
• Location of oedema: Inflammatory sacroiliitis typically affects the subchondral bone on both sides of the joint. Oedema confined to the iliac tuberosity (where muscles attach) or the sacral ala (away from the joint surface) is more suggestive of mechanical stress than inflammation.
• Erosions: Defects in the joint surface, seen as irregular cortical breaks filled with high T2/STIR signal. Early erosions are detected on MRI before they're visible on X-ray.
• Fat metaplasia: Areas of bright signal on T1 adjacent to the joint surface — indicating previous inflammation that has healed and been replaced by fatty marrow. This is a structural change (like a scar) rather than a sign of active disease.
• Sclerosis: Dense, dark signal on both T1 and T2 sequences adjacent to the joint. Represents chronic, established disease.
• Ankylosis: Complete bony fusion across the SI joint — the end stage of inflammatory sacroiliitis. The joint space is obliterated and replaced by continuous bone signal.
• Capsulitis and enthesitis: Inflammation of the joint capsule and ligament attachment points, seen as enhancement and oedema around the joint margin.
• Infection signs: Dramatic unilateral BMO, joint fluid, periarticular abscess, and soft tissue oedema extending into the gluteal muscles — these features distinguish septic sacroiliitis from inflammatory disease.

How to Prepare for the Scan

• No special preparation — eat, drink, and take medications as normal
• Wear comfortable clothing without metal belt buckles or button-fly jeans
• Remove all jewellery and hip/navel piercings
• If your back pain is severe, take your usual pain medication before the scan so you can lie still comfortably
• Let the department know if you have any metal implants in the pelvic area (hip replacements, spinal hardware)

What Happens During the Scan

You'll lie on your back on the scanner table. The SI joints are imaged using the body coil or a surface coil over the pelvis. The radiographer angles the imaging planes to align with the oblique orientation of the SI joints — this ensures the joint surfaces are seen in their true cross-section.

The scan takes approximately 20-30 minutes. It's a relatively straightforward scan with no breath-holds required. You simply lie still and let the scanner work.

Contrast injection is not routinely used for SI joint assessment. It may be added if infection is suspected or if the initial sequences are equivocal.

Some centres combine SI joint imaging with a lumbar spine or whole spine protocol — adding perhaps 15-20 minutes to the overall scan time.

What MRI Can't Show

• Pain source with certainty: Bone marrow oedema at the SI joint is common in healthy people, athletes, postpartum women, and those with degenerative changes. Not all BMO represents inflammatory sacroiliitis. The ASAS criteria emphasise that MRI findings must be interpreted in the clinical context — alongside symptoms, blood tests (HLA-B27, CRP/ESR), and clinical examination.
• Mechanical dysfunction: The subtle movement abnormalities that characterise SI joint dysfunction (hypermobility or hypomobility) aren't visible on static MRI. Diagnosis of mechanical SI joint dysfunction relies primarily on clinical provocative testing and diagnostic injection.
• Bony detail: CT is superior for showing cortical erosions (particularly early, subtle erosions), joint space narrowing, and established sclerosis. Some rheumatologists request both MRI (for soft tissue/active inflammation) and CT (for structural damage) in complex cases.
• Response timing: After starting biologic therapy, MRI changes lag behind clinical improvement. Repeating MRI too early (within 3-6 months) may not yet show the expected improvement.

Treatment Pathways After Diagnosis

• Inflammatory sacroiliitis (axial spondyloarthritis): NSAIDs are first-line (NICE recommends at least two different NSAIDs trialled for a total of 4 weeks). If NSAIDs are insufficient, biologic therapy is the next step — anti-TNF agents (adalimumab, certolizumab, etanercept, golimumab, infliximab) or IL-17 inhibitors (secukinumab, ixekizumab). JAK inhibitors (tofacitinib, upadacitinib) are newer alternatives. Regular exercise and physiotherapy are fundamental to long-term management.
• Mechanical SI joint dysfunction: Physiotherapy focusing on core stability, pelvic alignment, and muscle balance. An SI joint belt can provide temporary symptom relief. Diagnostic and therapeutic SI joint injection (local anaesthetic and corticosteroid under image guidance) confirms the joint as the pain source and provides temporary relief.
• SI joint fusion: For chronic, refractory mechanical SI joint pain confirmed by diagnostic injection, minimally invasive SI joint fusion (implanting small titanium devices across the joint) is an option with growing evidence of effectiveness.
• Septic sacroiliitis: Intravenous antibiotics (typically 4-6 weeks). If an abscess has formed, CT-guided drainage or surgical drainage may be needed.
• Postpartum SI joint pain: Usually self-limiting, resolving within months of delivery. Physiotherapy, an SI belt, and gradual return to activity are the mainstays. Persistent symptoms beyond 6-12 months warrant further investigation.

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