Diagnostic Approach to Sacroiliitis
Start with conventional radiography of the sacroiliac joints as your first imaging test, then proceed to MRI of the sacroiliac joints if radiographs are negative or equivocal but clinical suspicion remains high. 1, 2
Clinical Context Before Imaging
Look specifically for these features that suggest inflammatory sacroiliitis:
- Age of onset before 45 years 1
- Chronic back pain lasting ≥3 months with insidious onset 1
- Inflammatory characteristics: morning stiffness, pain improving with exercise but not rest, pain awakening in second half of night, alternating buttock pain 1
- Laboratory markers: HLA-B27 positivity, elevated C-reactive protein 1
Step 1: Conventional Radiography
Obtain plain radiographs of the sacroiliac joints as the initial imaging modality. 1, 2
- Radiography demonstrates chronic erosions, sclerotic changes, and ankylosis as sequelae of inflammatory sacroiliitis 2
- Major limitation: Low sensitivity (19-72%) for early disease detection, with radiographic changes often lagging 7+ years behind symptom onset 1, 2, 3
- Specificity is only 47% for active sacroiliitis 3
- Also obtain spine radiographs (at minimum cervical and lumbar) alongside sacroiliac joint films 2
Common pitfall: Radiographs miss more than half of patients with structural changes when compared to CT, and interobserver agreement is only fair to moderate 2
Step 2: MRI of Sacroiliac Joints (When Radiographs Negative/Equivocal)
If diagnosis cannot be established on clinical features and radiography, proceed to MRI of the sacroiliac joints. 1
MRI Protocol Requirements:
- T1-weighted sequences (coronal oblique) 1, 2
- Fat-suppressed fluid-sensitive sequences (T2-weighted fat-saturated or STIR, coronal oblique) 1, 2
- Axial oblique sequence perpendicular to the joint 2
- Joint-line-specific sequence for optimal bone-cartilage interface evaluation 4
What to Look For on MRI:
Active inflammatory lesions:
- Bone marrow edema (BME) - the primary finding (sensitivity 73-90%, specificity 90-97% in established disease) 1
- Deep BME extending ≥1 cm from articular surface is more specific for axial spondyloarthritis 2
Structural lesions:
- Bone erosions (sensitivity 11% alone, but combined with BME increases sensitivity to 96-98%) 1
- New bone formation, sclerosis, fat infiltration 1
MRI Performance:
- Overall sensitivity: 79-95% 2, 3
- Overall specificity: 89-100% 2, 3
- MRI picks up an additional 75% of early cases not diagnosed by plain radiography 3
Contrast administration: Generally not necessary, as non-contrast MRI has sensitivity of 79% and specificity of 89% 2. Contrast may be considered in select cases but does not significantly increase diagnostic accuracy 2
Critical caveat: Bone marrow edema can appear in non-inflammatory conditions and up to 30% of healthy controls, but deep lesions extending ≥1 cm are more specific 2
Step 3: Alternative Imaging (When MRI Unavailable or Contraindicated)
CT can provide additional information on structural damage if radiography is negative and MRI cannot be performed. 1
- Low-dose CT is preferred given patients are often young 4
- CT excels at evaluating structural lesions and subtle erosions 1, 4
- Dual-energy CT with virtual non-calcium images can depict bone marrow edema 4
Imaging NOT Recommended
Do not routinely use these modalities for diagnosis:
- Scintigraphy/SPECT: Low to moderate sensitivity (48%), limited value 1, 3, 5
- Ultrasound: Not recommended for sacroiliitis diagnosis 1, 2
- MRI of the spine: Not generally recommended for initial diagnosis of axial spondyloarthritis 1, 2
- PET/CT: Not routinely obtained as initial imaging, uncertain utility 2
Special Populations
Young patients and those with short symptom duration: MRI of the sacroiliac joints is an acceptable alternative first imaging method instead of radiography 1
Pediatric patients: Knowledge of normal imaging features (flaring, blurring, irregular articular surface appearance) is essential to avoid misinterpreting normal developmental changes as disease 4