Yes, an MRI of the sacrum will visualize the sacroiliac (SI) joints.
An MRI ordered as "MRI sacrum" will include the sacroiliac joints in the field of view, as the SI joints are anatomically adjacent to and partially formed by the sacrum itself. However, the imaging protocol and sequences used may differ from a dedicated SI joint MRI protocol, which could affect diagnostic accuracy for sacroiliitis.
Key Considerations for Optimal SI Joint Visualization
Protocol Matters for Diagnostic Accuracy
When evaluating for sacroiliitis or axial spondyloarthritis (axSpA), dedicated SI joint MRI protocols are superior to incidental sacral imaging 1, 2. The EULAR guidelines specifically recommend MRI of the SI joints (not just sacrum) when axSpA is suspected and conventional radiography is inconclusive 1.
For diagnosis of sacroiliitis, the MRI should include:
- STIR sequences (sufficient to detect inflammation; contrast is generally not needed) 1
- T1-weighted sequences (superior to radiography for detecting structural lesions like erosions and joint space changes, with 79% sensitivity vs 42% for X-ray) 3
- Proper angulation and slice orientation specific to SI joint anatomy 2, 4
What You'll See vs. What You Need
A standard "MRI sacrum" ordered for other indications (e.g., trauma, tumor evaluation) will show the SI joints anatomically, but:
- May lack the specialized sequences needed for inflammatory assessment
- May not have optimal slice orientation for detecting subtle erosions or bone marrow edema
- Could miss early sacroiliitis if not specifically protocoled for this indication 4
Clinical Pitfall to Avoid
If you're evaluating for sacroiliitis or inflammatory back pain, specifically order "MRI of the sacroiliac joints" rather than "MRI sacrum." This ensures the radiologist uses appropriate protocols including STIR sequences and proper angulation 1, 2. The SI joints are fully or partially displayed on many pelvic examinations, but incidental visualization doesn't equal diagnostic-quality imaging 2.
When SI Joints Are Incidentally Seen
If a sacral MRI performed for other reasons shows findings suggestive of sacroiliitis, the radiologist should recommend referral to a rheumatologist for further assessment and potentially dedicated SI joint imaging 2.