From the Guidelines
Magnetic Resonance Imaging (MRI) is the best imaging modality to assess the sacroiliac (SI) joints, particularly when evaluating for inflammatory conditions like sacroiliitis or ankylosing spondylitis. This recommendation is based on the most recent and highest quality study available, which emphasizes the importance of MRI in detecting early signs of inflammation and structural changes in the SI joints 1. MRI provides superior visualization of both bone marrow edema and soft tissue inflammation, which are early indicators of active inflammation before structural damage occurs.
Key Benefits of MRI
- Superior visualization of bone marrow edema and soft tissue inflammation
- Early detection of active inflammation before structural damage occurs
- Ability to detect chronic structural lesions such as sclerosis, erosions, fat deposition, and ankylosis
- Higher sensitivity and specificity compared to radiography and CT scans
Typical MRI Protocol
A typical MRI protocol for SI joints includes:
- T1-weighted sequences to evaluate anatomy and chronic changes
- Fluid-sensitive sequences (STIR or T2 fat-suppressed) to detect active inflammation
- Contrast enhancement with gadolinium may be added when needed to better characterize inflammatory changes
Comparison with Other Imaging Modalities
MRI is preferred over conventional radiography (X-rays), which can only detect structural damage that typically appears later in disease progression. While CT scans offer excellent bone detail for erosions or ankylosis, they cannot visualize active inflammation and involve radiation exposure. Ultrasound has limited utility for SI joints due to their deep location and complex anatomy. Nuclear medicine studies like bone scans may detect inflammation but lack the specificity and anatomical detail that MRI provides 1.
Clinical Implications
The use of MRI in assessing SI joints can lead to improved diagnostic confidence and treatment plans for patients with suspected axSpA. MRI findings have been shown to be predictive of both subsequent radiographic disease progression and the likelihood of response to therapy 1. Therefore, MRI should be the imaging modality of choice for assessing SI joints, particularly in patients with suspected inflammatory conditions like sacroiliitis or ankylosing spondylitis.
From the Research
Imaging Modalities for Sacroiliac Joints
To assess the sacroiliac (SI) joints, various imaging modalities can be used, each with its own advantages and limitations. The choice of the best imaging modality depends on the specific clinical question and the desired level of detail.
Available Imaging Modalities
- Computed Tomography (CT): CT scans provide detailed images of the bony structures of the SI joints and can be used to evaluate joint space narrowing, subchondral sclerosis, and erosions 2, 3.
- Magnetic Resonance Imaging (MRI): MRI is useful for evaluating the soft tissue structures surrounding the SI joints, including the ligaments and cartilage. Zero echo time (ZTE) MRI has been shown to be superior to T1-weighted fast spin echo (T1 FSE) for the recognition of SI joint structural lesions 4.
- X-ray: X-ray images can be used to evaluate the alignment and joint space of the SI joints. A study suggested that tilting the x-ray image intensifier 10 degrees caudal past the vertical anteroposterior (AP) view can provide an optimal approach to the SI joint's inferior limb 5.
Recommendations
- For evaluating the bony structures of the SI joints, CT scans are recommended 2, 3.
- For evaluating the soft tissue structures surrounding the SI joints, MRI is recommended, with ZTE MRI being a superior choice for recognizing structural lesions 4.
- For a basic evaluation of the SI joints, x-ray images can be used, with a suggested technique of tilting the x-ray image intensifier 10 degrees caudal past the vertical AP view 5.