The sacroiliac (SI) joint connects the sacrum, the triangular bone at the base of the spine, to the ilium, the large bone of the pelvis. These joints transfer the weight of the upper body to the legs and act as shock absorbers during movement. When these joints become inflamed, a condition known as sacroiliitis, it often causes deep pain in the lower back that can radiate into the buttocks and thighs. Since SI joint pain can mimic other common causes of low back pain, such as sciatica or disc issues, advanced imaging is necessary to accurately diagnose the source of the discomfort.
Why MRI is the Preferred Tool for SI Joint Pain
Magnetic Resonance Imaging (MRI) is the most sensitive imaging method for evaluating sacroiliitis, particularly in its early stages. Unlike X-rays or CT scans, which primarily visualize bone structure, MRI excels at showing soft tissues and the bone marrow itself. This capability allows the detection of active inflammation before structural damage becomes apparent on conventional radiographs. Early detection is important because symptoms can appear years before X-ray changes occur.
MRI’s ability to visualize high water content makes it a superior diagnostic tool for active disease. Specialized sequences, such as Short Tau Inversion Recovery (STIR) or T2-weighted fat-suppressed sequences, highlight areas where fluid has accumulated due to inflammation. This technique suppresses the signal from fat tissue, ensuring the bright signal seen is a direct result of inflammatory fluid. The early identification of inflammation is helpful in diagnosing conditions like axial spondyloarthritis, which often begins with sacroiliitis.
Identifying Active Inflammation on the MRI
The primary sign of active inflammation in the SI joint on an MRI is bone marrow edema (BME). BME appears as a bright signal intensity on fluid-sensitive sequences like STIR or T2 fat-suppressed images. This brightness indicates an accumulation of fluid and inflammatory cells within the bone marrow, signaling an active inflammatory process. For a finding to be considered evidence of sacroiliitis, the BME needs to be present in at least two consecutive slices or as two or more lesions in a single slice.
Radiologists also look for signs of inflammation in the joint’s soft tissues. Synovitis, inflammation of the joint lining, is seen as fluid or enhancement within the joint space itself. Capsulitis, inflammation of the joint capsule, is identified as thickening and contrast enhancement of the surrounding capsule. The use of an intravenous contrast dye, such as Gadolinium, can further enhance these active inflammatory areas on T1-weighted images. These findings confirm a diagnosis of active sacroiliitis, allowing clinicians to initiate treatment.
Understanding Chronic Damage Markers
An MRI also reveals signs of chronic structural damage resulting from long-standing inflammation. These markers indicate damage that has already occurred and do not necessarily mean the joint is currently inflamed. One common sign is erosions, which are areas where the bone has been destroyed near the joint space, appearing as marginal foci of bone loss. Erosions can be seen on both fluid-sensitive and T1-weighted sequences.
Another structural change is fat metaplasia, or fatty lesions, which is the replacement of inflammatory tissue in the bone marrow with fat. These areas appear bright on T1-weighted sequences and are considered a sign of post-inflammatory bone remodeling. Sclerosis, a hardening and thickening of the bone, shows up as dark, low-signal bands parallel to the joint margins on all MRI sequences. The most advanced chronic change is ankylosis, or joint fusion, where the joint space is bridged by bone, indicating end-stage disease.
Next Steps Following an MRI Diagnosis
MRI findings provide objective evidence that guides subsequent patient care. A positive MRI showing active sacroiliitis, especially with structural damage, raises suspicion of an inflammatory condition like axial spondyloarthritis. The radiological report is correlated with the patient’s clinical symptoms and relevant laboratory results, such as blood tests for inflammatory markers like C-reactive protein (CRP).
If imaging confirms an inflammatory cause, the patient is typically referred to a specialist, often a rheumatologist, for a definitive diagnosis and comprehensive treatment plan. While the MRI confirms the presence and extent of inflammation, the specialist integrates this information with the patient’s history to determine the underlying cause. This approach ensures the correct classification of the disease and the most appropriate management strategy.