The sacroiliac (SI) joint connects the spine to the pelvis, specifically where the triangular-shaped sacrum meets the top part of the ilium, a large bone in the pelvis. These two joints, located on either side of the lower back, play a role in absorbing shock between the upper body and the legs, providing stability, and transferring weight. When inflammation occurs in these joints, a condition known as sacroiliitis develops, often leading to pain in the lower back, buttocks, or even radiating into the legs. Accurately identifying the cause of this pain is an important step toward effective management.
The Role of MRI in Diagnosis
Magnetic Resonance Imaging (MRI) is a sensitive imaging technique often used to diagnose inflammation in the sacroiliac joints. Unlike X-rays, which primarily show bone, MRI excels at visualizing soft tissues, cartilage, and bone marrow. This capability allows MRI to detect early inflammatory changes that other imaging methods might miss.
MRI can reveal active inflammation, such as fluid accumulation in the bone marrow, and structural changes that develop over time. It is considered the imaging choice for characterizing sacroiliac joint disease and assessing its severity and activity.
What an MRI Reveals
An MRI scan of an inflamed sacroiliac joint can show several specific findings, each indicating different aspects of the disease process. One of the earliest and most common signs of active inflammation is bone marrow edema. This appears as areas of increased signal on certain MRI sequences, reflecting fluid accumulation within the bone marrow adjacent to the joint surfaces. It signifies active inflammation and is a key diagnostic feature for sacroiliitis.
Over time, chronic inflammation can lead to structural changes. Erosions are areas where the cartilage or bone has been worn away due to the ongoing inflammatory process, appearing as marginal foci of bone loss, often with a low signal on T1-weighted images. Joint space narrowing can also be observed, where the space between the joint surfaces becomes thinner, indicating cartilage degradation.
In more advanced stages, ankylosis may occur, which is the complete fusion of the joint, where bone bridges form across the joint space. This represents a severe, late-stage finding. Additionally, MRI can show capsulitis, which is inflammation of the joint capsule, or synovitis, inflammation of the joint lining. These findings often present with increased signal intensity and may show enhancement after contrast administration, especially when adjacent to bone marrow edema.
The MRI Process
You will typically lie flat on a movable table that slides into the MRI scanner. Before the scan, remove all metallic objects, including jewelry, watches, and any clothing with metal fasteners. Inform the medical staff about any medical implants, such as pacemakers or certain surgical clips, as these can affect scan safety or quality.
During the scan, the MRI machine produces loud knocking or buzzing noises, so earplugs or headphones are usually provided for comfort. It is important to remain as still as possible throughout the scan to ensure clear images, which typically lasts between 20 to 30 minutes.
In some cases, a contrast dye, often containing gadolinium, may be injected intravenously to enhance the visibility of inflamed tissues. This helps to highlight areas of active inflammation more clearly.
After the scan, most individuals can resume their normal activities immediately.
Understanding Your MRI Report
Following your MRI scan, a radiologist, a medical doctor specializing in interpreting medical images, will analyze the images and create a detailed report. This report typically includes sections describing the “Findings,” which detail the observations from the images, and an “Impression” or “Conclusion,” which summarizes the most significant findings. The report will describe any signs of inflammation or structural changes in your sacroiliac joints.
The MRI report is a diagnostic tool, not a definitive diagnosis on its own. Its findings must be integrated with your clinical symptoms, a physical examination, and other laboratory tests.
Your treating physician, such as a rheumatologist or orthopedist, will review the report with you, explain what the findings mean for your specific condition, and discuss the next steps. They will use all available information to establish a precise diagnosis and develop a personalized treatment plan.