On MRI, ankylosing spondylitis shows up primarily as bright patches of bone marrow edema along the sacroiliac joints and spine, signaling active inflammation. As the disease progresses, the MRI picture shifts from these inflammatory signals to structural damage: erosions eating into bone, fatty deposits replacing normal marrow, bony spurs bridging vertebrae, and eventually fusion of joints. What your MRI shows depends largely on how early or late in the disease you are, and which MRI sequences are used to capture different types of tissue change.
Two MRI Sequences, Two Different Views
Radiologists use two main types of MRI sequences to evaluate ankylosing spondylitis, and each one highlights different aspects of the disease. STIR (short tau inversion recovery) images are designed to detect water and inflammation. Active inflammatory lesions light up as bright white areas on these scans. T1-weighted images, by contrast, are better at showing the anatomy of bone and revealing chronic structural damage like erosions and fatty deposits.
Most imaging protocols include both sequences because relying on just one would miss half the picture. STIR catches what’s actively inflamed right now. T1 catches what inflammation has already done to the bone over months or years. In some uncertain cases, a contrast dye (gadolinium) is injected to better distinguish between inflamed tissue and normal joint fluid, since STIR images alone can’t always tell them apart.
Active Inflammation in the Sacroiliac Joints
The sacroiliac joints, where the spine meets the pelvis, are almost always the first place ankylosing spondylitis appears on MRI. Active inflammation shows up as bone marrow edema: bright signal on STIR images in the bone just beneath the joint surface. This edema is the MRI equivalent of an acute inflammatory lesion, and it’s the hallmark finding that clinicians look for when considering a diagnosis.
For diagnostic purposes, the international classification group ASAS divides each sacroiliac joint into quadrants on MRI. A scan is considered definitively positive for active disease when bone marrow edema appears in at least four quadrants across both joints, or when edema at the same location shows up on three or more consecutive image slices. These thresholds exist because small amounts of edema can occur in healthy people, especially athletes, so a certain volume of inflammation is needed before it’s considered disease-related rather than mechanical stress.
Beyond bone marrow edema, active inflammation can also appear as bright signal in the joint capsule (capsulitis) or at the points where ligaments attach to bone (enthesitis). These findings add supporting evidence but are less specific on their own.
What Inflammation Looks Like in the Spine
In the spine, inflammation tends to target the corners of the vertebral bodies, where ligaments and the outer ring of the disc attach. These corner lesions appear as bright spots on STIR images at the front or back edges of the vertebrae. They’re known clinically as Romanus lesions and are found in roughly 67% of people with spondyloarthritis. When more than three vertebral corners are inflamed and there are no degenerative changes like bone spurs that might explain them, the specificity of this finding is 81% for spondyloarthritis. In patients under 40, that specificity climbs to 97%.
A second type of spinal lesion, called an Andersson lesion, involves inflammation at the endplates adjacent to the disc itself. On MRI, this looks like bright signal along the upper or lower surface of a vertebra, sometimes extending into the disc space in a way that can mimic a bacterial disc infection. About 33% of patients with spondyloarthritis show these lesions, though they’re less specific than corner inflammation.
Signs of Structural Damage
As inflammation persists over time, it leaves behind structural changes that are best seen on T1-weighted images. These chronic findings often coexist with active inflammation, giving the MRI a layered appearance of old and new disease.
Erosions appear as dark areas on T1 images where the bright normal bone marrow and the dark cortical bone surface have been destroyed. They can occur at vertebral corners, along the endplates between vertebrae, or at the small facet joints in the back of the spine. In the sacroiliac joints, erosions create an irregular, moth-eaten appearance along the joint surface. An erosion is considered “large” when it extends across more than 25% of the endplate width or vertebral height.
Fatty deposits (fat metaplasia) show up as unusually bright areas on T1 images within the bone marrow near joints or vertebral corners. These represent a tissue repair response after inflammation resolves, and they carry particular significance. Research has shown that fat metaplasia is an intermediary step between active inflammation and new bone formation. When inflammation fades and fatty tissue replaces it, that site becomes a likely location for future bony bridging and fusion. Patients who develop fat metaplasia as their inflammation resolves appear to be at higher risk for progressive structural damage than those whose inflammation simply clears without leaving fatty deposits behind.
Bone spurs appear as bright projections on T1 images growing from one vertebral endplate toward the next. These are the MRI equivalent of the syndesmophytes seen on X-ray, the bony bridges that gradually connect vertebrae together.
Ankylosis, or complete fusion, is the end stage. On MRI, fused joints or vertebrae show continuous bone signal connecting one structure to the next. This can occur between vertebral corners, along endplates, or across the facet joints. Ankylosis typically appears dark on all MRI sequences because mature bone has low water and low fat signal.
The Progression From Inflammation to Fusion
MRI captures ankylosing spondylitis at different points along a fairly predictable path. First, bone marrow edema appears at the sacroiliac joints or vertebral corners. As that inflammation resolves, either naturally or with treatment, fat metaplasia often develops in the same location. If erosions were present, the fatty tissue can fill in the erosion cavity, a process called “backfill,” making the erosion appear to heal on imaging. But this repair tissue is the seed for new bone. Fat metaplasia and backfill are followed by bony spurs, then bridging bone, and ultimately ankylosis.
Importantly, inflammation alone does not predict new bone formation. Studies have found that when inflammation resolves without leaving behind fat metaplasia, the risk of progressive structural damage is much lower. This means the MRI findings that matter most for predicting your long-term outcome aren’t just the bright inflammatory signals on STIR, but also whether fatty deposits are developing on T1 as that inflammation settles.
How Inflammation Is Measured on MRI
Radiologists and rheumatologists often use a standardized scoring system called SPARCC to quantify how much inflammation is present in the sacroiliac joints. The score is calculated from STIR images by dividing the joints into quadrants across six consecutive image slices. Each quadrant is scored for the presence or absence of bone marrow edema, with additional points for particularly intense signal or edema extending deeper than 1 centimeter from the joint surface. The maximum possible score is 72.
SPARCC scores are especially useful for tracking disease over time or measuring treatment response. Studies have consistently shown that scores decrease after treatment with biologic therapies, and the scoring system has good reliability between different readers. Your rheumatologist may reference this score when discussing whether your inflammation is improving.
Findings That Can Mimic the Disease
Not every bright spot near the sacroiliac joints means ankylosing spondylitis. Bone marrow edema in this area shows up in 20 to 36% of young athletes, particularly recreational runners and elite ice hockey players. This exercise-related edema tends to cluster in the back and lower part of the iliac bone, near where deep ligaments insert, and is often associated with normal anatomical features like small fluid-filled cysts or blood vessel effects near the joint.
Standard MRI protocols image the sacroiliac joints in a single plane (semi-coronal). Adding a second perpendicular plane (semi-axial) substantially reduces false-positive readings by helping radiologists distinguish mechanical stress edema from true inflammatory sacroiliitis. Degenerative joint disease, pregnancy-related joint changes, and a benign condition called osteitis condensans ilii can also produce edema patterns that overlap with spondyloarthritis. This is one reason the ASAS criteria require a minimum threshold of edema before calling an MRI positive, rather than flagging any trace of signal as disease.
What a Positive MRI Means for Diagnosis
For a definitive structural finding on MRI, the current ASAS thresholds require erosions in at least three sacroiliac joint quadrants, or fat lesions in at least five quadrants. Alternatively, erosion at the same location visible on two or more consecutive slices, fat lesions at the same location on three or more consecutive slices, or a deep fat lesion extending more than one centimeter qualifies. These cutoffs are designed to separate genuine disease from incidental or age-related changes.
MRI is the most sensitive imaging tool for ankylosing spondylitis because it can detect inflammation years before any changes appear on conventional X-rays. A normal X-ray with a positive MRI is common in early disease, and this combination is what led to the broader diagnostic category of “non-radiographic axial spondyloarthritis,” which captures the disease at an earlier, potentially more treatable stage.