Table 1: Summary General MRI Imaging Findings
| T1 | T2 or STIR | |
|---|---|---|
| Joint space (Normal) | Low SI | Low but can have a thin band of high SI when normal joint fluid is present |
| Joint Effusion | Low SI | High SI |
| Joint Synovitis | Low SI | High SI similar to joint fluid.
Note: can be differentiated on T1FS sequence post contrast whereby synovitis enhances and becomes high SI whereas the joint fluid remains low SI |
| Articular cartilage (Normal) | Intermediate to low SI | Intermediate SI |
| Subchondral Bone Plate (Normal) | Low SI | Low SI |
| Erosion | Focal defect in subchondral bone plate, Low SI | Low SI in chronic inactive disease
In active disease the erosion may be filled with fluid or synovitis, both high SI on T2, on its articular aspect and have reactive osteitis/edema within the adjacent subchondral marrow |
| Marrow (Normal) | Intermediate to high (varies with age and the extent of hemopoeitic marrow. In younger patients there is more abundant red marrow which is of intermediate SI and slightly higher than muscle SI which is replaced by increasingly fatty marrow as one ages and becomes higher in SI. The normal subchondral marrow should be of similar SI to the sacral marrow at the level of the sacral foramina. | Low SI due to suppression of fat within normal marrow |
| Edema | Low SI | High SI |
| Sclerosis | Low SI | Low SI |
| PIFA
(Post Inflammatory Fat Accentuation) |
High SI | Low SI |
| Enthesitis | Low SI | High SI (at tendon/ligament insertion) |
| Capsulitis | Low SI (may be mildly thickened) | High SI |
Table 2: Summary Pathology In Sacroiliitis
| Sacroiliac Pathology | Radiograph | MRI |
|---|---|---|
| Edema (Osteitis) | ND | Periarticular high signal intensity on STIR or contrast enhanced T1FS |
| Enthesitis | ND | High signal intensity on STIR or contrast enhanced T1FS at site ligament or tendon attachment to bone |
| Synovitis | ND, may be associated with joint space widening | Intra-articular high signal intensity on contrast enhanced T1FS |
| Effusion | ND, may be associated with joint space widening | Intra-articular high signal intensity on STIR, no enhancement on contrast enhanced T1FS |
| Capsulitis | ND | Capsular high signal intensity on STIR or contrast enhanced T1FS. May extend medially or laterally into periosteum as an enthesitis |
| Erosions | Cortical defect within cartilaginous compartment joint. Low signal intensity on all imaging sequences. In active erosions may be high signal intensity within, active synovitis, or deep to erosion, osteitis | |
| Fat deposition | ND | Periarticular high signal intensity on T1, low signal intensity T1FS and STIR |
| Sclerosis | Subchondral area of high attenuation, appears “white” | Periarticular low signal intensity on T1, low signal intensity T1FS and STIR |
| Ankylosis | Partial or complete bony fusion across joint. Discernible joint space is lost | Partial or complete bony fusion across joint. Discernible joint space is lost. Follows bone marrow signal intensity on all imaging sequences |
ND-Not Detectable
Table 3: Summary Pathology In Spondylitis
| Spinal Pathology | Radiograph | MRI |
|---|---|---|
| Romanus Lesion | Anterior vertebral body corner erosion. May heal with sclerosis, the shiny corner. | Focal osteitis, high SI on STIR, low T1, anterior corners vertebral body. Chronic lesions are depicted as foci of fat signal intensity, high on T1 and T2. |
| Anderson lesion/Spondylodiscitis | Types 1,2 and 3. Localised or generalized discovertebral destruction with surrounding ill-defined sclerosis. Fractured posterior elements in type 3. | Irregularity to frank erosion of cortical endplate, adjacent osteitis of increased SI on STIR vertebral body, low SI disc and sclerosis deep to osteitis. Assess for posterior element fracture |
| Spondylitis | Anterior (Romanus) or posterior vertebral body corner erosion. May heal with sclerosis, the shiny corner. | Corner or non-corner osteitis vertebral body, high SI STIR, low on T1. Chronic lesions are depicted as foci of fat signal intensity, high on T1 and T2 |
| Facet joint arthritis | Erosions, subchondral sclerosis and eventually joint ankylosis. | Erosions, osteitis pedicles, subchondral sclerosis and eventually joint ankylosis. |
| Costovertebral arthritis | Erosions, sclerosis and fusion. Difficult to detect on radiographs | Erosions, subchondral osteitis, subchondral sclerosis and eventually joint ankylosis, involves costotransverse and costovertebral joints. |
| Enthesitis | May be normal or erosion and osteopenia, reactive sclerosis | High SI within ligament and osteitis at bony attachment, may develop erosion and sclerosis, commonly seen in posterior element involvement. |
| Syndesmophytes | New bone formation within the outer fibres of the annulus fibrosis of the intervertebral disc commencing at the juncture with the vertebral body, eventually bridging between vertebral bodies | May be difficult to identify on MRI, radiographs more sensitive. Non/bridging bone following marrow SI on all imaging sequences |
| Ankylosis | Bony fusion, partial or complete, across a joint space | Bony fusion, partial or complete, across a joint space. May have bone marrow SI (easier to identify on MRI) or be sclerotic and low SI on all sequences, |