Development of international consensus on a standardised image acquisition protocol for diagnostic evaluation of the sacroiliac joints by MRI: an ASAS-SPARTAN collaboration

Abstract

Background: A range of sacroiliac joint (SIJ) MRI protocols are used in clinical practice but not all were specifically designed for diagnostic ascertainment. This can be confusing and no standard diagnostic SIJ MRI protocol is currently accepted worldwide.

Objective: To develop a standardised MRI image acquisition protocol (IAP) for diagnostic ascertainment of sacroiliitis.

Methods: 13 radiologist members of Assessment of SpondyloArthritis International Society (ASAS) and the SpondyloArthritis Research and Treatment Network (SPARTAN) plus two rheumatologists participated in a consensus exercise. A draft IAP was circulated with background information and online examples. Feedback on all issues was tabulated and recirculated. The remaining points of contention were resolved and the revised IAP was presented to the entire ASAS membership.

Results: A minimum four-sequence IAP is recommended for diagnostic ascertainment of sacroiliitis and its differential diagnoses meeting the following requirements. Three semicoronal sequences, parallel to the dorsal cortex of the S2 vertebral body, should include sequences sensitive for detection of (1) changes in fat signal and structural damage with T1-weighting; (2) active inflammation, being T2-weighted with fat suppression; (3) bone erosion optimally depicting the bone-cartilage interface of the articular surface and (4) a semiaxial sequence sensitive for detection of inflammation. The IAP was approved at the 2022 ASAS annual meeting with 91% of the membership in favour.

Conclusion: A standardised IAP for SIJ MRI for diagnostic ascertainment of sacroiliitis is recommended and should be composed of at least four sequences that include imaging in two planes and optimally visualise inflammation, structural damage and the bone-cartilage interface.

Keywords: Arthritis; Axial Spondyloarthritis; Magnetic Resonance Imaging.

Figure 1. The dorsal cortex of the S2 vertebra is an easily identified straight line (A) on a midline T1-weighted sagittal image of the sacrum from which the semicoronal sequences can be orientated. The sacral side of the SIJ primarily arises from the S1 and S2 vertebrae with a smaller component at S3. The shaded area (dashed outline in B) represents the outer perimeter of the cartilage compartments of both SIJ superimposed over the sagittal image. All of the shaded area must be included when planning the semicoronal sequences. The stack of semicoronal images (white box in B) should be orientated parallel to the dorsal cortex of S2 and the centre of the stack should be at the anterior border of the S1/2 vestigial disc. Even when severe deformity is present at L5/S1, such as may occur with spondylolisthesis and/or disc degeneration (C) the straight dorsal cortex of S2 can still be readily identified (arrows in C). SIJ, sacroiliac joint.

Figure 2. Recommended SIJ MRI protocol (3T Magnetom Vida, Simens, Erlangen): Semicoronal T1-weighed spin echo. (A) (TR 530 ms, TE 12 ms, 3 mm thick), Semicoronal T2-weighted spin echo with fat suppression (B) (TR 5940 ms, TE 88 ms, 3 mm thick), Semicoronal T1-weighted three-dimensional gradient echo (VIBE) with fat suppression (C) (TR 4.04 ms, TE 1.57 ms, 1 mm thick) all show normal sacroiliac joints. An axial T2-weighted spin echo Dixon water-only image (TR 4360 ms, TE 93 ms, 4 mm thick) completes the standard protocol. (D) The L5/S1 intervertebral disc is very degenerate with marked decrease in height and decreased signal intensity on T2FS. (B) Note that the 1 mm thick VIBE image (C) defines the bone-cartilage interface of the irregular vertebral endplates better than the spin echo sequences due to superior tissue contrast and a thinner slice. TE, time to echo; TR, repetition time; VIBE, volumetric interpolated breath-hold examination.

Figure 3. Comparison between 3 mm thick T1-weighted spin echo images (T1 SE) and 1 or 2 mm thick T1-weighted three-dimensional (3D) gradient echo with fat saturation images (T1 3D GE FS) using three different 3D gradient echo techniques (examples taken from the North American cohort of the CLASSIC study). (A, B) T1 SE appears to show probable erosion of the articular surface of the left ilium (arrows in A). However, 2 mm thick 3D FAME confirms that no erosion is present with mild subchondral sclerosis noted (arrowheads in B). (C, D) T1 SE is suggestive for a single erosion in right SIJ (arrowhead in C), however, the image is challenging to interpret. In the 1 mm thick 3D VIBE image, the contrast between bone and cartilage/erosion is much greater and multiple erosions of the right ilium are obvious (arrows in D). (E, F) On the T1 SE, Iliac subchondral bone marrow signal is heterogeneous but no definite erosion is seen and the sacrum appears normal. (E) By comparison, 3D LAVA shows small but definite erosions in the left ilium (arrows in F) and probable erosion in the right ilium. In addition, small but definite right sacral erosions visible on the 3D LAVA (arrowheads in F) are invisible on T1 spin echo (E). FAME, fast acquisition with multiphase elliptical fast gradient echo; LAVA, liver acceleration volume acquisition; VIBE, volumetric interpolated breath-hold examination.