MRI Protocol for the Assessment of Juvenile Idiopathic Arthritis of the Wrist: Recommendations from the OMERACT MRI in JIA Working Group and Health-e-Child

To the Editor:

Magnetic resonance imaging (MRI) is the only imaging tool that allows us to assess all relevant structures in juvenile idiopathic arthritis (JIA): the synovium, cartilage, bone, ligaments, and tendon sheaths. The interpretation of the MRI of the wrist in patients with JIA is challenging because of the complex anatomy and the presence of normal variants mimicking pathology^1,2. There is a need for a consensus of MRI interpretation in children with JIA and a universal protocol for MRI acquisition, which can enable uniformity of identification of all involved structures.

From 2012 onward, an international collaborative network of clinical and radiological experts on imaging in JIA has set out to standardize the challenging MRI acquisition and interpretation of JIA disease activity at the wrist³. For this purpose, experts from the Outcome Measures in Rheumatoid Arthritis Clinical Trials (OMERACT) Working Group “MRI in JIA” and the Health-e-Child Radiology group have joined forces and met twice a year⁴. The group addressed the MRI acquisition and made recommendations on a core set of mandatory protocol settings (Table 1). In Table 2, general optional requisites for an MRI of the wrist are reported. Agreement on the MRI protocol was reached through previous research and plenary group discussions. Consensus-based recommendations on MRI acquisition protocols should facilitate comparison of MRI studies conducted in different centers across the world.

One of the major differences in our suggested protocol compared to the core set of MRI sequences suggested by OMERACT for rheumatoid arthritis is the cartilage-specific sequence⁵. Because cartilage represents the main target of the destructive process in JIA, cartilage-specific sequences should be included in the MRI protocol for a more accurate and comprehensive evaluation of structural damage. Further, cartilage-specific MRI sequences may help us discriminate normal, growth-related bony depressions from pathologic bone erosions⁶. Examples of suitable cartilage-specific sequences are proton density sequences and gradient echo sequences with water-selective excitation specific for cartilage (WATSc)⁷. Together with the Dixon sequence, the WATSc is a newer MRI technique that uses chemical shift for differentiation between water and fat. Whereas Dixon calculates the difference between water and fat based on carefully chosen image acquisition timepoints, WATSc creates a different signal for water and fat by another radiofrequency pulse to selectively excite the water. Dixon is considered very promising and superior to other fat-suppression (FS) techniques in musculoskeletal imaging, especially in children and for complex anatomy because of the high signal-to-noise ratio and homogeneous FS — this perfectly applies for the patient with JIA with wrist involvement⁸. The Dixon FS technique is also time-saving because both T1 turbo spin echo (TSE) without FS, used to assess bone marrow, and T1 TSE with FS, used to compare postcontrast images, are gained in 1 acquisition. If the Dixon FS technique is not used, identical precontrast and postcontrast T1 FS sequences must be obtained for comparison of findings.

To date, the administration of intravenous gadolinium is necessary for proper appreciation of the inflamed synovium⁹. Preliminary results for research on diffusion-weighted imaging in patients with JIA raise the suggestion that next to intravenous contrast, this technique could also be valuable in differentiating inflamed synovium for joint effusion¹⁰.