Intravoxel incoherent motion magnetic resonance imaging of the knee joint in children with juvenile idiopathic arthritis.

Background: MRI of synovitis relies on use of a gadolinium-based contrast agent. Diffusion-weighted MRI (DWI) visualises thickened synovium but is of limited use in the presence of joint effusion.

Objective: To investigate the feasibility and diagnostic accuracy of diffusion-weighted MRI with intravoxel incoherent motion (IVIM) for diagnosing synovitis in the knee joint of children with juvenile idiopathic arthritis.

Materials And Methods: Twelve consecutive children with confirmed or suspected juvenile idiopathic arthritis (10 girls, median age 11 years) underwent MRI with contrast-enhanced T1-weighted imaging and DWI at 1.5 T. Read-out segmented multi-shot DWI was acquired at b values of 0 s/mm, 200 s/mm, 400 s/mm and 800 s/mm. We calculated the IVIM parameters perfusion fraction (f) and tissue diffusion coefficient (D). Diffusion-weighted images at b=800 s/mm, f parameter maps and post-contrast T1-weighted images were retrospectively assessed by two independent readers for synovitis using the Juvenile Arthritis MRI Scoring system.

Results: Seven (58%) children showed synovial hypertrophy on contrast-enhanced imaging. Diagnostic ratings for synovitis on DWI and on f maps were fully consistent with contrast-enhanced imaging, the diagnostic reference. Two children had equivocal low-confidence assessments on DWI. Median f was 6.7±2.0% for synovitis, 2.1±1.2% for effusion, 5.0±1.0% for muscle and 10.6±5.7% for popliteal lymph nodes. Diagnostic confidence was higher based on f maps in three (25%) children and lower in one child (8%), as compared to DWI.

Conclusion: DWI with IVIM reliably visualises synovitis of the knee joint. Perfusion fraction maps differentiate thickened synovium from joint effusion and hence increase diagnostic confidence.