Synovial signal intensity on static contrast-enhanced MRI for evaluation of disease activity in juvenile idiopathic arthritis – A look at the bright side of the knee

Background: Knowledge on the role of synovial signal intensity (SI) grading on static contrast-enhanced (CE) MRI of the knee for assessment of disease activity in juvenile idiopathic arthritis (JIA) is lacking. Objectives: To assess the value of synovial SI on static CE-MRI of the knee for evaluation of disease activity in children with JIA. Materials and methods: Children with clinically inactive and clinically active JIA who underwent static CE-MRI of the knee were included. Synovial SI was evaluated on post-contrast T1-weighted fat-saturated images using a 0.02 cm2 region of interest drawn in the area of the synovium that contained visually the highest SI. To control for potential time-dependent post-contrast enhancement variability, a ratio between the SI of the synovium to the musculus gastrocnemius was calculated. Results: We included 427 JIA patients (clinically inactive JIA: 150 [35,1%]; clinically active JIA: 277 [64.9%]), 65.3% female, with a mean age of 13.3 ± 3.2 years. Mean SI synovium-to-muscle ratio was 2.1 ± 0.7 in patients with clinically inactive JIA versus 2.2 ± 0.8 in patients with clinically active JIA. Subgroup analysis showed no significant difference in SI synovium-to-muscle ratio between JIA patients with clinically inactive disease and JIA patients with clinically active disease (p-value 0.22). Conclusions: Evaluation of the brightness of the synovium on static CE-MRI of the knee for assessment of JIA disease activity should be avoided, as this might lead to incorrect clinical conclusions.