Background Diffusion-weighted imaging (DWI) of synovitis has been suggested as a possible non-invasive altative to contrast-enhanced T1w imaging (ce-T1w).We aimed to study DWI for diagnosing synovitis in the knee joint of pediatric patients,to quantify inter-observer agreement on DWI and ce-T1w and to calculate quantitative measures of synovial diffusivity and conspicuity.Methods Forty consecutive patients with known or suspected arthritis of the knee (25 girls,median age 12 years) underwent routine 1.5T MRI with ce-T1w and transverse DWI with b values 50 and 800 s/mm2.Mean apparent diffusion coefficient (ADC) values and signal intensity of inflamed synovium,joint effusion and muscle were measured with regions of interest retrospectively.Post-contrast T1 w images (diagnostic standard) and diffusion-weighted images at b =800 s/mm2 with ADC map were separately rated by three independent and blinded readers with different levels of expertise for the presence and degree of synovitis along with the level of diagnostic confidence.Results Thirty-one (78％) patients showed at least some synovial contrast enhancement,17 (43％) children were diagnosed with synovitis on ce-T1w.Ratings by the 1st reader on ce-T1w and on DWI for synovitis showed very good agreement (kappa =0.90).Inter-observer agreement on DWI ranged from moderate to substantial with kappa values between 0.68 and 0.79 (all P ＜ 0.001).Agreement and diagnostic confidence were generally lower in patients with mild and without synovial enhancement,compared to patients with synovitis.DWI yielded higher signal of inflamed synovium vs.muscle tissue,but lower signal vs.joint effusion,compared to ce-T1 w (all P ＜ 0.001).Conclusions Diffusion-weighted imaging is a promising,though reader-dependent altative to contrast-enhanced imaging in patients with arthritis of the knee,based on our preliminary findings.It holds potential for increasing patient safety and comfort.