Figure 1.

MRI markers of inflammation in OA. Fluid sensitive sequences are capable of delineating intraarticular joint fluid. However, a distinction between true joint effusion and synovial thickening is not possible as both are visualized as hyperintense signal within the joint cavity. For this reason the term effusion-synovitis was introduced, which in the MOAKS system is scored based on the distension of the joint capsule and is graded from 0 to 3 in terms of the estimated maximal distention of the synovial cavity with 0=normal, grade 1=<33% of maximum potential distention, grade 2=33%–66% of maximum potential distention and grade 3=>66% of maximum potential distention. Axial dual-echo at steady-state (DESS) MR images show A. Grade 2 effusion-synovitis (asterisk), and B. Grade 3 effusion-synovitis (asterisk). In addition, signal changes in Hoffa’s fat pad are commonly used as a surrogate for synovitis on non-contrast enhanced MRI. Although synovitis can only be visualized directly on contrast-enhanced sequences, it has been shown that Hoffa’s signal changes are a sensitive but non-specific surrogate of synovitis. C. Sagittal intermediate-weighted fat-suppressed MRI shows a discrete ill-defined hyperintense signal alteration in Hoffa’s fat pad consistent with grade 2 Hoffa-synovitis (arrow). D. Severe, grade 3 signal alterations almost occupying the entire fat pad are seen in this image (arrows).