Update on recent developments in imaging of inflammation in osteoarthritis: a narrative review

Synovitis is an important component of the osteoarthritis (OA) disease process, particularly regarding the inflammatory phenotype of OA. Imaging plays an important role in the assessment of synovitis in OA with MRI and ultrasound being the most deployed imaging modalities. Contrast-enhanced (CE) MRI, particularly dynamic CEMRI (DCEMRI) is the ideal method for synovitis assessment, but for several reasons CEMRI is not commonly performed for OA imaging in general. Effusion-synovitis and Hoffa-synovitis are commonly used as surrogate markers of synovitis on non-contrast-enhanced (NCE) MRI and have been used in many epidemiological observational studies of knee OA. Several semiquantitative MRI scoring systems are available for the evaluation of synovitis in knee OA. Synovitis can be a target tissue for disease-modifying OA drug (DMOAD) clinical trials. Both MRI and ultrasound may be used to determine the eligibility and assess the therapeutic efficacy of DMOAD approaches. Ultrasound is mostly used for evaluation of synovitis in hand OA, while MRI is typically used for larger joints, namely knees and hips. The role of other modalities such as CT (including dual-energy CT) and nuclear medicine imaging (such as positron-emission tomography (PET) and its hybrid imaging) is limited in the context of synovitis assessment in OA. Despite research efforts to develop NCEMRI-based synovitis evaluation methods, these typically underestimate the severity of synovitis compared to CEMRI, and thus more research is needed before we can rely only on NCEMRI.

Automated summary

Synovitis is an important component of osteoarthritis and can be assessed using MRI and ultrasound. Contrast-enhanced MRI, particularly dynamic CEMRI, is the ideal method for synovitis assessment, but is not commonly performed for osteoarthritis imaging. Non-contrast-enhanced MRI can use effusion-synovitis and Hoffa-synovitis as surrogate markers for synovitis. Both MRI and ultrasound can be used to evaluate synovitis and assess the efficacy of disease-modifying approaches. Other imaging modalities such as CT and nuclear medicine imaging have limited roles in synovitis assessment. More research is needed before non-contrast-enhanced MRI can be relied upon for synovitis evaluation.