Osteoarthritis and fibroblast activation: an imaging study assessing feasibility and reliability of whole-body 68 Ga-FAPI PET-CT for structural osteoarthritis evaluation

Objective Primary aim was to evaluate whether presence of osteoarthritis (OA), as assessed by ordinal grading on whole-body computed tomography (CT), is associated with 68 Ga fibroblast activation protein inhibitor (FAPI) positron emission tomography (PET) tracer uptake as a measure of fibroblastic activation. Secondary aim was to evaluate whether OA disease severity is positively correlated with increased tracer uptake and to evaluate reliability. Material and methods In a retrospective study design, patients who had undergone 68 Ga-FAPI PET-CT for a spectrum of clinical reasons were included. Whole-body CT was assessed for OA using the OsteoArthritis Computed Tomography‐Score in multiple joints and the spine. Maximum standard uptake value (SUV max ) was determined correspondingly. Logistic regression and correlation analyses were used to describe associations between structural OA and 68 Ga-FAPI PET activity. Results Fifty-four patients were included. Presence of OA in the acromioclavicular joints (ACJ) was associated with odds of SUV max being in the highest tertile. Increased odds were seen for one location of the cervical spine (OR 5.7, 95% confidence interval [1.6, 20.4]) and both locations of the lumbar spine (OR 8.3, 95% confidence interval [1.5, 46.6]; 5.0, 95% confidence interval [1.3, 18.7]). Moderate correlations were observed for OA severity and 68 Ga-FAPI PET uptake for most joints and spine. Reliability for OA scoring (overall percent agreement) ranged from 40 to 100% (intra-reader) and from 66.3 to 100% (inter-reader). Mean intraclass correlation coefficients for SUV max were 0.76 and 0.82, respectively. Conclusion Presence of OA was associated with increased SUV max uptake for the ACJ and the cervical and lumbar spine. In this feasibility study, CT-based OA scoring and 68 Ga-FAPI PET SUV max determinations could be achieved with good reliability.