68 Ga-FAPI PET/CT for Rheumatoid Arthritis: A Prospective Study.

Background In rheumatoid arthritis (RA), fibroblast-like synoviocyte cells, which are involved in inflammation of the articular cartilage and bone, overexpress fibroblast activation protein (FAP). This is a feature that could be leveraged to improve imaging assessment of disease. Purpose To determine the performance of gallium 68 ( 68 Ga)-labeled FAP inhibitor (FAPI) in assessing joint disease activity of RA and to compare with fluorine 18 ( 18 F) fluorodeoxyglucose (FDG) imaging. Materials and Methods Twenty participants with RA (15 women; mean age, 55 years ± 10 [SD]) were prospectively enrolled from September 2020 to December 2021 and underwent clinical and laboratory assessment of disease activity and dual-tracer PET/CT ( 68 Ga-FAPI and 18 F-FDG) imaging. Imaging-derived variables of PET joint count (the number of joints positive for RA at PET) and PET articular index (a sum of the points of the joints using a three-point scale) were correlated to clinical and laboratory variables of disease activity. Results The combined output of both PET/CT techniques helped detect 244 affected joints, all of which showed positive results at 68 Ga-FAPI PET/CT. However, fifteen of 244 (6.1%) FAPI-avid joints in six of 20 (30%) participants were not detected at 18 F-FDG PET/CT. The maximum standardized uptake value of the most affected joint in each participant was higher in 68 Ga-FAPI than in 18 F-FDG PET/CT (9.54 ± 4.92 vs 5.85 ± 2.81, respectively; P = .001). The maximum standardized uptake values of the joints at both 68 Ga-FAPI and 18 F-FDG PET/CT were positively correlated with laboratory evaluation of C-reactive protein levels ( r = 0.49 [ P = .03] and 0.54 [ P = .01], respectively). The PET joint count and PET articular index scores at 68 Ga-FAPI PET/CT were also positively correlated with most clinical disease activity variables and radiographic progression of joint damage ( P < .05). Conclusion In participants with rheumatoid arthritis who underwent gallium 68 fibroblast activation protein inhibitor PET/CT, the extent of joint involvement correlated with clinical and laboratory variables of disease activity and showed a greater amount and degree of affected joints than at fluorine 18 fluorodeoxyglucose PET/CT. Clinical trial registration no. NCT04514614 © RSNA, 2023 Supplemental material is available for this article. See also the editorial by Williams and Ahlman in this issue.