Comparison of [18F]FAPI-42 and [18F]FDG PET/CT in the evaluation of systemic vasculitis

Purpose The role of fibroblast activation protein (FAP)-targeted imaging in systemic vasculitis is currently unclear. We aimed to evaluate the clinical value of fluorine-18-labeled FAP inhibitor 42 ([F-18]FAPI-42) in patients with systemic vasculitis and to compare with [F-18]fluorodeoxyglucose (FDG) imaging. Methods Patients with systemic vasculitis who underwent dual-tracer PET/CT([F-18]FDG and [F-18]FAPI) imaging from September 2020 to March 2022 were retrospectively analyzed. Positive lesions are defined as vascular/extravascular lesions with increased tracer uptake above surrounding background, which cannot be attributed to the physiologic biodistribution of the radiotracer. The vascular/extravascular lesion detection rate and semiquantitative values (SUVmax, TBRblood and TBRliver) of [F-18]FAPI and [F-18]FDG were compared, and the correlation between the extent and range of tracer uptake and levels of inflammatory markers was investigated. Results Thirty patients (13 males and 17 females; mean age, 52.5 +/- 17.2 years) with systemic vasculitis were included (17 large vessel vasculitis, 10 anti-neutrophil cytoplasmic antibody-associated vasculitis, 2 Behcet's disease and 1 polyarteritis nodosa). [F-18]FDG PET/CT had positive findings in 93.3% (28/30) of patients, while [F-18]FAPI PET/CT had positive findings in all patients (100%, P = 0.500). Compared with [F-18]FDG PET/CT, [F-18]FAPI PET/CT detected more lesions (161/168 vs. 145/168, P = 0.005), and more extensive vascular involvement in 60% (18/30) of patients. Although SUVmax did not differ significantly between [F-18]FAPI and [F-18]FDG (median, 5.94 vs. 5.46, P = 0.517), [F-18]FAPI had higher TBRliver (median, 9.59 vs. 3.15, P < 0.001) and TBRblood (median, 5.45 vs. 4.20, P = 0.006). The total number of positive lesions in FAPI PET/CT show a moderate correlation with erythrocyte sedimentation rate (r(s) =0.478, P = 0.008) and C-reactive protein (r(s) =0.486, P = 0.006). After treatment, follow-up FAPI PET/CT of 6 patients showed decreased SUVmax, TBR and number of detected lesions, paralleling the clinical remission. Conclusion [F-18]FAPI PET/CT imaging is a promising imaging modality for the diagnosis and therapeutic monitoring of systemic vasculitis.