Molecular imaging findings with ¹⁸F-FDG and ⁶⁸Ga-FAPI PET/CT in a case of chronic polyarthralgia

Substantial activation of tissue-resident fibroblasts occurs in immune-mediated disorders, which is the basis of tissue damage.[1] Immunological tissue remodeling can be seen in vivo using fibroblast activation protein inhibitors (FAPI) PET.[2] Clinical assessment of disease activity in immune-mediated rheumatologic disorders majorly depends on physical examination, functional parameters, and a patient’s self-reporting of disease activity.[3] Progression of disease is diagnosed when tissue destruction occurs between two patient examinations, thus disease activity is only measured indirectly by the progression of existing tissue damage. Direct measurement of tissue destruction can be possible using ⁶⁸Ga-fibroblast activation protein inhibitor (⁶⁸Ga-FAPI) positron emission tomography computed tomography (PET/CT), which in turn will serve as a reproducible and objective indicator of disease activity.[4] A prospective study by Luo et al.[5] in 20 patients of rheumatoid arthritis found that both ¹⁸F-fluorodeoxyglucose and ⁶⁸Ga-FAPI PET/CT detect disease-affected joints, but ⁶⁸Ga-FAPI is more sensitive in this regard. Furthermore, the SUV_max correlated with the disease activity and radiological progression of joint damage. This finding is also demonstrated in our case [Figure 1], where ¹⁸F-FDG PET/CT showed uptake in large joints, while ⁶⁸Ga-FAPI PET/CT showed more extensive uptake, including paraspinal muscle involvement. Dorst et al.[6] used anti-fibroblast activation protein-targeted photodynamic therapy in rheumatoid synovial explants which showed no significant side effects on macrophages of neighboring fibroblast, thus the aberrant synovial cells identified by FAPI PET/CT can be selectively targeted. Thus, FAPI PET/CT imaging modality also has the potential to be used for the visualization and quantification of treatment response to antifibrotic therapies.[7]

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Figure 1 A 68-year-old male with complaints of joint pain in bilateral upper and lower limbs and lower back ache for 1 year underwent ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) and ⁶⁸Ga-fibroblast activation protein inhibitors (⁶⁸Ga-FAPI) positron emission tomography–computed tomography (PET/CT) within 1 week. Maximum intensity projection (MIP) image of ¹⁸F-FDG PET/CT showed abnormal FDG uptake in all the large joints of bilateral upper and lower limbs (a). MIP image of ⁶⁸Ga-FAPI PET/CT showed abnormal FAPI uptake in all the large joints of bilateral upper and lower limbs and in the paraspinal region (b)

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