FOLR2 and rheumatoid arthritis: Since activated macrophages constitute the major inflammatory cell type in RA lesions and these inflammatory macrophages (but not tissue resident macrophages in healthy tissues) express FRβ that can be imaged with [18F]Fluoro-PEG-Folate, the authors hypothesized that the intensity of an RA joint’s inflammation might be assessed by quantitating its uptake of the folate-targeted PET tracer mentioned above.