TSPO and rheumatoid arthritis: To assess active inflammation in RA, 99mTc can be also labeled with ligands or receptors overexpressed in RA such as integrin αvβ3, antibodies, cytokines, folate receptor, translocator protein (TSPO), or the macrophage mannose receptor (MMR) [77,85,86,87,88,89,90,91,92].