Fialuridine I-124 is an isotope-labeled form of fialuridine, or 1-(2-deoxy-2-fluoro-1-D-arabinofuranosyl)-5-iodouracil (FIAU), a thymidine nucleoside analog with activity against various herpesviruses and hepatitis B virus (HBV) in vitro and in vivo. Bacterial or herpes virus thymidine kinase (TK) considered to be a target of FIAU. Fialuridine I-124 is a positron emission tomography/computed tomography (PET/CT) tracer that was developed for imaging of cells transfected with the herpes simplex virus 1 (HSV1) thymidine kinase reporter gene. This enzyme transfers a γ phosphate group fom ATP to the 5’ hydroxyl group of pyrimidine deoxynucleosides. The lipophilic tracer diffuses into the cell and is trapped in the cell with HSV1-TK activity, because the phosphorylated tracer cannot pass the plasma membrane. TK gene of bacteria was sufficiently similar to that of the viral TK of HSV1 and fialuridine I-124 could also be phosphorylated by the endogenous bacterial TK. Once phosphorylated by TK, fialuridine I-124 becomes trapped within bacteria and can be detected with positron emission tomography/computed tomography (PET/CT).