EF5 [2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl) acetamide] is a 2-nitroimidazole-based hypoxia marker that selectively binds hypoxic cells. It is a compound developed at the University of Pennsylvania by Dr. Cameron Koch and Dr. Sydney Evans. Upon injection into animal tissues, EF5 selectively binds to hypoxic cells and forms adducts. 2-(2-Nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl) acetamide ([18F]EF5) is a specific positron emission tomography tracer approved by the US Food and Drug Administration for in vivo detection of hypoxia. EF5 has been used to detect hypoxia in rodent and human tumors, and it has shown good correlation with other hypoxia markers, such as hypoxia-inducible transcription factor. EF5 undergoes a biochemical reduction by nitroreductases in viable cells and forms covalent adducts to intracellular macromolecules in the absence of oxygen Therefore, only viable hypoxic cells are detectable with EF5. EF5 is a lipophilic compound and has better pharmacokinetic properties than other nitroimidazole compounds. [18F]EF5 positron emission tomography imaging has been introduced as a promising tool for the detection of hypoxia in tumor tissues in patients and in experimental models. Large atherosclerotic plaques in mice contained hypoxic areas and showed uptake of [18F]EF5. Despite its slow blood clearance, the high uptake of [18F]EF5 in plaques suggested that plaque hypoxia is a potential target for identifying high-risk plaques noninvasively.