A radioconjugate consisting of EF5, a fluorinated derivative of etanidazole, conjugated to fluorine F 18, a positron emitting isotope. EF5 binds to hypoxic tissue; conjugation to Fluorine F 18 allows imaging of hypoxic regions within tumors. Or A substance being studied in positron emission tomography (PET) imaging to detect tumor hypoxia (a low level of oxygen in the tumor). This may help predict how the tumor will respond to treatment. It is a type of radiopharmaceutical. Also called 18F-EF5.