A radioactive substance being studied as an imaging agent in head and neck cancers and other types of cancer. It binds to large molecules in tumor cells that have a low level of oxygen. Radiation given off by FMISO is detected by a PET scan. The amount of FMISO in the tumor may help decide the best treatment and help predict whether the cancer will come back after treatment. FMISO is a type of radioimaging agent. Also called 18F-fluoromisonidazole and 18F-MISO.