An imaging technique using compounds labelled with short-lived positron-emitting radionuclides (such as carbon-11, nitrogen-13, oxygen-15 and fluorine-18) to measure cell metabolism. It has been useful in study of soft tissues such as CANCER; cardiovascular system; and brain. SINGLE-PHOTON EMISSION-COMPUTED tomography is closely related to positron emission tomography, but uses isotopes with longer half-lives and resolution is lower.