PET
Positron Emission Tomography. An imaging technique not unsimilar to SPECT or CT. In PET very short-lived radionuclides, mostly the fluoride isotope18F, are injected - attached to a carrier molecule with an affinity to a specific organ/structure. The radionuclides used in PET will send out a positron (very small particle similar to electrons, but with a positive electrical charge) which immediately collides with an electron resulting in 2 high energy rays in exactly opposite directions each time an atom decays. When these rays hit a circular detector a computer can calculate the straight line along which the source of the positrons lies. By counting a large number of cross-bearings the exact size and position of the organ/structure that has taken up the radionuclide can be determined.
GE Healthcare Glossary