emission computed tomography emission computed tomography emission computed tomography (ect) is a type of tomography involving radioactive emissions. emission computed tomographyicd-9-cm92.0-92.1meshd014055ops-301 code3-72-3-75 types include positron emission tomography (pet) and single-photon emission computed tomography (spect). the imaging agent used in spect emits gamma rays, as opposed to the positron emitters (such as 18f) used in pet. there are a range of radiotracers (such as 99mtc, 111in, 123i, 201tl) that can be used, depending on the specific application.