Positron-emission tomography is a NUCLEAR MEDICINE diagnostic technique that works by identifying positrons – positively charged electrons – given off by substances labelled with radioactive varieties of elements. The result is three-dimensional images that identify metabolic and chemical activities of tissues, especially brain tissues. The images provide information about tissue and organ functions, and can be collated with structural images using COMPUTED TOMOGRAPHY or magnetic resonance imaging (MRI). The technique is especially valuable in the assessment of neurological disorders (see also ISOTOPE).