Single photon emission computed tomography – An imaging technique that measures gamma rays emitted after the injection of a gamma-emitting radioisotope. Typically this isotope is attached to a substance that is taken up by a tissue of interest and the intensity of gamma emission reflects the degree of uptake (metabolic activity) of that tissue. The concentration of emitted rays is recorded three dimensionally allowing for the production of a three dimensional image of the tissue that resembles a CT scan.