Single photon emission computed tomography (SPECT) is a radiologic imaging technique. It is a nuclear imaging modality, as is another technique called positron emission tomography (PET). In both techniques, a radioactive substance is injected into the patient’s bloodstream. The radioactive substances have short half-lives, so they decay quickly in patients’ bodies. The radioisotopes used for SPECT have slightly longer half-lives than those used for PET.
When they decay, the radioactive substances emit positrons. A positron is an antimatter particle that has the same mass as an electron, but a positive charge. The emitted positron contacts an electron in an atom within the body and there is annihilation (just like the matter-antimatter reactions that power the Enterprise in “Star Trek”). The annihilation process emits gamma rays; SPECT isotopes emit one gamma ray, while PET isotopes emit two. SPECT equipment places a ring of detectors around your body to detect the gamma rays. The detectors convert the gamma ray events into electrical signals that a computer processes to form an image. This helps physicians more easily see areas of increased or decreased blood flow — an important factor in studying brain injuries, for example