The term tomography comes from the Greek words tomos (τοµος), which means “to cut or section” and grapho (γραφω),“to write or record.” In this imaging technique, the nuclear technician injects a technetium-99m compound (or other gamma ray–emitting medical radionuclide) into the bloodstream of the patient. As the blood flows through the organ of medical interest, a rotating gamma camera carefully measures the radioactivity at short intervals, providing the data for a three-dimensional (3-D) computer-reconstructed image of the target organ. Physicians use the SPECT procedure to obtain multiple views of the target organ, as it undergoes metabolic functions within a patient. With SPECT, a doctor can diagnose a potential problem before it becomes life threatening or evaluate the impact of a particular treatment on a diseased or malfunctioning organ. Another very effective role for radioactive isotopes in nuclear medicine is the use of short-lived positron (β+) emitters, such as carbon-11, nitrogen 13, oxygen-15, or fluorine-18, in a process known as positron emission tomography (PET).
In PET, the medical technician incorporates a positronemitting radionuclide into a suitable chemical compound that once introduced into the body selectively migrates to specific organs. Medical diagnosis depends on the important physical phenomenon of pair production, during which a positron and an ordinary electron annihilate each other and two gamma rays of identical energy (typically 0.51 MeV each) depart the annihilation reaction spot in exactly opposite directions. Radiation sensors mounted on a ring around the patient detect the pair of simultaneously emitted annihilation gamma rays and reveal a line on which the annihilation reaction occurred within the organ under study. When a large number of their gamma ray pairs have been detected, physicians use a computer system to reconstruct the PET image of an organ as it metabolizes the positron-emitting radioactive compound. For example, by attaching a positron emitter to a protein or a glucose molecule and allowing the body to metabolize it, doctors can study the functional aspect of an organ, such as the human brain. Figure 4.27 contains a PET image that shows where glucose is being absorbed by a human brain.
Nuclear Technology and Saving Lives: Medical Application of the Mortal Energy
