Nuclear medicine is based on labeling a pharmaceutical with a suitable radioactive isotope. After application to a patient, the radiopharmaceutical (see Chapters 14, 15, 16, 17 and 18) takes part in physiologic processes. Radioactive decays are continuously taking place and can be detected from the outside by the emitted gamma rays, allowing monitoring of the distribution of the tracer concentration. Several properties are required for a radioactive isotope to be suitable for nuclear medicine. The half-life of the radiopharmaceutical must be long enough to be delivered to the site of use, if it is not locally produced, and to permit acquisition of patient data; yet it should be short for radiation-protection reasons. The energy of the emitted photons must be high enough to penetrate the body tissues but low enough to be stopped and detected in the detectors of the gamma and positron emission tomography (PET) cameras. Finally, the chemical properties of the isotope should be such that it can be easily bound into stable molecules of interest.

A simplified atomic model is used for outlining the most relevant processes that eventually result in gamma rays usable for emission imaging in nuclear medicine. An atom consists of a nucleus surrounded by electrons (Fig. 2.1). The nucleus is formed by a number Z of positively charged protons and a number N of neutrons. Z is called the atomic number and determines the chemical properties of the atom; therefore, the atomic number has a one-to-one relation to a chemical symbol and a corresponding name. For instance, all atoms with Z = 6 are carbon C atoms. The term “nuclide” is used when referring to an atomic species with a defined number of protons and neutrons. The same number of protons may be combined with different numbers of neutrons. For carbon, configurations with five, six, seven, or eight neutrons are denoted as ¹¹₆C, ¹²₆C, ¹³₆C, and ¹⁴₆C and are called C isotopes. Whether a nucleus is stable depends on the relative number of protons and neutrons: the N/Z ratio. When plotting the stable N/Z configurations as a function of Z, a “line of stability” starts with 1 and increases with the atomic number, reaching 1.54 for lead 208 (Pb) (1). In nature, about 270 stable configurations occur in elements and hence lie along this line. Unstable nuclides are positioned above the line of stability (neutron rich) or below (proton rich). They undergo transitions also called “radioactive decay processes” to reach a more stable configuration and by means of which particles or gamma rays are emitted. Hence they are called “radionuclides.” Of the 2,700 known radionuclides, about
60 occur naturally; the others are produced using different techniques, mostly in reactors or cyclotrons.