Thomas J. Vogl Wolfgang Reith - Diagnostic and Interventional Radiology

The component unit of particle radiation, the corpuscle, has a rest mass (m0) and can carry a charge. Its energy is composed of the so-called rest energy (E0) and kinetic energy (Ekin):

The rest energy is derived from the rest mass and the speed of light (c):

Electromagnetic waves consist of an electric and a magnetic field. In quantum theory, electromagnetic waves are attributed with properties of particles. These particles, photons, carry neither mass nor charge, but only the energy of the radiation.

Waves are described by their wavelength (), frequency (f) and amplitude (A). Wavelength and frequency are linked via the velocity of propagation (c):

Electromagnetic radiation always propagates at the speed of light. The energy of electromagnetic radiation is proportional to its frequency (f):

h is Plancks constant. Electromagnetic waves also include visible light, infrared radiation, ultraviolet radiation, radio waves, X-rays, -rays and microwaves. They differ only in the frequency of the radiation and thus by their energy.

The atom is composed of protons, neutrons, and a shell of electrons. According to Rutherfords atomic model, atoms consist of a shell of negatively charged electrons and a positively charged nucleus. The electrons are bound to the nucleus via electromagnetic interactions. The nucleus consists of nucleons, the positively charged protons, and the uncharged neutrons. The nucleons are bound together by the nuclear force. In the range of the short distance from the atomic nucleus, this attractive force is much stronger than the repulsive electromagnetic force acting between the positively charged protons.

The atoms of a chemical element are characterised by the number of protons, which is known as the atomic number (Z) (Fig.). Atoms with the same number of protons, but a different number of neutrons (N) are referred to as isotopes of an element. An atomic species, which is characterised by a certain number of protons and neutrons is referred to as a nuclide. The sum of protons and neutrons is called the mass number (A). An atom is electrically neutral if the number of electrons in the shell corresponds to the number of protons in the nucleus. If the number of shell electrons differs from the number of protons, the atom is electrically charged. It is no longer referred to as an atom, but rather as an ion. This condition is depicted by indicating the charge state e.g. Na+, Cl, Fe2+.

The theory of quantum mechanics developed at the beginning of the 20th century postulated that electrons can only move in shells of a certain energy (energy levels). The number of electrons per shell is thus restricted; the shells are arranged in order of increasing energy and referred to as K, L, M, N shells and so on. Their energy value corresponds to the energy required to completely separate the respective electron from the atom. Instead of leaving an atom (ionisation), an electron can pass to a shell of higher energy (excitation). The energy difference must be supplied to the electron, e.g. via radiation. If an excited electron returns to a shell of lower energy, the energy difference will be converted to characteristic radiation (X-rays) or an Auger electron in the case of a light atomic material. The emission of light is referred to as luminescence. By measuring the energy of the characteristic radiation (spectroscopy), the chemical element can be conclusively identified. The electron shell determines the electronic properties of an atom. Atoms with filled shells (e.g. the noble gases helium and neon) are chemically inert (Fig.).

The nucleus also has a shell structure, although like nucleons, the shell transitions are only possible with the absorption (nuclear excitation) or release (nuclear decay) of energy in the form of energy radiation.

In 1896, Becquerel discovered that a photographic plate becomes blackened upon exposure to uranium crystals. This led to the discovery of radioactive decay . In radioactive decay, the nucleus of a chemical element is spontaneously converted to the nucleus of another chemical element, thus emitting radiation. This property is referred to as radioactivity. Nuclides with this property are called radioactive nuclides. For most elements, stable and unstable (radioactive) isotopes can be distinguished. The newly created nucleus, which was created through radioactive decay, may itself be unstable, thus resulting in a chain of decay. The radioactive decay is classified according to the radiation emitted. If the same type of radiation is always released during the radioactive decay of an unstable nucleus, this is referred to as a pure emitter. For some radioactive nuclides, various types of decay occur (e.g. 64Cu).