chapter 1, X-Ray Production, Interaction, and Detection in Diagnostic Imaging

Excerpt

1.1 X-ray production

1.1.1 Definitions and mechanisms

X rays and γ rays are forms of electromagnetic radiation that are energetic enough that when interacting with atoms, they have the potential of liberating electrons from the atoms that bind them. When an atom or molecule is stripped of an electron, an ion pair forms, consisting of the negatively charged electron (e⁻) and the positive atom or molecule. X rays and γ rays, therefore, are forms of ionizing radiation, and this feature fundamentally distinguishes these rays from the rest of the electromagnetic spectrum.

An electromagnetic wave of frequency v has an energy proportional to v, with the constant of proportionality given by Plank's constant, h:

X rays and γ rays have different spectral characteristics, but fundamentally an x ray of energy E is exactly the same as a γ ray of energy E. By definition, γ rays originate from the nucleus of the atom, whereas x rays originate at the atomic level of the atom. Gamma rays are given off by radioactive isotopes such as technetium 99m, thallium 201, and iodine 131. A complete discussion of γ rays, which are the rays of interest in nuclear medicine imaging, is beyond the scope of this chapter.

X rays can be produced by several different methods, such as by synchrotrons, by channeling sources, by free electron lasers, etc. The most common x-ray production technology used in the vast majority of the radiology departments around the world, however, is the standard x-ray tube which emits bremsstrahlung as well as characteristic x rays. These processes are discussed below.

1.1.2 Bremsstrahlung radiation

According to classical theory, if a charged particle is accelerated it will radiate electromagnetic energy. When energetic electrons are incident upon a metal target (as in an x-ray tube), the electrons interact with the coulomb field of the nucleus of the target atoms and experience a change in their velocity, and hence undergo deceleration.