X-rays and their ability to penetrate human tissues were discovered by Röentgen in 1895. He called them X-rays because their nature was then unknown. They are in fact a form of high-energy electromagnetic radiation and are part of the electromagnetic spectrum, which also includes low-energy radiowaves, television and visible light (see Table 2.1).
|Radio, television and radar waves||3 × 104 m to 100 µm||4.1 × 10−11 eV to 1.2 × 10−2 eV|
|Infra-red||100 µm to 700 nm||1.2 × 10−2 eV to 1.8 eV|
|Visible light||700 nm to 400 nm||1.8 eV to 3.1 eV|
|Ultra-violet||400 nm to 10 nm||3.1 eV to 124 eV|
|X-and gamma-rays||10 nm to 0.01 pm||124 eV to 124 MeV|
X-rays are described as consisting of wave packets of energy. Each packet is called a photon and is equivalent to one quantum of energy. The X-ray beam, as used in diagnostic radiology, is made up of millions of individual photons.
Atoms are the basic building blocks of matter. They consist of minute particles – the so-called fundamental or elementary particles – held together by electric and nuclear forces. They consist of a central dense nucleus made up of nuclear particles – protons and neutrons – surrounded by electrons in specific orbits or shells (see Fig. 2.1).
• In the neutral atom, the number of orbiting electrons is equal to the number of protons in the nucleus. Since the number of electrons determines the chemical behaviour of an atom, the atomic number (Z) also determines this chemical behaviour. Each element has different chemical properties and thus each element has a different atomic number. These form the basis of the periodic table.
X-rays are produced inside machines called X-ray generating equipment, described in more detail in Chapter 3. A typical dental X-ray machine is shown here in Fig. 2.2A. The X-ray generating part is referred to as the tubehead (Fig. 2.2B), within which is a small evacuated glass envelope called the X-ray tube (Figs 2.2C and D). X-rays are produced inside the X-ray tube when energetic (high-speed) electrons bombard the target and are suddenly brought to rest.
• A high-voltage (kilovoltage, kV) connected between the cathode and anode accelerates the electrons from the negative filament to the positive target. This is sometimes referred to as kVp or kilovoltage peak, as explained in Chapter 3.