2: The production, properties and interactions of X-rays

The production, properties and interactions of X-rays

Introduction

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).

Table 2.1

The electromagnetic spectrum ranging from the low energy (long wavelength) radio waves to the high energy (short wavelength) X- and gamma-rays

Radiation Wavelength Photon energy
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.

To understand the production and interactions of X-rays a basic knowledge of atomic physics is essential. The next section aims to provide a simple summary of this required background information.

Atomic structure

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).

Main features of the atomic particles

Nuclear particles (nucleons)

Summary of important points on atomic structure

• 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.

• Atoms in the ground state are electrically neutral because the number of positive charges (protons) is balanced by the number of negative charges (electrons).

• If an electron is removed, the atom is no longer neutral, but becomes positively charged and is referred to as a positive ion. The process of removing an electron from an atom is called ionization.

• If an electron is displaced from an inner shell to an outer shell (i.e. to a higher energy level), the atom remains neutral but is in an excited state. This process is called excitation.

• The unit of energy in the atomic system is the electron volt (eV):

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X-ray production

X-rays are produced inside machines, so called X-ray generating equipment, which are described in more detail in Chapter 3. However, a typical dental X-ray machine is shown in Fig. 2.2A. The X-ray generating part is referred to as the tubehead (see Fig. 2.2B), within which is a small evacuated glass envelope called the X-ray tube (see Fig. 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.

Main features and requirements of an X-ray tube

• The cathode (negative) consists of a heated filament of tungsten that provides the source of electrons.

• The anode (positive) consists of a target (a small piece of tungsten) set into the angled face of a large copper block to allow efficient removal of heat.

• A focusing device aims the stream of electrons at the focal spot on the target.

• 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 later in Chapter 3.

• A current (milliamperage, mA) flows from the cathode to the anode. This is a measure of the quantity of electrons being accelerated.

• A surrounding lead casing absorbs unwanted X-rays as a radiation protection measure since X-rays are emitted in all directions.

• Surrounding oil facilitates the removal of heat.

Practical considerations

The production of X-rays can be summarized as the following sequence of events:

Jan 12, 2015 | Posted by in Oral and Maxillofacial Radiology | Comments Off on 2: The production, properties and interactions of X-rays
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