The production, properties and interactions of X-rays

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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), 1eV = 1.6 × 10−19 joules.

X-ray production

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.

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 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:

Interactions at the atomic level

The high-speed electrons bombarding the target (Fig. 2.3) are involved in two main types of collision with the tungsten atoms:

Heat-producing collisions

X-ray-producing collisions

• The incoming electron penetrates the outer electron shells and passes close to the nucleus of the tungsten atom. The incoming electron is dramatically slowed down and deflected by the nucleus with a large loss of energy which is emitted in the form of X-rays (Fig. 2.5A).

• The incoming electron collides with an inner-shell tungsten electron displacing it to an outer shell (excitation) or displacing it from the atom (ionization), with a large loss of energy and subsequent emission of X-rays (Fig. 2.5B).

X-ray spectra

The two X-ray-producing collisions result in the production of two different types of X-ray spectra:

Continuous spectrum

The X-ray photons emitted by the rapid deceleration of the bombarding electrons passing close to the nucleus of the tungsten atom are sometimes referred to as bremsstrahlung or braking radiation. The amount of deceleration and degree of deflection determine the amount of energy lost by the bombarding electron and hence the energy of the resultant emitted photon. A wide range or spectrum of photon energies is therefore possible and is termed the continuous spectrum (see Fig. 2.6).

Characteristic spectrum

Following the ionization or excitation of the tungsten atoms by the bombarding electrons, the orbiting tungsten electrons rearrange themselves to return the atom to the neutral or ground state. This involves electron ‘jumps’ from one energy level (shell) to another, and results in the emission of X-ray photons with specific energies. As stated previously, the energy levels or shells are specific for any particular atom. The X-ray photons emitted from the target are therefore described as characteristic of tungsten atoms and form the characteristic or line spectrum (see Fig. 2.7). The photon lines are named K and L, depending on the shell from which they have been emitted.

Summary of the main properties and characteristics of X-rays

• X-rays are wave packets of energy of electromagnetic radiation that originate at the atomic level.

• Each wave packet is equivalent to a quantum of energy and is called a photon.

• An X-ray beam is made up of millions of photons of different energies.

• The diagnostic X-ray beam can vary in its intensity and in its quality:

• The factors that can affect the intensity and/or the quality of the beam include:

• In free space, X-rays travel in straight lines.

• Velocity in free space = 3 × 108 m s−1

• In free space, X-rays obey the inverse square law:

< ?xml:namespace prefix = "mml" />Intensity=1/d2

image

    Doubling the distance from an X-ray source reduces the intensity to image (a very important principle in radiation protection, see Ch. 7).

• No medium is required for propagation.

• Shorter-wavelength X-rays possess greater energy and can therefore penetrate a greater distance.

• Longer-wavelength X-rays, sometimes referred to as soft X-rays, possess less energy and have little penetrating power.

• The energy carried by X-rays can be attenuated by matter, i.e. absorbed or scattered (see later).

• X-rays are capable of producing ionization (and subsequent biological damage in living tissue, see Ch. 7) and are thus referred to as ionizing radiation.

• X-rays are undetectable by human senses.

• X-rays can affect film emulsion to produce a visual image (the radiograph) and can cause certain salts to fluoresce and to emit light – the principle behind the use of intensifying screens in extraoral cassettes and digital sensors (see Ch. 4).

Interaction of X-rays with matter

When X-rays strike matter, such as a patient’s tissues, the photons have four possible fates, shown diagrammatically in Fig. 2.9. The photons may be:

Interaction of X-rays at the atomic level

There are four main interactions at the atomic level, depending on the energy of the incoming photon, these include:

Only two interactions are important in the X-ray energy range used in dentistry:

Photoelectric effect

The photoelectric effect is a pure absorption interaction predominating with low-energy photons (see Fig. 2.10).

Summary of the stages in the photoelectric effect

1. The incoming X-ray photon interacts with a bound inner-shell electron of the tissue atom.

2. The inner-shell electron is ejected with considerable energy (now called a photoelectron) into the tissues and will undergo further interactions (see below).

3. The X-ray photon disappears having deposited all its energy; the process is therefore one of pure absorption.

4. The vacancy which now exists in the inner electron shell is filled by outer-shell electrons dropping from one shell to another.

5. This cascade of electrons to new energy levels results in the formation of very low energy radiation (e.g. light) which is quickly absorbed.

6. Atomic stability is finally achieved by the capture of a free electron to return the atom to its neutral state.

7. The high-energy ejected photoelectron behaves like the original high-energy X-ray photon, undergoing many similar interactions and ejecting other electrons as it passes through the tissues. It is these ejected high-energy electrons that are responsible for the majority of the ionization interactions within tissue, and the possible resulting damage attributable to X-rays.

Important points to note

• The X-ray photon energy needs to be equal to, or just greater than, the binding energy of the inner-shell electron to be able to eject it.

• As the density (atomic number, Z) increases, the number of bound inner-shell electrons also increases. The probability of photoelectric interactions occurring is ∝ Z3. Lead has an atomic number of 82 and is therefore a good absorber of X-rays – hence its use in radiation protection (see Ch. 7). The approximate atomic number for soft tissue is 7 (Z3 = 343) and for bone is 12 (Z3 = 1728) – hence their obvious difference in radiodensity, and the contrast between the different tissues seen on radiographs (see Ch. 19).

• This interaction predominates with low energy X-ray photons – the probability of photoelectric interactions occurring is ∝ 1/kV3. This explains why low kV X-ray equipment results in high absorption (dose) in the patient’s tissues, but provides good contrast radiographs.

• The overall result of the interaction is ionization of the tissues.

• Intensifying screens, described in Chapter 4, function by the photoelectric effect – when exposed to X-rays, the screens emit their excess energy as light, which subsequently affects the film emulsion.

Compton effect

The Compton effect is an absorption and scattering process predominating with higher-energy photons (see Fig. 2.11).

Summary of the stages in the Compton effect

1. The incoming X-ray photon interacts with a free or loosely bound outer-shell electron of the tissue atom.

2. The outer-shell electron is ejected (now called the Compton recoil electron) with loss of some of the energy of the incoming photon, i.e. there is some absorption. The ejected electron then undergoes further ionizing interactions within the tissues (as before).

3. The remainder of the incoming photon energy is deflected or scattered from its original path as a scattered photon.

4. The scattered photon may then:

5. Atomic stability is again achieved by the capture of another free electron.

Important points to note

• The energy of the incoming X-ray photon is much greater than the binding energy of the outer-shell or free electron.

• The incoming X-ray photon cannot distinguish between one free electron and another – the interaction is not dependent on the atomic number (Z). Thus, this interaction provides very little diagnostic information as there is very little discrimination between different tissues on the final radiograph.

• This interaction predominates with high X-ray photon energies. This explains why high-voltage X-ray sets result in radiographs with poor contrast.

• The energy of the scattered photon (Es) is always less than the energy of the incoming photon (E), depending on the energy given to the recoil electron (e):

Es=Ee

image

• Scattered photons can be deflected in any direction, but the angle of scatter (θ) depends on their energy. High-energy scattered photons produce forward scatter; low-energy scattered photons produce back scatter (see Fig. 2.12).

• Forward scatter may reach the film and degrade the image.

• The overall result of the interaction is ionization of the tissues (see Ch. 7).

To access the self assessment questions for this chapter please go to www.whaitesessentialsdentalradiography.com