E-Book, Englisch, 493 Seiten
E-Book, Englisch, 493 Seiten
ISBN: 978-3-86867-333-3
Verlag: Quintessenz
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
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Weitere Infos & Material
Chapter 01: The Limitations of Conventional Radiography and Adjunct Imaging Techniques
Chapter 02: Radiation Physics
Chapter 03: Cone Beam Computed Tomography
Chapter 04: Using CBCT: Dose, Risks and Artefacts
Chapter 05: Dentoalveolar Anatomy
Chapter 06: Assessment of Root Canal Anatomy
Chapter 07: Apical Periodontitis
Chapter 08: Non-surgical and Surgical Re-treatment
Chapter 09: Traumatic Dental Injuries
Chapter 10: Root Resorption
Chapter 11: Vertical Root Fractures
Chapter 2
Radiation Physics
Simon C Harvey Introduction
The aim of this chapter is, firstly, to explain what X-ray radiation is and, secondly, to describe the production and interaction of X-ray radiation. The electromagnetic wave
The electromagnetic wave describes a wave of energy that has an electric field alternating (between positive and negative) along one axis. At right angles to this, a magnetic field alternates between north and south (Fig 2-1). The two are often drawn as one wave to make their depiction easier. All electromagnetic waves travel at the same speed in a vacuum, irrespective of their energy—the speed of light = 299 792 458 ms-1. The speed of any wave is related to its wavelength and frequency by the following equation: speed = wavelength × frequency. As the speed is known and constant (speed of light = c), the wavelength and frequency of different electromagnetic waves must change accordingly. At one end of the spectrum, the waves have a very long wavelength (and therefore low frequency) and are lower in energy. At the other end, the waves have a very short wavelength, high frequency, and are very high in energy (Fig 2-2). Fig 2-1 The electromagnetic wave. The electromagnetic spectrum is continuous. Although we name different parts of the spectrum and provide cut-offs, these are arbitrary, and the different categories of waves differ only in the energy they possess. Fig 2-2 The electromagnetic spectrum (NASA). Fig 2-3 A rotating anode X-ray tube. Fig 2-4 Bremsstrahlung radiation production. It is noticeable that visible light only makes up a narrow band in the spectrum. Waves with frequencies below 4 × 1014 Hz are not visible to the human eye, and frequencies above 8 × 1014 Hz are equally invisible. Above a certain energy level, the waves can become ionising and cause damage to biological tissues. Higher-energy ultraviolet waves, X-rays, and gamma rays all have enough energy to damage human cells. Individual photons or continuous waves? We have seen that electromagnetic waves are a continuous wave: however, we often refer to ‘photons’, which have a particulate form and particulate properties. This is an alternative way of describing the interactions of electromagnetic waves more easily, and will appear throughout the book. It should be noted, however, that the photons have no mass, and even though they have particulate properties and can be described individually, they are in fact discrete packets of energy. X-ray production
X-rays are high-energy electromagnetic waves or photons. They occur naturally and are emitted from some radioactive atoms; however, this is not amenable to everyday imaging, as the radioactive source would deplete, and be constantly irradiating, and the amount and energy of the radiation could not be easily controlled. Therefore, an artificial production method is needed. An X-ray tube contains several essential components, as illustrated in Figure 2-3 and listed in Table 2-1, with a description of their purpose. The X-rays are produced in two ways: Bremsstrahlung An incoming electron emitted from the Tungsten filament is accelerated through a vacuum towards the Tungsten anode. As it strikes and passes through the anode, it may be attracted to the positive nucleus of an individual Tungsten atom. This attraction will simultaneously deflect the trajectory of the fast-moving electron and cause it to slow down rapidly. This rapid deceleration and change of path results in energy loss, which is emitted as an X-ray photon. The greater the deflection and slowing of the electron, the greater the resultant X-ray photon energy. As each interaction between an individual electron and a nucleus of the Tungsten atom in the anode is different and the energy loss is dissimilar, the energy profile of the X-rays produced (the spectrum) is over a wide range. The majority of X-rays—approximately 80%—from an X-ray tube are produced in this method. It should be noted that the interaction here is between an incoming electron released by the filament and the nucleus of the Tungsten atoms in the target (Fig 2-4). Characteristic radiation If the incoming electron passes close to the nucleus and has enough energy, it can knock out a tightly bound inner shell electron (K shell) from the Tungsten atom. This leaves a vacant inner shell, which is filled quickly by an outer shell (L or M shell) electron from the same atom. As the outer shell electron ‘jumps down’ energy shells, it loses energy in the form of X-ray radiation. In this case, the energy the outer electron needs to lose when ‘jumping’ to the inner shell is a known amount for each different atom; so, the X-ray produced has exactly that amount of energy. The outer shell electron may come from an L or M shell, so the energy will differ slightly between the two. This is known as characteristic radiation—it is characteristic of that particular atom (Fig 2-5). For Tungsten, the values for characteristic radiation are 58 keV and 68 keV. Fig 2-5 Characteristic radiation production. It should be noted that for characteristic radiation to be produced, the incoming electron must have enough energy to knock out the inner K shell Tungsten electron. The inner Tungsten electron needs 70 keV of energy to be knocked out; so only electrons with this amount of energy or more have the chance to produce characteristic radiation with a Tungsten target. This means that X-ray tubes operating below 70 kV will have no chance of producing characteristic radiation. Cone beam computed tomography (CBCT) sets generally use 80 to 120 kV, which is enough for characteristic radiation production with a Tungsten target. Heat The two interactions described above result in X-ray production; however, this is not the fate of every electron released by the cathode that strikes the anode in the X-ray tube. About 99% of energy is converted to heat, so only 1% of energy results in X-ray production. Therefore, X-ray tubes are very inefficient at X-ray production. This large amount of heat energy is the reason for heat removing devices such as the rotating anode (see motor in Fig 2-3) and outer cooling oil. Table 2-1 X-ray tube components and their purpose. Component Purpose Notes Tungsten filament Produces a supply of electrons by thermionic emission Heats up via a low-voltage circuit to approx 2200°C Tungsten anode/target for electrons Large potential difference accelerates the electrons to a high speed, causing them to smash into the anode Component Vacuum Ensures the electrons can be accelerated uninterrupted This means excess heat created in the tube cannot be lost by convection as there is no convection medium (air) Lead casing Prevents X-rays leaving in other directions Oil in outer case Helps with heat dispersal and insulates the unit electrically An oil leak is very serious and the tube must not be used Rotating motor Rotates the anode, allowing a greater heat loading Spins at up to 10 000 rpm High-frequency generator Provides a near constant high kV and therefore direct current Older, smaller dental sets may use mains AC (alternative current), which is inefficient at X-ray production Tube window The only part of the lead casing that lets out X-rays Often aluminium, and contributes towards filtration Fig 2-6 Bremsstrahlung spectrum profile. Fig 2-7 Bremsstrahlung plus characteristic radiation. Fig 2-8 Filtered profile; note the lower-energy photons to the left have been removed. Spectrum profile Bremsstrahlung radiation is produced over a wide range of energies up to the maximum tube potential, as depicted in Figure 2-6. If we use a tube operating at over 70 kV, then we also have characteristic X-rays, which are at specific values (see Fig 2-7). Filtering Only the higher-energy photons that have the potential to pass through the patient and record at the receptor are useful for imaging. The lower-energy photons are absorbed by the patient and only contribute to dose. This is discussed again later on. Filtering is the process whereby lower-energy photons are removed. The X-ray tube itself does some filtering by its inherent properties; the rest is added, usually in the form of...
