Chapter 22 Factors affecting X-ray beam quality and quantity

22.1 Aim

The aim of this chapter is to consider the various factors which have an influence on the quantity and/or the quality of the beam of radiation from the X-ray tube.

Definitions

The quantity of radiation in an X-ray beam is a measure of the number of photons in the beam. The terms quantity and exposure are often interchanged in radiography as the higher the quantity or amount of radiation, the greater the exposure to a structure. In fact, probably the simplest method of comparing the quantity of two beams of radiation is to compare the exposure received by a structure. As we shall see in Chapter 27, the exposure is measured using the unit of air kerma. As the quantity of radiation increases, so does the intensity of the beam.

The quality of a beam of X-rays is a measure of its penetrating power. As we saw in Section 21.6, the quality of the beam is related to its average photon energy. In Sections 20.7 and 20.8 we saw that a monochromatic beam of radiation is exponentially absorbed by a uniform medium and so the penetrating power of two beams may be compared by comparing their half-value thickness – the higher the value of the half-value thickness, the more penetrating the beam. Although the beam of X-rays from the tube is not monochromatic, but has a continuous spectrum over a wide range of energies, the half-value layer is a useful way of comparing the penetrating power of X-ray beams.

However, changing the quality of the radiation beam also affects the intensity of the beam. For a given quantity of radiation, the higher the quality of the radiation, the greater the intensity of the radiation beam.

The intensity of a beam of X radiation is defined as the total amount of energy – measured at right angles to the direction of the beam – passing through unit area in unit time. Although measured in units of joules per metre squared per second, in radiography we tend to use one of its effects – the ionization of air or of air kerma – as a measurement of radiation beam intensity.

As can be seen from the definitions of quantity and quality, any factors which change the quantity or the quality of the radiation beam will bring about a change in the beam’s intensity.

22.2 Introduction

In Chapter 21 we considered the mechanisms by which X-rays were produced at the anode of the X-ray tube. In this chapter we will consider the various factors which influence the quantity and/or the quality of the X-ray beam and hence its intensity at a given point. Before we look at this in any more detail, it is first important to ensure that we understand the meaning of the terms quantity, quality and intensity as applied to a beam of X radiation.

22.3 The effect of mA on the X-ray beam

If the current through the X-ray tube (mA) is, for example, doubled, the number of electrons flowing across the tube in unit time is doubled. If all the other factors remain unchanged, each electron will have the same chance of creating X-ray photons and so the number of photons of each energy produced per unit time will be doubled. If the mA is halved, the same argument can be used to show that the number of X-ray photons of each energy is also halved. Thus we can say that the quantity of the X-ray beam per unit time (or the beam intensity) is directly proportional to the mA through the tube.

Equation 22.1

The effect on the X-ray beam of altering the mA is shown in Figure 22.1. Note that the area under the graph for 200 mA is half the area under the graph for 400 mA. The maximum photon energy and the minimum photon energy are the same in each case and the average photon energy remains unaltered.

Figure 22.1 The effect of the mA on the X-ray spectrum. Note that the quantity of the radiation changes – as shown by the alteration of the area under each curve – but the quality of the radiation is unaltered – as shown by the maximum photon energy and the peak photon energy being at the same energy for each graph. Thus we can say that the mA selected for an exposure affects the quantity of the X-ray beam but does not affect the quality of the beam – an increase in the mA will produce an increase in the quantity of radiation from the target.

22.4 The effect of kVp on the X-ray beam

The kVp across the X-ray tube influences the force of attraction experienced by an electron released by the filament as it moves towards the anode. Thus, if the kVp is increased, then the kinetic energy of the electron at the point when it starts to interact with the target will be increased. As we saw in Section 21.4.2, the efficiency of X-ray production by Bremsstrahlung is proportional to E² and so this improved efficiency means that:

Equation 22.2

As we already discussed in Section 21.4.2, increasing the kVp will also increase the energy of the maximum-energy photons in the beam – if the kVp is 50, then the maximum photon energy is 50 keV and if the kVp is 100, then the maximum photon energy is 100 keV. As the average photon energy is approximately 30–50% of the maximum photon energy, increasing the maximum photon energy will also increase the average photon energy.

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