Chapter 25 The radiographic image

25.1 Aim

The aim of this chapter is to consider the major factors involved in the production of a radiographic image. The chapter will first consider the attenuation patterns in a patient that will produce an X-ray image and will then consider how this reacts with a recording medium to produce a radiograph. The chapter also summarizes how the selection of exposure factors affects the quality of the radiographic image produced.

25.2 Introduction

The quality of the radiographic image is affected by a number of geometrical factors that determine the magnification of the image and the amount of geometrical unsharpness produced. The radiographic image depends on more than geometrical considerations. This chapter considers the other factors which contribute to the image quality. To understand this, it is necessary to understand the contribution of photoelectric absorption and Compton scattering to the final radiographic image (see Ch. 23).

It is easier to understand the final image quality if we consider image formation as a two-stage process:

1. The production of an X-ray image pattern as the beam of radiation is attenuated by the patient.

2. The production of a radiographic image as this radiation pattern interacts with a recording medium.

Therefore, this chapter will consist of two halves, each looking at one of these stages.

25.3 The X-ray image pattern

25.3.1 Attenuation of the X-ray beam by the body

We will assume, for simplicity, that the radiation beam from the X-ray tube striking the body is of uniform intensity across the beam. When this beam interacts with the body substance, different structures will cause different amounts of attenuation and a ‘pattern’ of radiation intensities is transmitted to the imaging device. If all structures in the beam attenuated the radiation by the same amount, there would be no pattern and no image of any structures would be seen on the radiograph. A simple example of such differential absorption is shown in Figure 25.1, where two separate rectangular blocks of bone and soft tissue are shown interacting with the X-ray beam. The profiles of the incident (I₀) and the transmitted (I_T) radiation intensifies are also shown. If again we assume, for simplicity, that the attenuation of the radiation beam is exponential, then:

Figure 25.1 A comparison of the attenuation of an X-ray beam by an equal thickness of bone and soft tissue. Note that the attenuation of X-rays by bone is greater than that by soft tissue.

Equation 25.1

where I₀ is the intensity of the X-ray beam before it enters the patient, μ(B) is the total linear attenuation coefficient for bone and μ(T) is the total linear attenuation coefficient for soft tissue. I_B is the intensity of the radiation transmitted through a thickness d of bone and I_T is the intensity transmitted through a similar thickness of soft tissue. As can be seen from Figure 25.1, I_B is less than I_T. This is because μ(B) is greater than μ(T). There are two physical reasons for this:

1. The density of bone is approximately twice that of soft tissue (ρ_B=1.8; ρ_T=1.0).

2. The average atomic number of bone is approximately twice that of soft tissue (Z_B=14; Z_T=7.5).

The total linear attenuation coefficient is proportional to the number of atoms present in unit volume and the density of the medium. As the density of bone is twice that of soft tissue, then there must be twice as many atoms in unit volume and so, all other things being equal, the linear attenuation coefficient for bone would be twice that for soft tissue.

To appreciate the importance of the difference in atomic number, we must consider the attenuation process occurring. The equations for each process are summarized below:

Equation 25.2

where τ is the linear attenuation coefficient for the photoelectric effect, σ is the linear attenuation coefficient for Compton scattering, ρ is the density of the attenuator, Z is its atomic number and E is the photon energy. The higher atomic number of bone means that it will greatly attenuate suitable radiation by the photoelectric effect.

As the total attenuation is a combination of both the photoelectric effect and Compton scattering, in the diagnostic energy ranges, a given thickness of bone will attenuate radiation approximately 12 times the level of an equal thickness of soft tissue.

The findings are summarized in Table 25.1. The process of photostimulation is (See page 185). The essential points to be taken from the table are that in the diagnostic range of photon energies, the higher atomic number of bone results in photoelectric absorption being the main attenuation process, whereas the lower atomic number of soft tissue means that Compton scattering is the main attenuation process. (In the therapy range of photon energies, the dominant attenuation processes are Compton scattering and pair production, both of which are less dependent on the atomic number of the attenuator.)

Table 25.1 Comparison of linear attenuation in bone and soft tissue

ATTENUATOR	PHOTOELECTRIC τ ∝ ρ×Z³/E³	COMPTON SCATTERING σ ∝ ρ (ELECTRON DENSITY)/E	TOTAL ATTENUATION μ=τ+σ
Bone Z=14 ρ=1.8	Photoelectric absorption is high when photon energy is low: 12–16 times greater than soft tissue	Predominates at high photon energies 500 keV to 5 MeV	Mainly photoelectric absorption at diagnostic energies
Soft tissue Z=7.5 ρ=1.0	Significant at low photon energies <25 keV	predominates at photon energies > 30 keV	Compton scattering is the dominant process if the average photon energy is greater than about 30 keV

A more realistic example of attenuation is given in Figure 25.2. This simulates the presence of a piece of bone surrounded by soft tissue. A profile of the transmitted radiation intensity is also shown and it can be seen that its minimum corresponds to the maximum thickness of the bone (point A in the figure).