Chapter 10 Effects of radiation
The beam of X-ray photons produced from the X-ray tube will lose its energy by interacting with atoms as it passes through various materials. These include the wall of the X-ray tube and its surrounding oil, the added filter, the collimators, the air, the exposed part of the patient, the couch, the image receptor and the floor. X-ray photons travel through air at virtually the velocity of light (3 × 108 m s−1) so they interact with these materials and transfer their energy almost instantaneously. Some of this energy will be absorbed in the various materials but some will also be scattered in different directions from the primary beam and will lose energy by interacting with other adjacent structures, such as parts of the patient not in the primary beam, the walls of the room and the lead-glass screen.
As each X-ray photon can be considered as a tiny packet of energy, the beam of X-ray photons carries energy from the X-ray target into the matter through which it passes. On penetrating matter, X-ray photons transfer energy by interacting with its atoms; this transfer of energy is called attenuation. The beam of X-ray photons is attenuated differently in various materials: in general, the denser the matter, the greater the attenuation. Denser materials include metals (particularly lead) and bone.
Attenuation is partly due to some X-ray photons being totally absorbed and partly to the energy of some X-ray photons being partially absorbed while the remainder is scattered in various directions (Fig. 10.1). Some X-ray photons are transmitted through the material unchanged, without interacting with any atoms.
The number of X-ray photons transmitted compared to the number attenuated in any particular type of material depends on the thickness of that material. In general, the thicker the material, the greater the attenuation. However, it is not a linear relationship where the same numbers of X-ray photons are attenuated in an equal thickness of material, but an equal percentage is attenuated in equal thickness. For example, 20% of photons may be attenuated in the first centimetre of material, then 20% of what is left in the second centimetre, and 20% of the remainder in the third centimetre, etc. This is called an exponential relationship and the percentage attenuated in each thickness is known as the linear attenuation coefficient (LAC or μ) for the specific material. This relationship is used in practice during quality control checks on X-ray equipment to measure the half value thickness/layer (HVT or HVL) of an X-ray unit. The measurement gives the thickness of aluminium that will attenuate 50% of the X-ray photons at a specific kV setting. This gives an indication of the penetrating power of the beam: the thicker the aluminium required to attenuate half of the beam, the more penetrating it is and it can be related (using published tables) to the total filtration present in the beam (see pp. 100, 107).
Photoelectric (PE) absorption occurs when an X-ray photon interacts with a bound electron, usually in the inner shell of an atom, when its energy exceeds the binding energy of the electron (Fig. 10.2). The atom may be in the patient (an atom of calcium in bone, for example) or it might be an atom of carbon in the carbon-fibre tabletop; or an atom of lead in the lead-glass screen.
The X-ray photon disappears, by transferring all its energy to the bound electron. This energy overcomes the binding energy of the electron, which then escapes the atom as a photoelectron, carrying any extra energy as kinetic energy.
As a result, the atom is ionised but will quickly regain stability as electrons rearrange within the atom to restore the original electron configuration, resulting in small bursts of electromagnetic radiation being released. This process is similar to that in the X-ray target following ionisation of target atoms by high-speed electrons to release characteristic radiation. In tissue, where elements have low proton numbers and correspondingly low binding energies, the characteristic radiation energies are extremely low and are usually absorbed within the atom with negligible effect.
An X-ray photon is more likely to undergo photoelectric absorption in dense matter containing atoms of higher proton number. It is also more likely to occur with a bound electron whose binding energy is just below the X-ray photon’s energy: this means PE absorption is much more likely to occur in tissue with lower energy X-ray photons of less than 25 keV.
Bone has a fairly high density and contains atoms of calcium and phosphorus, giving an effective proton number (Zeff) of approximately 12. Soft tissues, such as muscle and fat, are lower in density and contain atoms of lower proton number (particularly carbon, hydrogen and oxygen), giving an effective proton number of approximately 7. Air is primarily nitrogen and oxygen, giving a similar effective proton number to soft tissue, but has very low density (Table 10.1).
|Material||Density (kg m−3)||Effective proton number|
|Soft tissue (muscle)||1000||7.6|
|Soft tissue (fat)||900||6.5|