Effects of radiation

Chapter 10 Effects of radiation





KEY POINTS















ATTENUATION


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.




When a beam of X-ray photons passes through the body, the difference between parts through which X-ray photons are transmitted and those where they are absorbed results in an image.





PHOTOELECTRIC ABSORPTION


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.




THE IMPLICATIONS OF PHOTOELECTRIC ABSORPTION IN PRACTICE


The main implications to consider are:








Feb 20, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Effects of radiation
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