A beam of xrays may be:
 Transmitted: pass through unaffected or with a lower energy
 Absorbed: transfer all energy to matter and not pass through the patient to the film
 Scattered: diverted with or without energy loss
Attenuation
Attenuated xrays are those that are absorbed, transmitted with a lower energy or scattered. It is an exponential process and, therefore, the beam intensity never reaches zero. Attenuation of the beam can be represented numerically by:
 Half value layer
 Linear attenuation coefficient
 Mass attenuation coefficient
Half value layer (HVL)
This is the measure of the penetrating power of the xray beam and is the amount of matter required to attenuate the beam to half its energy value. It differs for different materials and strengths of beams. To calculate the factor of reduction use: 2HVL
e.g. if the HVL of a beam is 2 mm, by what factor is the beam attenuated if it passes through 8 mm of material?
8 mm = 4 HVLs
24 = 16
The beam is attenuated by a factor of 16
Linear attenuation coefficient (LAC)
This is the probability of the material to attenuate the beam. It can also be expressed as the amount of energy transferred to the material per unit of track length of the particle. The LAC (μ) is calculated by:
μ = 0.693 / HVL
Key:
μ = LAC, units: cm1
Mass attenuation coefficient
The MAC is a measure of the rate of energy loss by a photon beam as it travels through an area of material. By dividing LAC by the density of the material the effect of density is removed. The MAC is, therefore, independent of density and depends only on the atomic number of the material and the photon energy.
MAC = μ / ρ
Key:
μ = LAC, units: cm1
MAC units: cm2g1
ρ = density
ρ = density
Effect of beam quality on attenuation
The above only really apply to a monoenergetic (one energy value) beam of xrays from a point source (infinitely small area) travelling in a vacuum. In reality, the xray beam focus is not a fine point and contains photons of different energies that, once the leave the xray tube, do not travel in a vacuum.
Wider beam
Increased width of beam = increased scatter produced and measured = larger measured HVL
Heterogeneous beam
 The beams produced by xray tubes are photons of a wide range of energies.
 The lowerenergy photons are attenuated proportionally more than the higherenergy photons and are removed, leaving behind higher energy photons aka "beam hardening".
 The resulting beam is of a higher average energy.
 It can, therefore, penetrate tissue easier and the HVL is increased.
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Written by radiologists, for radiologists with plenty of diagrams to explain complicated concepts in an easytofollow way. An excellent resource for radiology physics revision.
Written by radiologists, for radiologists with plenty of diagrams to explain complicated concepts in an easytofollow way. An excellent resource for radiology physics revision.
Interactions with matter
Three processes may occur and contribute to attenuation:
 Compton effect
 Photoelectric absorption
 Elastic scatter
Compton effect
 Photon hits free electron
 Electron absorbs some of the photon's energy and is deflected
 The photon, having lost some energy, is deflected and scattered
The greater the angle of scatter the:
 Lower the residual deflected photon energy
 Higher the subsequent electron energy
Compton Linear Attenuating Coefficient (LAC)
Compton scatter is dependent on electron density and physical density but not on atomic number. The amount of scatter decreases with increasing photon energies but not significantly within the range of diagnostic xray energies.
Compton scatter = density / energy
The Compton LAC does not change over a large range of photon energies.
Photoelectric effect
In low Z materials (e.g. tissue and bone) the high energy photon collides with a bound electron. The released photon has very little energy and is absorbed immediately with the ejection of a further, lowenergy or "Auger" electron and all the energy is said to have been absorbed by the material.
Photoelectric LAC
The probability of a photoelectric effect increases:
 When electrons are more tightly bound (i.e. higher Z)
 When photon energy closer to binding energy. The higher the photon energy the less likely the photoelectric effect.
τ = ρZ3 / E3
Key:
τ = photoelectric LAC
ρ = density
Z = atomic number
E = photon energy
ρ = density
Z = atomic number
E = photon energy
Absorption edges
When the photon energy is as strong as the binding energy of the kshell electrons (Ek; for iodine this is 33 keV) the probability of photoelectric absorption jumps to a higher value resulting in a sharp increase in attenuation (the kedge).
For iodine this kedge is in the diagnostic xray energy range and is utilised to massively increase the photoelectric effect and therefore give greater tissue contrast. Soft tissues and bone also have a kedge but these occur at 1 keV or less so the kedge doesn't appear in the diagnostic energy range. As you can see from the graph above, the Compton effect does not change significantly over the range of photon energies for tissues.
Elastic scatter
Aka coherent, classical, unmodified or Rayleigh scattering.
 Photon bounces off an electron that is firmly bound to its parent atom
 Occurs if photon energy less than binding energy of electron
 No secondary electron is set moving and no ionisation or other effect is produced in the material
 Little significance in radiology
Σ Summary
 Attenuation is an exponential process  beam intensity never reaches zero
 Penetrating power of a beam is measured by its half value layer (HVL)  the depth of material that results in a 50% reduction in the beam intensity  factor of reduction = 2HVL
 Mass attenuation coefficient independent of density of material  depends only on atomic number of material and photon energy
 Wide beam  increases measured HVL due to increased scatter
 Heterogeneous beam  HVL increases with distance travelled due to beam hardening
Compton effect  Photoelectric effect 
Interactions with free / outer shell electrons  Interactions with inner shell electrons 
Part of electron energy transferred  Whole of photon energy transferred 
Depends on:
 Depends on:

More important in low density structures (e.g. air, water, soft tissues) and with high energy photons  More important in high Z structures (e.g. iodine, lead) and with low photon energy 
Both processes occur equally at:

Interaction with matter
Reviewed by Radiology Madeeasy
on
March 31, 2018
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