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Absorption of X-Rays

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Absorption of X-Rays

While X-rays can pass through many body tissues, the majority are not completely transparent to X-rays. Most tissues are translucent to these wavelengths, with different tissues having different degrees of opacity. As shown by Wilhelm Conrad Röntgen, a fundamental property of X-rays is that they are absorbed (or attenuated) at different rates as they pass through different materials.

Why is X-ray absorption important?

X-ray absorption (or attenuation) allows us to use X-Rays to produce images. The dark and light areas on a radiograph image represent the intensity of X-Rays that reach the detector plate. They indicate the level of attenuation caused by the tissues between the source and detector. Because different tissues attenuate X-rays by different amounts, this is what produces image contrast, which enables us to distinguish structures inside the body.

Different types of X-ray absorptions

There are four main mechanisms of X-ray attenuation, and they depend on the energy of the incident photon. The four types consist of two scattering and two absorbing X-ray absorptions.

Simple (Rayleigh) scattering

Absorption of X-rays Simple Rayleigh scattering StudySmarterDiagram showing how simple scattering deflects X-ray photons, Ross MacDonald - StudySmarter Originals.

Simple scattering affects low-energy photons in the range of one to 20 kiloelectronvolts (keV). These photons do not have sufficient energy to displace an electron when colliding with atoms. The oscillating electric field of an X-ray photon interacts with an atom in the tissue, which induces a force between them. This force alters the trajectory of the photon and causes scattering.

Because the mass of the photon is much smaller than the atom, it is deflected from its path and scattered with no change in momentum. Typically, the photon continues to travel in a scattered forward direction; however, there is a low chance of it being deflected backwards after a head-on collision with an atom nucleus. This type of scattering only makes a minor contribution to the attenuation coefficient because X-rays used in imaging typically have energy higher than 20keV.

Compton scattering

Absorption of X-rays Compton scattering StudySmarterDiagram showing how Compton scattering produces a scattered low-energy photon and free-electron, Ross MacDonald - StudySmarter Originals

Compton scattering occurs when an X-ray photon with energy between 30keV and five MeV (megaelectronvolts) collides with an electron of a tissue atom. These photons have sufficient energy to eject the electron from its atomic orbit by exceeding its binding energy. The photons also transfer some of their energy to the electron in the process. The new lower-energy photon is also scattered by the interaction, resulting in this type of scattering, which produces both a free electron and a scattered, lower-energy X-ray photon.

Check out our explanation on Binding Energy.

Photoelectric effect absorption

Absorption of X-rays Photoelectric effect StudySmarterDiagram showing how the photoelectric effect allows an atom to absorb an incident photon to eject an electron and emit a new X-ray photon, Ross MacDonald - StudySmarter Originals

The photoelectric effect affects photons with energies less than 100keV. In this range, X-ray photons can have an energy equal to the shell binding energy of atoms in the tissue. This allows the photon and its energy to be absorbed by the atom, and energy is transferred to an electron that is ejected from the atom. This creates an ionised atom in a higher energy state, which returns to its ground state by emitting X-ray(s) with a wavelength characteristic of the atom type. These emitted X-rays are at a different energy level than the incident photon and will not travel coherently with X-ray photons from the source.

Pair production absorption

Absorption of X-rays Pair production absorbtion StudySmarterDiagram showing how pair production absorbs a high-energy photon to produce an electron-positron pair and subsequently emit two 511keV photons, Ross MacDonald - StudySmarter Originals.

This mechanism affects very high-energy photons over 1.022MeV. At these energy levels, a photon can interact with an atoms nucleus, transferring all its energy to produce an electron and a positron. These antiparticles may travel for a short distance before interacting with each other (or other nearby electrons/positrons), where they are annihilated and transformed into a pair of 511keV photons. The pair of newly produced photons travel from the point of annihilation in diametrically opposite directions, ensuring momentum is conserved. The effect of pair production absorption grows as the photon energy increases, meaning it is the dominant mechanism at high energies.

A little summary for you: There are four main mechanisms of X-ray attenuation: two, which scatter photons, and two, which absorb photons. The contribution of each of these mechanisms depends on the photon energy E and material (atomic number Z) of the tissue.

Overview of the different types of X-ray absorptions

Low-energy photons are more easily attenuated than those with higher energy during X-ray scans. This is because the probability of photoelectric absorption (the primary attenuation mechanism at X-ray scan energy levels) is proportional to (Z/E)3, where Z is the atomic number of the atoms in the tissue and E is the X-ray photon energy.

This also means that the lower-energy photons in the X-ray beam are, on average, absorbed sooner as they pass through the patient, resulting in increasing average photon energy from the front to the back of the patient. Since low-energy photons are more likely to be absorbed, the energy deposition dose is highest at the patients skin and decreases as the beam passes through.

Attenuation mechanism

Photon energy range

Variation of u with E

Variation of u with Z

Simple scatter

1-20keV

Photoelectric effect

< 100keV

Compton scattering

0.5-5.0MeV

Slowly falls as E increases

Independent

Pair production

> 1.022MeV

Rises as E increases

Implications of attenuation on the X-ray procedure

Because most energy deposition occurs near the skin, one of the risks of X-rays is skin injuries. This risk is greater for larger patients, as they will require higher doses for the beam to penetrate body parts and produce a helpful image.

Attenuation coefficients can estimate what types of tissue different regions represent based on the amount of attenuation of the initial beam intensity.

Absorption of X-rays formula: attenuation coefficient

The four main attenuation mechanisms outlined above show that for photons with a given energy, the material (influences Z) and tissue thickness control the amount of attenuation the X-ray beam undergoes. The intensity of X-Rays transmitted through a substance relative to the initial beam intensity is given by the equation below.

I0 is the initial intensity of the photons, x is the tissue thickness (distance travelled), and μ is the linear attenuation coefficient for the photon energy. Larger values of μ indicate greater X-ray attenuation, meaning substances like bone have a larger coefficient than soft tissues. The SI unit of attenuation coefficients is m-1.

Larger values of μ indicate greater X-ray attenuation.

Effect of attenuation on dose

If we want to produce an X-ray image with a good level of detail, the digital detector plate needs to measure a large enough number of photons to stand out against the background noise. The noise comes from photons that have been scattered as they travel through the body, or it can randomly arrive from an alternative source. The ratio of unattenuated photons in the X-ray beam (signal) to background noise is the signal-to-noise ratio (SNR). In X-rays, the SNR is related to the number of photons N in the X-ray dose.

The SNR improves as the number of photons increases, producing an image with more useful detail.

We can increase the number of photons in two ways: prolonging the exposure time (mA) or increasing the accelerating voltage in the X-ray tube (as N ∝ KV3).

Increasing the photon energy level also results in a lower proportion being attenuated by the patient’s tissue, which offsets the higher photon energy and results in a lower overall dose being absorbed. However, since the X-ray energy level increases and the attenuation rate decreases, the level of contrast in the image produced is poorer due to attenuation creating the contrast between tissue types. Therefore, balancing the image contrast, noise, and patient dose requires a trade-off between the photon energy/accelerating voltage and exposure time.

Contrast mediums

Some soft tissues have attenuation coefficients too low to create enough contrast in a radiograph image, so we can use contrast mediums to improve the visibility of these structures. Bromine or iodine compounds are the two most commonly used contrast mediums as they are harmless to humans and have large atomic numbers (Z), representing large atoms with many electrons.

The primary attenuation mechanism for X-ray imaging is the photoelectric effect. As this relies on the incoming photon colliding with an electron, larger atoms with a greater number of electrons are more likely to cause photoelectric scattering than smaller ones. Because of this, the photoelectric attenuation coefficient is proportional to the cube of the atomic number (μ∝Z3), making iodine or bromine far more absorbent than soft tissues, which primarily contain smaller atoms. This allows these compounds to be injected into blood vessels or the digestive tract to capture X-ray images of soft tissue structures.

Absorption of X-Rays - Key takeaways

  • The absorption (or attenuation) of X-rays as they pass through tissues in a patients body is what produces contrast in the image and allows us to distinguish tissues.
  • There are four main mechanisms of X-ray attenuation: two, which absorb photons, and two, which scatter photons. The contribution of each of these mechanisms depends on the photon energy E and material (atomic number Z) of the tissue.
  • We can use the attenuation coefficient μ to calculate the expected attenuation for a given material, thickness, and photon energy.
  • Achieving an image with sufficiently good contrast, low noise, and reasonable patient dose requires balancing the photon energy and exposure time.
  • Contrast mediums are compounds with large atomic numbers that can be introduced into soft tissues to raise their attenuation coefficient, improving image contrast.

Frequently Asked Questions about Absorption of X-Rays

The amount of absorption (or attenuation) of an X-ray beam is affected by the energy of the X-ray photons E and the atomic number(s) Z of the substance the beam is penetrating. The photon energy determines the relative contribution of the four main attenuation mechanisms, while substances with higher atomic numbers (and larger atoms) are more likely to absorb or scatter the beam, resulting in greater attenuation.

There are four main mechanisms of X-ray beam absorption (or attenuation). There are two scattering mechanisms, simple scattering and Compton scattering, and two absorption mechanisms, the photoelectric effect and pair production.

Final Absorption of X-Rays Quiz

Question

Why is X-ray absorption/attenuation a useful phenomenon for medical imaging?

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Answer

Different tissues attenuate X-rays by different amounts, producing variation in the X-ray intensities that reach the detector. This produces contrast in the image and allows us to distinguish tissues.

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Question

How many main X-ray attenuation mechanisms are there?

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Answer

There are four key mechanisms – two absorb the X-ray photons, while two scatter incoming photons.

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Question

List the four attenuation mechanisms in order of minimum required photon energy.


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Answer

1. Simple scatter 

2. Photoelectric effect 

3. Compton scattering 

4. Pair production

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Question

Outline how simple (Rayleigh) scattering attenuates the X-ray beam.

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Answer

The oscillating electric field of the photon interacts with tissue atoms, creating a force acting between the two particles. This force deflects the photon (which has negligible mass compared to the atom) and scatters it within the tissue. This reduces the intensity of the remaining X-ray beam, which passes through the tissue.

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Question

Outline how Compton scattering attenuates the X-ray beam.

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Answer

Photons with energy between 0.5-5.0MeV can transfer sufficient energy to electrons to eject them from their atomic shells. A portion of the photon energy is transferred during electron ejection, and the new lower-energy photon is scattered in a new direction, reducing the intensity of the remaining X-ray beam, which passes through the tissue.

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Question

Outline how the photoelectric effect attenuates the X-ray beam.

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Answer

X-ray photons with energy <100keV can have an energy equal to the shell binding energy of electrons around atoms in the material. The photon can therefore be absorbed by the atom and its energy transferred to an electron, which escapes and ionises the atom. As the atom drops from its ionised state back to ground energy, it emits an X-ray photon. The original photon is destroyed and the emitted photon does not travel coherently with the beam, attenuating its intensity.

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Question

Outline how the pair production effect attenuates the X-ray beam.


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Answer

Photons with energies >1.022MeV are sufficient to be absorbed by the nucleus of an atom and produce an electron-positron antiparticle pair. These antiparticles are quickly annihilated, which produces a pair of 511keV photons travelling in opposite directions from the annihilation point. As the original photon is absorbed and the emitted photons are not coherent with the beam, its intensity is attenuated.

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Question

Simple scattering has an effect on X-ray attenuation with which photon energy range?


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Answer

A photon energy range between 1-20keV.

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Question

The photoelectric effect has an effect on X-ray attenuation with which photon energy range?


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Answer

A photon energy range of < 100keV.

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Question

Compton scattering has an effect on X-ray attenuation with which photon energy range?


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Answer

A photon energy range between 0.5–5.0MeV.

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Question

Simple pair production has an effect on X-ray attenuation with which photon energy range?


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Answer

A photon energy range > 1.022MeV. 

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Question

Why do materials consisting of atoms with larger atomic numbers (Z) attenuate the X-ray beam more during medical scans?


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Answer

Medical X-rays typically operate in the range of 30-100keV, which means the photoelectric effect is the dominant mechanism. As this effect occurs when the photon collides with an electron orbiting an atom, atoms with larger atomic numbers (which also have more electrons) are more likely to produce a collision. This results in a higher rate of X-ray beam attenuation.

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Question

Why does the majority of the X-ray energy deposition dose occur near the skin?


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Answer

Photons with lower energy are more likely to be absorbed by tissue due to the photoelectric effect, as the probability of absorption is proportional to (Z/E)3. This means that the lower-energy photons are absorbed quickly as they penetrate through the skin, while higher-energy photons are more likely to pass all the way through the patient without being absorbed.

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Question

Why may larger patients have a higher risk of sustaining skin injuries from an X-ray?


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Answer

X-ray attenuation is proportional to the thickness of the substance the beam must penetrate, meaning larger patients can require a higher intensity beam to achieve good images.

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