The x-ray interactions are Photoelectric, Compton and Coherent. Photoelectric is mainly responsible for image contrast, Compton contributes to artifacts in the images, and Coherent scattering has little influence in most diagnostic (x-ray/CT) procedures.
Overview of the Physics Behind X-Ray Interactions
When x-rays interact with the human body during an x-ray exposure, they form an image that is highly dependent on the type of interactions of matter and x-rays. Diagnostic x-ray interactions are dominated by two different physical interactions – the photoelectric effect and Compton scatter.
Understanding the impact of the photoelectric effect and Compton scatter and their behavior as a function of energy can greatly improve your ability to select the best technical parameters for a given clinical situation.
We start with a high level summary graphic that demonstrates the differences between the x-ray interactions of: photoelectric, compton and coherent scattering and then go into detail on each of the interactions.
The Photoelectric Effect
The photoelectric effect is the dominant contributor to the generation of signal in an x-ray image as the x-ray is coming in and will be stopped and deposit its energy locally.
The photoelectric effect occurs when an x-ray interacts with an electron in the matter. The photo is completely absorbed and its energy is transferred to an electron that is removed from the electron cloud.
Since the electrons that are in the inner shells are at a more stable configuration the electrons in the outer shells will transition to an inner shell and a characteristic x-ray will be emitted. These secondary events are very low energy and are absorbed relatively locally and do not contribute to the measured image signal.
The likelihood of such interactions with inner shells depends strongly on atomic number Z (i.e. Z3), or how many protons are in nucleus.
Therefore, image contrast in x-ray and CT is much better for materials with high Z elements.
During this interaction, electrons which move to the inner shell, preserve energy and emit secondary x-ray photon.
Another important point is that the likelihood of interaction is much higher for lower diagnostic x-ray energies, i.e. (1/E3), where E is the energy of the x-ray photons.
Therefore, when possible it is typically beneficial to use lower energy photons for a given imaging task, provided that they can penetrate the patient.
Rad Take-home Point: In the photoelectric effect an x-ray comes in and deposits its energy locally mostly in an energetic electron (which then deposits its energy locally).
Compton Scattering
Compton Scattering is the second dominant effect in x-ray imaging. In this case, the x-ray photon interacts with an electron in the outer shell, and hence the likelihood of Compton Scattering doesn’t depend on Z.
As shown in the Figure the X-Ray photon knocks the electron out. Then the photon goes out in an opposing direction from the knocked out electron in order to conserve momentum.
It is important to remember here is that unlike in the photoelectric effect, the energy is not all deposited locally.
The scattered photon may still have a significant fraction of the energy of the incoming photon. It can still travel through the patient and potentially could have a secondary scatter effect or could get measured on the detector.
For more information on the impact of x-ray scatter on image quality and the effect of technical parameters on x-ray scatter please see our post on x-ray scatter.
Rad Take-home Point: In the Compton effect an x-ray interacts with a weakly bound electron and the electron and photon both continue on in opposing directions.
Coherent (Classical) Scatter
Coherent Scattering, it is one of the 3 interactions that can take place with diagnostic X-rays and the body. It also has other names ‘Elastic Scattering’ and ‘Rayleigh Scattering’.
Coherent Scattering happens when an X-Ray photon comes in, interacts with electron cloud and goes out. The X-Ray is scattered after this interaction but it has the same energy as it leaves.
If you imagine a rubber band ball and throw it against the wall, it will come off with approximately the same energy it had going in. That’s what we call elastic scattering. That’s why this interaction is called ‘Elastic Scattering’. For diagnostic imaging Coherent scattering only occurs at energies below 10keV.
For a lot of energy spectra used in diagnostic imaging; there are not very many photons below 10keV that pass through the pre-patient attenuators. Therefore,this effect is less relevant that Compton and Photoelectric Effect for diagnostic imaging.
For completeness we will mention that the likelihood is dependent on the number of protons (i.e. Z). So, if you have more protons, you’re more likely to have coherent scattering and it’s inversely proportional to 1 over Energy squared.
As the energy increases of the X-rays this effect is less likely. This is why there is not a big effect for most diagnostic X-ray exams.
Rad Take-home Point:
- Rad Take Home Point: Coherent scattering is an additional interaction to Compton Scattering and Photoelectric.
- It only occurs at very low energies. So, it’s not as important as the other two.
Energy Dependence of Interactions
In different parts of the body and at the different energy levels, photoelectric effect and Compton scattering have different contributions.
From the perspective of an image scientist or medical physicist the human body can usually be approximated as a bag of water for the soft tissue and with some bone distributed throughout.
The photoelectric and Compton effects have similar behavior as a function of energy but the energy where the transition occurs between photoelectric being dominant to Compton being dominant is at a higher energy in the case of bones.
In water photoelectric is dominant up to level of 26 keV, while in bones, it is dominant up to 45 keV. Beyond those transition points Compton scatter occurs more often than photoelectric.
As discussed above the likelihood of photoelectric interactions is proportional to Z3. This is what is driving the dominance of the photoelectric up to higher energies as the bones contain Ca and other high Z elements.
Rad Take-home Point: Photo-electric effect is dominant at low energies and for high Z materials the transition energy where Compton becomes dominant is significantly higher.
Summary
The regions in an x-ray image with the most attenuation are typically shown as bright in an x-ray image. These regions attenuate or absorb the x-rays at a higher rate than other regions.
The primary interactions dominating diagnostic x-ray imaging are the photoelectric effect and Compton Scattering.
In general, maximizing the contribution of photoelectric interactions will lead to the highest image contrast. This can be achieved by using high Z materials as contrast agents and/or using lower energy x-rays where the photoelectric effect becomes more likely.
To conclude this post we provide a summary table so you have a study guide with all of the information just in one place.
| Coherent Scatter | Photoelectric | Compton Scatter | |
|---|---|---|---|
| Products | X-Ray photon (=energy) | Electron Characterictic X-Ray (low energy) | Electron Scattered Photon (low energy) |
| Outcome | Direction change of x-ray | X-Ray stops and deposits energy locally | Some energy deposited, scattered x-ray in different direction |
| Energy Summary | Less than 10 keV | Dominant Below ~30 keV (1/E) | Dominant Above~30 keV (1/E) |
| Z Dependence | Z | Z^3 | Independent of Z |
| Impact on X-Ray Image | No Significant | Primary Contrast | Background Haze |
| Impact on Patient Dose | No Significant | Electrons, characteristic x-rays Deposit Dose | Electrons Deposit Dose |
| Impact on Staff Dose | No Significant | Not sign, except for interventions if in beam | Dominant Source of stray dose |
Coherent Scattering: In a coherent scattering event you have a diagnostic X-ray come in and then the X-ray goes out with a different angle but the same energy.
Photoelectric Effect: The diagnostic X-ray comes in and then that X-ray is stopped locally and an electron and a characteristic lower energy photon are emitted. Those both deposit their energy relatively locally.
Compton Effect: Compton is somehow in between Coherent Scattering and the photoelectric effect. The X-ray comes in, it’s scattered and an electron is scattered as well. The electron deposits its energy locally. So, for Compton you’ll have some energy going forward and some energy being deposited.
Dominant Interactions: Photoelectric and Compton are the dominant interactions. Coherent scattering really only plays impact at really low energies.
Outcome of photoelectric is that, that photon stopped and that energy is deposited locally. So, if we think of that from a dose perspective or from an imaging perspective, this is how contrast is generated on images. In terms of the Compton scattering, you have some energy deposited locally while the x-ray is changing its direction. So, some energy is going to keep going as the X-ray continues to traverse the matter.
Energy Summary: Less than 10keV for Coherent scattering and then depending on the material type, around 30keV is the transition between Photoelectric and Compton being dominant. So Photoelectric is dominant at lower energies and Compton is dominant at higher energies. Depending upon what the energy spectrum either photoelectric or Compton will be dominant.If the spectral has a lot of low energy photons, it’s going to be dominated by the Photoelectric effect.
Z-Dependence: The Z dependence is directly dependent on Z for coherent scattering. For photoelectric, it’s dependent on Z^3, so it’s very strongly dependent on Z. That’s why images of bone are exquisite on X-ray imaging because bones have relatively higher Z.
Compton scattering, Compton is independent of Z.
Impact on Image Contrast: Coherent scattering, does not a significant impact on an X-ray image. Especially with a standard diagnostic energy spectrum. Photoelectric, is the primary contrast source in your image. Compton scattering leads to a background haze in an X-ray image. It can lead to different more structured artifacts in a CT image.
Impact on Dose: Coherent scattering does not have a significant impact on patient dose.
Impact on Patient Dose: The electrons deposit the energy locally. So, both Photoelectric and Compton scattering lead to significant contribution there in terms of Patient dose.
Impact on Staff: Coherent and photoelectric are not significant. Compton scattering on the other hand is the dominant source of the background radiation in the room.
We think that the table above provides a good summary of the contributions of the different physical interactions of X-rays with matter.
With this material you can make either flash cards or make a table and cover up the different squares as you’re going through and just test yourself so that you can identify the different areas that these different interactions impact in terms of dose, and image contrast in X-ray imaging.