A Summary of the Physics Associated with X-Ray Interactions

 

The type of interactions between matter and x-rays that occur when x-rays interact with the human body during an x-ray exposure greatly influences the image that results. The photoelectric effect and Compton scatter are the two main physical interactions that dominate diagnostic x-ray interactions.

Your capacity to choose the appropriate technical parameters for a specific clinical condition can be significantly improved by comprehending the effects of the photoelectric effect, Compton scatter, and their behavior as a function of energy.

We begin with a high-level overview graphic that highlights the differences between photoelectric, compton, and coherent scattering as x-ray interactions, and we then go into detail on each interaction.

 

The photoelectric effect

 

Once the x-ray enters, is stopped, and deposits its energy locally, the photoelectric effect is the primary factor in the creation of a signal in an x-ray image.

When an electron in the matter interacts with an x-ray, the photoelectric effect happens. As an electron is taken out of the electron cloud, the photo is totally absorbed and its energy is transferred to that electron.

The electrons in the outer shells will change to an inner shell because they are in a more stable configuration in the inner shells, and a distinctive x-ray will be released. These secondary events have very low energies, are absorbed relatively locally, and do not affect the picture signal that is being measured.

The atomic number Z (i.e., Z3), or the number of protons in the nucleus, has a significant impact on the likelihood of such interactions with inner shells.

As a result, materials with high Z components have substantially greater image contrast in x-ray and CT scans.

The electrons that travel to the inner shell during this contact maintain their energy and release secondary x-ray photons.

Another crucial aspect is that lower diagnostic x-ray energies, or (1/E3), where E is the energy of the x-ray photons, have substantially greater interaction probabilities.

For a particular imaging task, it is therefore normally advantageous to employ lower-energy photons whenever possible, provided that they can pass through the patient.

X-ray 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

 

The second important x-ray imaging phenomenon is Compton scattering. Because an electron in the outer shell interacts with the x-ray photon in this instance, the chance of Compton scattering is independent of Z.

The electron is ejected by an X-ray photon, as shown in the image. In order to maintain momentum, the photon then exits in the opposite direction from the knocked-out electron.

It’s important to keep in mind that, in contrast to the photoelectric effect, not all of the energy is localized here.

The energy of the incoming photon may still be present in considerable amounts in the dispersed photon. It may still pass through the patient and maybe cause a secondary scatter effect or cause the detector to register the signal.

X-ray 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.

(Classical) Coherent Scatter

 

One of the three possible interactions between the body and diagnostic X-rays is coherent scattering. Elastic scattering and Rayleigh scattering are additional names for it.

When an X-ray photon enters, interacts with an electron cloud, and then exits, coherent scattering occurs. After this collision, the X-ray is scattered but retains its original energy.

Imagine a rubber-band ball and throw it against a wall. It will bounce off with roughly the same energy it had when it was thrown. Elastic scattering is what we refer to as such. Elastic Scattering is the name given to this interaction because of this. the use of diagnostic imaging Only energies below 10 keV result in coherent scattering.

There aren’t many photons below 10 keV that make it through the pre-patient attenuators for a lot of the energy spectra utilized in diagnostic imaging. Hence, for diagnostic imaging, this effect is less important than the Compton and photoelectric effects.

To be thorough, we’ll explain that the likelihood varies with the number of protons (i.e., Z). As a result, coherent scattering is inversely proportional to energy squared and is more likely to occur when there are more protons.

The likelihood of this impact decreasing as X-ray energy rises As a result, the majority of diagnostic X-ray scans have little impact.

X-ray 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

 

The contributions of the photoelectric effect and Compton scattering vary depending on the location of the body and the energy level.

The human body can typically be compared to a bag of water for the soft tissue and some scattered bone from the viewpoint of an imaging scientist or medical physicist.

The photoelectric and Compton effects behave similarly as a function of energy, although in the case of bones, the energy at which the switch from photoelectric to Compton dominance occurs is higher.

In contrast to bones, where it dominates up to 45 keV, water exhibits photoelectric dominance up to a threshold of 26 keV. Compton scatter is more common than photoelectric scatter outside of those transition sites.

Before, it was mentioned that the probability of photoelectric contacts is inversely proportional to Z3. Given that bones contain calcium and other high-Z elements, this is what is causing the photoelectric to predominate at higher energies.

X-ray Take Home Point:

 

• The photo-electric effect is dominant at low energies, and for high-Z materials, the transition energy where Compton becomes dominant is significantly higher.

Frequently Asked Questions

 

 

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