compton scattering VS photoelectric VS coherent scattering

 X-Ray Interactions for Radiography and CT Technologists

Compton Scattering

In x-ray imaging, Compton Scattering is the second most important impact. The x-ray photon interacts with an electron in the outer shell in this situation, and so the chance of Compton Scattering is unaffected by Z.

The electron is knocked out by the X-Ray photon, as indicated in the diagram. To conserve momentum, the photon then travels in the opposite direction as the knocked-out electron.

It's vital to keep in mind that, unlike the photoelectric effect, the energy isn't fully deposited locally in this case.

A considerable portion of the energy of the entering photon may still be present in the scattered photon. It can still pass through the patient, where it may cause a secondary scatter effect or be detected by the detector.

Photoelectric Effect

The photoelectric effect is the most important factor in the formation of signal in an x-ray image because the x-ray will be stopped and its energy will be deposited locally.

When an x-ray interacts with an electron in matter, the photoelectric effect occurs. The photon is totally absorbed, and the energy of the photon is transferred to an electron that has been withdrawn from the electron cloud.

The electrons in the outer shells will transition to an inner shell because the electrons in the inner shells are in a more stable state, and a characteristic x-ray will be released. These secondary events have a low energy, are absorbed locally, and do not contribute to the image signal assessed.

The likelihood of such interactions with inner shells depends strongly on atomic number Z (i.e. Z3), or how many protons are in nucleus.

As a result, for materials with high Z components, picture contrast in x-ray and CT is substantially improved.

Electrons that travel to the inner shell keep their energy and release secondary x-ray photons during this interaction.

Another key factor to remember is that the possibility of contact is substantially higher for lower diagnostic x-ray energies, i.e. (1/E3), where E is the x-ray photons' energy.

As a result, it is usually preferable to use lower energy photons for a given imaging task when possible, provided that they can penetrate the patient.

Coherent Scattering

Coherent Scattering is one of three possible interactions between diagnostic X-rays and the human body. Elastic Scattering and Rayleigh Scattering are two more names for it.

When an X-Ray photon enters, interacts with the electron cloud, and then exits, it is called coherent scattering. The X-Ray is scattered as a result of this collision, but it retains its energy.

If you imagine a rubber band ball and toss it against a wall, it will bounce back with roughly the same amount of energy as it went in. This is referred to as elastic scattering. This interaction is referred to as 'Elastic Scattering.' For the purpose of diagnostic imaging Coherent scattering occurs only at energies less than 10 keV.

There aren't many photons below 10 keV that pass through the pre-patient attenuators for many of the energy spectrum utilised in diagnostic imaging. As a result, this effect is less important for diagnostic imaging than the Compton and Photoelectric Effects.

For the sake of completeness, we'll state that the probability is proportional to the number of protons (i.e. Z). So, the likelihood of coherent scattering increases as the number of protons increases, and it is inversely proportional to 1 over Energy squared.

As the energy of the X-rays increases, this effect becomes less frequent. As a result, most diagnostic X-ray scans have little effect.