Compton Scattering (Radiography Physics)
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Remember that there are three primary ways x-ray radiation can interact in the human body:
Photoelectric Effect
Coherent Scattering
Compton Scattering
Compton Scattering is extremely significant in medical imaging.
• It’s the most common radiation interaction at most energy ranges
• It affects the radiation dose to the patient
• It affects the radiation dose to x-ray techs, and…
• It affects radiography image quality.
Like all x-ray interactions, Compton scattering can happen when an x-ray photon leaves the x-ray tube and enters the patient’s body.
• With Compton scattering, this incident photon collides with an electron, usually a loosely bound outer shell electron.
• Two things happen as a result of this collision… 1) the electron is ejected out of orbit (called ionization), and 2) the photon decreases in energy and changes direction (called scattering).
• That’s why this interaction is called “scattering” – the photon is scattered off its original path.
The difference between Compton scattering and the photoelectric effect is actually pretty simple…
• With the PE effect, the incoming photon is completely absorbed by the electron. The electron is ejected out of the atom, but there’s no scattered photon.
• With Compton scattering, the energy of the incoming photon is only partially absorbed by the electron. The electron is knocked out of the atom, but a scattered photon also comes out of the atom.
• Basically, PE effects result only in an ionized electron, but Compton scattering results in an ionized electron AND a scattered photon.
What happens to the scattered photon and the ejected electron? The destination of these two Compton products is what makes Compton scattering so significant.
• The ejected electron is eventually absorbed by the body, which is why Compton scattering does affect the patient’s radiation dose.
A lot of things can happen to the scatted photon:
• It may also be absorbed by the body and increase the patient’s dose.
• This photon might also be scattered out of the patient and towards the radiographer! This is why Compton scattering is the most important source of occupational dose.
• Finally, the scattered photon might be scattered out of the patient and towards the receptor.
• Compton scatter photons decrease image quality by decreasing image contrast. Use of radiographic grids helps to prevent scattered photons from reaching the receptor.
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There are specific terms used to describe the direction of the scattered photon.
• Compton photons scattered back towards the x-ray tube are called “backscatter” or “large angle scatter”
• Photons scattered towards the receptor are called “forward scatter” or “small-angle scatter”
• Photons scattered perpendicular to the path of the main beam are called “side scatter”
The direction of the scattered photon affects its energy.
• Backscattered (“large angle scatter”) photons have the lowest energy
• Forward scattered (“small-angle scatter”) photons have the highest energy
There are few things that affect the amount of Compton scattering and the direction of Compton scattering:
• Increasing patient thickness increases the number of Compton interactions taking place in the body. Maybe you’ve heard the saying, “More matter more scatter”. The more scatter produced the more scatter will be on the image.
• Increasing the field size (or receptor size) also increases the number of scattered photons reaching the receptor.
• Increasing kVp doesn’t affect the amount of scattering produced, but it causes more of the Compton photons to be scattered towards the image receptor. That’s part of the reason why increasing kVp decreases image contrast. More kVp means more scatter on the receptor.
To keep Compton scattering to a minimum, the x-ray technologists should use a low kVp, a small receptor size, and only take x-rays of small patients…
In summary…
• Compton scattering is one of the three major x-ray interactions
• This interaction results in a scattered photon and an ionized electron
• It affects patient dose, occupational dose, and image quality
• It is the main source of dose to technologists
• It is the main reason for scatter radiation on the image receptor
