Photoelectric and Compton Effect
Photoelectric Effect
As photons that make up the primary x-ray beam pass into the body, some of them will be completely absorbed by the body, which means they do not pass through to the image receptor. This complete absorption of the photon happens when it has enough energy to eject an inner-shell electron from the atom it comes into contact with. The ejected electron is called a photoelectron and the ability to remove these electrons is known as ionization. This interaction of the x-ray photon and the inner-shell electron of the atom is known as the photoelectric effect. The ejection of the inner-shell electron causes the atom to become unstable, and an electron from an upper-level shell will drop down into the inner shell to fill the vacancy. The outer shell electron that falls into the inner shell to fill the void must give up some of its energy to make the transition. The energy that it gives up is in the form of a secondary x-ray photon. This secondary x-ray photon is classified as scatter radiation which may exit the patient or interact with other tissue electrons.
Compton Effect
An x-ray photon may not be absorbed as it passes through the body. It may, however, lose energy when it interacts with atoms in the body tissues. This process creates scatter radiation and is known as the Compton effect. When a photon ejects an outer shell electron from an atom, the ejected electron is called a Compton electron or a secondary electron. The original x-ray photon loses some of its energy and changes direction. It may continue to interact with other atoms, and it may pass through the anatomic part to interact with the image receptor. Compton interactions occur within all diagnostic x-ray energies.
Notes:
- The main difference between the Photoelectric effect and Compton effect is that in the photoelectric effect, the original photon gives up all of its energy when it comes into contact with the inner-shell electron. In the Compton effect, the original photon only gives up part of its energy when encountering an outer-shell electron.
- Photoelectric interactions generally decrease at higher kVp.
- Compton interactions generally increase at higher kVp.
Transmission occurs when when the incoming x-ray photon passes through the body without any interactions with the anatomic structures. More transmission occurs at higher kVp.