What is the photoelectric effect?
The photoelectric effect is an interaction in which an X-ray photon transfers all of its energy to an inner-shell electron, ejecting that electron from the atom. The photon is completely absorbed, and the ejected electron is called a photoelectron.
The photoelectric effect occurs when an X-ray photon is completely absorbed while ejecting an inner-shell electron, and it is a major contributor to radiographic image contrast.
This interaction is an important mechanism of X-ray attenuation in diagnostic imaging and contributes significantly to image contrast, particularly between materials with different atomic numbers.
Understanding the physics
In the photoelectric effect, an incoming X-ray photon interacts with a tightly bound electron in one of the atom’s inner shells, most commonly the K-shell. For the interaction to occur, the photon must have energy at least equal to the binding energy of that electron.
When the photon collides with the electron, it transfers all of its energy to the electron and disappears. The electron is ejected from the atom with kinetic energy given by:
Ek = Ephoton − Ebinding
where Ephoton is the energy of the incident photon and Ebinding is the binding energy of the electron.
The removal of an inner-shell electron leaves the atom in an unstable state with a vacancy in the electron shell. An electron from a higher shell subsequently falls into this vacancy, releasing energy in the form of characteristic radiation or Auger electrons.
The probability of the photoelectric effect occurring depends strongly on both atomic number and photon energy, and can be approximated by:
PPE ∝ Z3/E3
where is the atomic number of the absorber and is the photon energy.
This relationship shows that the photoelectric effect is much more likely in materials with higher atomic numbers and at lower photon energies.
Where this matters clinically
The photoelectric effect plays a key role in producing image contrast in radiography. Because the probability of this interaction increases strongly with atomic number, tissues containing higher atomic number elements attenuate the X-ray beam more strongly.
For example, bone contains calcium, which has a higher atomic number than the elements that make up soft tissues. As a result, photoelectric absorption occurs more frequently in bone, causing it to attenuate the X-ray beam more strongly and appear brighter on radiographic images.
Contrast agents such as iodine and barium also rely on this effect. Their high atomic numbers greatly increase photoelectric absorption, making structures containing these agents more visible on imaging.