As the kilovoltage peak (kVp) is increased, what happens to the photoelectric effect?

As kilovoltage peak (kVp) increases, the photoelectric effect decreases. This phenomenon can be attributed to the nature of the interaction between X-ray photons and matter. The photoelectric effect predominately occurs at lower energies, where X-ray photons have sufficient energy to be completely absorbed by the atom, leading to the ejection of inner-shell electrons.

When kVp is increased, the energy of the X-ray photons is elevated, making them less likely to interact with matter through the photoelectric effect. At higher energies, the dominant interaction mode shifts towards Compton scattering, where the photon transfers only part of its energy to an outer-shell electron and continues on with reduced energy. This is because higher energy photons have a greater likelihood of penetrating the tissue without being absorbed, thus reducing the incidence of the photoelectric effect.

Consequently, for imaging techniques that utilize high kVp settings, the contribution from the photoelectric effect is diminished, leading to a decrease in the overall contrast of the images produced. This understanding helps radiologic technologists to optimize imaging parameters for the desired diagnostic outcomes.