Photoelectric absorption in tissue (X-ray attenuation)

The **photoelectric absorption** mechanism is a key process by which X-rays are attenuated (i.e., reduced in energy or intensity) as they pass through tissues. In this process, an incident X-ray photon is entirely absorbed by an inner-shell electron of an atom, ejecting the electron as a photoelectron. The probability of this effect is highly dependent on the atomic number (Z) of the absorbing material—proportional to Z^3—and inversely on the photon energy (E^3), making it most likely at lower photon energies and in materials with higher atomic numbers such as bone or contrast agents like iodine. The effect is responsible for most tissue contrast in diagnostic X-ray and CT imaging. It is not a drug target, but is manipulated in medicine by selecting imaging parameters and using contrast agents that increase Z locally within the body. Safety concerns include patient radiation dose and, where contrast is used, the risk associated with contrast media.

Other names

Photoelectric effect in tissuesX-ray photoelectric absorptionX-ray attenuation (via photoelectric absorption)

Not applicable (Contrast agents increase X-ray attenuation by increasing the local probability of photoelectric absorption due to their high atomic number.)