What happens when X-rays interact with matter?
When X-rays pass through matter, they may be transmitted, absorbed, or scattered. These interactions determine how much of the X-ray beam reaches the detector and therefore form the physical basis of radiographic image formation.
When X-rays pass through matter they may be transmitted, absorbed, or scattered, and these interactions determine attenuation and image contrast.
In diagnostic radiology, the two most important interactions are the photoelectric effect and Compton scattering, both of which contribute to attenuation of the X-ray beam.
Understanding the physics
As X-ray photons travel through the patient, they interact with atoms in the tissues along their path. The probability that an interaction will occur depends on the energy of the photon and the composition and density of the material it encounters.
If a photon passes through the material without interacting, it is transmitted and may reach the detector to contribute to image formation.
If the photon interacts with the material, it may be absorbed or scattered. Absorption occurs when the photon transfers all of its energy to the atom and disappears. This is the case in the photoelectric effect, where the photon ejects an inner-shell electron and is completely absorbed.
In other interactions, the photon is scattered, meaning it changes direction after transferring part of its energy to an electron. The most important example in diagnostic imaging is Compton scattering, where the photon interacts with a loosely bound outer-shell electron. After the interaction, the photon continues in a different direction with reduced energy.
These interactions collectively cause attenuation of the X-ray beam. Attenuation refers to the reduction in beam intensity as photons are absorbed or scattered while passing through matter.
The reduction in beam intensity follows the exponential attenuation relationship:
I = I0e−μx
where I0 is the initial intensity, I is the transmitted intensity, μ is the linear attenuation coefficient of the filter material, and is the thickness of the absorbing material.
Because different tissues attenuate X-rays to different degrees, varying amounts of radiation reach the detector, creating the contrast that forms the radiographic image.
Where this matters clinically
The interactions between X-rays and tissues determine how anatomical structures appear on radiographic images. Structures that strongly attenuate the X-ray beam, such as bone, allow fewer photons to reach the detector and therefore appear white. Structures that attenuate the beam less, such as air-filled lungs, allow more photons to reach the detector and appear darker.
Scatter interactions also influence image quality because scattered photons reaching the detector can reduce image contrast.
Understanding how X-rays interact with matter therefore explains both image formation and radiation dose to the patient.