X-ray interaction with matter: Rad Tech

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X-rays are used in radiology because of how they interact with matter. An X-ray beam is made of photons, which are packets of electromagnetic energy.
As photons pass through the body, they may be transmitted without interacting, scattered, or absorbed. When they interact, they do so through 5 processes coherent scattering, Compton scattering, the photoelectric effect, pair production, and photo disintegration.
As a radiologic technologist, understanding these processes helps you predict what reaches the image receptor, how contrast is created, and how energy deposition affects patient dose.
All right, let's start with coherent scattering, also called classical scattering or Thompson scattering. It occurs mainly with low energy X-ray photons.
An incoming X-ray photon called the incident photon briefly excites an atom. The atom then releases a scattered photon with the same energy, but in a different direction, usually forward.
No electron is ejected, so the atom is not ionized. Because essentially no net energy is transferred, coherent scattering contributes very little to the absorbed dose.
A few scattered photons may reach the image receptor where they contribute to image noise that slightly reduces contrast, but overall coherent scattering has a minimal effect on the image.
Next is Compton scattering, which is one of the main interactions in soft tissue. An incident photon transfers part of its energy to a loosely bound outer shell electron, ejecting it as a Compton electron and ionizing the atom.
The transferred energy contributes to patient dose, and the scatter created adds to the dose to the radiologic technologist.
The photon continues with less energy in a new direction. If it reaches the image receptor, it creates unwanted exposure and reduces contrast.
Thicker body parts and larger fields produce more scatter, especially in abdominal imaging. Scatter is also a significant source of occupational exposure for technologists.
In the photoelectric effect, an incident X-ray photon strikes a tightly bound inner shell electron. This is most likely to occur when photon energy is just above the electron's binding energy.
The photon is completely absorbed. Some energy frees the electron, and the rest becomes its kinetic energy.
The ejected electron is called a photoelectron. Photoelectric interactions occur in low atomic number atoms, including those in soft tissue, but are more likely in tissues with a higher effective atomic number, such as bone.
Photoelectric absorption contributes to greater attenuation of the bone, meaning fewer photons remain in the primary beam after it passes through bone.
As a result, fewer photons reach the image receptor and bones appear brighter on the image. X-ray beams with lower photon energies favor this effect and can improve image contrast.
Complete absorption also contributes to patient dose. Together, the photoelectric effect and Compton scattering are responsible for creating the image in diagnostic imaging.
Next, let's look at two high energy interactions, pair production and photo disintegration. Pair production requires energies more than 1.02 megaelectron volts or MEV.
Near a nucleus, the photon disappears and creates an electron and a positron. In photo disintegration, a high energy photon is absorbed by a nucleus, which releases a particle, often a neutron.
The required energy varies by nucleus but is greater than 10 MeV. Diagnostic X-rays are in the kiloelectron volt range, far below these energies, so neither occurs in diagnostic imaging.
However, both can be relevant to radiation therapy. Now, differential absorption is the difference in how much tissues attenuate the X-ray beam and create image contrast.
In diagnostic imaging, photoelectric absorption and Compton scattering are the main processes that remove photons from the primary beam.
The remaining photons are transmitted to the image receptor. Structures that attenuate more X-rays are radio opaque and appear lighter because fewer photons reach the receptor.
Bone is an example. Structures that transmit more X-rays are radiolucent and appear darker because more photons reach the receptor.
Air-filled regions, such as the lungs are relatively radiolucent, while soft tissue appears in shades of gray. A tissue's effective atomic number affects differential absorption.
As it increases, photoelectric interactions become more likely. Compton scattering depends mainly on electron density and much less on atomic number.
The mineral content of the bone gives it a higher effective atomic number than soft tissue, and this contributes to greater attenuation and a brighter appearance on the radiograph.
Positive contrast agents containing high atomic number elements such as iodine or barium also attenuate more X-rays. Making certain structures easier to see.
Mass density also affects attenuation. In general, a denser material contains more atoms and more electrons within a given volume, giving an X-ray photon more opportunities to interact.
Air has very low density and attenuates very little of the beam, so air-filled regions appear dark. Aerated lungs are also relatively low in density and usually appear dark gray.
Soft tissue is denser and attenuates more, so it appears in various shades of gray. Bone is even more dense, so appears brighter than soft tissue.
Thickness matters too. A longer path through the material causes more attenuation.
In X-ray imaging, contrast agents make structures more radio opaque than nearby tissue. Many soft tissues have similar effective atomic numbers and densities, so their attenuation and therefore their shades of gray can be similar.
Iodine and barium contain high atomic number elements that greatly increase photoelectric absorption. Iodinated contrast can outline blood vessels and enhance organs, while barium sulfate can outline the gastrointestinal tract.
Because these agents attenuate more of the beam, fewer photons reach the image receptor behind them, making the opacified structure appear brighter and easier to distinguish.
Now, keep in mind that attenuation is not a separate interaction. It's the reduction in the number of photons remaining in the primary beam as it passes through tissue, caused by absorption and scattering.
Absorption is an all or nothing outcome for the incident X-ray photon. The incident photon disappears, transferring all its energy to matter.
In diagnostic imaging, the photoelectric effect is the main absorption process. Scattering also reduces the primary beam because the photon changes direction.
The photon may transfer some of its energy to matter, but it continues as a scattered photon, even though the scattered photon still exists and may reach the image receptor by another path.
It's not part of the primary beam any longer. Transmission is different.
It means a photon passes through without interacting and remains in the primary beam. Alright, as a quick recap, X-ray photons can be absorbed, scattered, or transmitted, and these interactions determine what reaches the image receptor.
Coherent scattering redirects a low energy photon without ionizing the atom and has little effect on image quality or patient dose.
Compton scattering is the main interaction in soft tissue. It reduces image contrast and contributes to technologist dose.
In the photoelectric effect, the incident photon is completely absorbed. This interaction contributes to image contrast, especially in materials with a higher effective atomic number, such as bone, but it also contributes to patient dose.
Pair production and photo disintegration require photon energies far above those used in diagnostic imaging. Absorption and scattering combine to attenuate the primary beam while transmitted photons continue toward the image receptor.
Effective atomic number, mass density, and tissue thickness affect attenuation and radiographic appearance. Air-filled regions are radio lucent and appear dark, soft tissues appear in shades of gray, and bone is radio opaque and appears light.
Positive contrast agents containing iodine or barium attenuate more X-rays and make certain structures easier to see. Understanding these interactions helps you connect technique choices with image contrast, scatter, and patient dose.