X-ray interaction with matter: Rad Tech

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 five processes: coherent scattering, Compton scattering, the photoelectric effect, pair production, and photodisintegration. 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.

Alright, let's start with coherent scattering, also called classical scattering or Thomson 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 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 photodisintegration. Pair production requires energies more than 1.02 mega-electron volts, or MeV. Near a nucleus, the photon disappears and creates an electron and a positron. In photodisintegration, 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 kiloelectronvolt 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 radiopaque 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.