INTERACTIONS OF X-RAYS
Absorption
X-rays are a form of energy. As X-rays enter the body, many of them will interact with the atoms within the body. As they interact, they transfer energy to the atom. This transfer of energy is called absorption. The amount of energy absorbed is called absorbed dose.
As these photons enter the body, they may do one of 3 things:

- X-rays can pass completely through the body and strike the image receptor. These areas appear black on a radiograph.
- X-rays can transfer some of its energy through partial absorption. The photon has less energy as it strikes the image receptor. These interactions give us our greys.
- X-rays can be completely absorbed by the body. The areas of the image were x-rays did not make their destination appear white.
Scatter Radiation
In addition to the above examples, the x-rays may also scatter. Scatter is a term used to describe a photon that travels in a direction other than that of the primary beam. This happens when an x-ray photon interacts with atoms within the body. A photon may knock out an inner electron, which causes it to lose most of it's energy, and get completely absorbed. A photon may also strike an outer electron, which causes the photon to change directions. Many times these photons will leave the body, which then can strike anybody else who happens to be in the room during exposure. There are 2 main types of scatter:
- Photoelectric interaction = Occurs in lower energy x-rays interacting with matter. Because they are lower energy, the chances of the patient absorbing the radiation before it can escape the body is much higher. Photoelectric interactions are associated with patient dose.
- Compton interaction = The most common interaction in medical imaging. Occurs in higher energy x-rays. Because the energy is higher there is greater chance that it will leave the body. Compton interactions, or Compton Scatter, is responsible for occupational dose.
Every time a photon interacts with matter, it loses energy. General rule of thumb it to allow a photon to scatter twice in order to reduce the energy to the point considered safe.
What are some factors that control scatter?
- Patient size - the larger the patient, the more scatter they will produce.
- Body part thickness - for the same reason as above.
- Body composition - Parts that are dense will produce more scatter. For instance, muscle will scatter more than fat.
- Collimation - The larger the x-ray field size, the greater the area being exposed, so greater the chance of scatter occurring.
- Increased mAs - Increase mAs means increase number of photons, which means more photons are available to interact with the body.
- Increased kVp - The higher the energy of the primary beam, the greater the chance of scatter.