RAD 120 - Ch 13 X-Ray Interactions - Review Questions

Define attenuation:

Is the reduction in the number of x-ray photons in the beam, and the subsequent loss of energy, as the beam passes through matter.

Describe the photoelectric absorption:

Photoelectric absorption results when a x-ray photon interacts with an inner shell electron. The incident photon ejects the electron from its inner shell and is totally absorbed in the interaction. The result is an ionized atom, because of the missing inner shell electron, and an ejected electron, called a photoelectron.

The ionized atom is in an unstable state with an inner-shell electron missing. The vacancy is instantly filled by an electron from an outer shell or a free electron. In the vast majority of cases, the electron transfers from an outer shell to an inner shell and, as it does, it releases energy in the form of a characteristic photon known as secondary radiation.

Describe coherent scatter interactions:

Coherent scatter is an interaction that occurs between very-low-energy x-ray photons and matter. When a very-low energy photon, below approximately 10keV, interacts with the electron(s) in an atom it may cause the electron(s) to vibrate at the same frequency as the incident photon. The vibrating or exciting atom immediately releases this excess energy by producing a secondary photon that has the same energy and wavelength as the incident photon but that travels in a different direction from the initial photon. The result is a scattered photon that possesses the same energy, frequency, and wavelength as the initial photon but that is traveling in a different direction. Because there is no energy transferred in the interaction, the atom is not ionized in the process.

Describe Compton scatter interactions:

Compton scattering occurs when an incident x-ray photon interacts with a loosely bound outer shell electron, removes the electron from its shell, and then proceeds in a different direction as a scattered photon. Part of the energy of the incident photon is used to remove the outer-shell electron and impart it to kinetic energy. The dislodged electron is called a compton or recoil electron. The photon which exits the atom in a different direction, is called a Compton scattered photon. It possesses less energy than the incident photon and therefore has a lower frequency and longer wavelength.

What is back scatter?

When a scattered photon is deflected back toward the source, it is traveling in the direction opposite to the incident photon. These photons are called backscatter radiation.

What are the two interactions that have a significant impact on the radiographic image?

The two interaction that have a significant impact on an x-ray image are photoelectric absorption and compton scattering.

What type of radiographic contrast will result if the prevalent interaction is photoelectric absorption?

When the photoelectric effect is more prevalent, the resulting radiographic image will possess high contrast.

What type of radiographic contrast will result if the prevalent interaction is Compton scatter?

When Compton interactions prevail, the resulting radiograph will possess lower-contrast.

How do changes in technical factor selections impact x-ray interactions?

As kVp increases the number of photons transmitted without interaction increases. The percentage of photoelectric interactions decreases with increased kVp and the percentage of Compton interactions increases with increased kVp.