Photon Interactions with Matter and SI Units of Measurement

Flashcards covering photon interactions with matter (Compton, Photoelectric, Coherent), attenuation, subject density, and SI units of radiation measurement.

What are the possible outcomes when an x-ray photon passes through matter?

It can pass through without interaction, change direction (scatter), or be absorbed by the atom.

What happens to a photon that is absorbed by an atom?

The photon no longer exists.

What are the characteristics of a scattered photon compared to the incident photon?

The scattered photon still exists but possesses less energy.

As kVp increases, how does the total number of photons transmitted without interaction change?

The total number of photons transmitted without interaction increases.

What interaction is most common between x-rays and tissue at low energies?

Photoelectric interactions.

With an increase in kVp, how are the percentages of photoelectric and Compton interactions affected?

The percentage of photoelectric interactions decreases while the percentage of Compton interactions increases.

What two processes make up the product of attenuation?

Absorption and scattering.

What is the definition of Binding Energy?

The energy required to remove an orbital electron from its shell.

Which shell's electrons possess the highest binding energy in any given atom?

The K-shell electrons.

How does the atomic number of an element affect its K-shell binding energy?

Elements with a higher atomic number have a higher K-shell binding energy.

What is the relationship between an electron's distance from the nucleus and its total energy?

Electrons further from the nucleus require less energy to remove but possess a greater total energy.

What type of electron does a relatively high-energy x-ray photon eject in the Compton effect?

A loosely bound outer-shell electron.

What is the ejected electron called in a Compton interaction?

A Compton or recoil electron.

What is the fate of the incident photon in the Compton effect?

It is deflected and becomes a Compton scattered photon that exits the body.

What is the mathematical expression for the Compton effect according to Carlton/Adler?

$$Ei = Es + Eb + Eke$$

Which interaction is responsible for the majority of scatter reaching the Image Receptor (IR)?

The Compton effect.

How does Compton scatter affect the scale of contrast on an image?

It adds radiation fog and lowers the scale of contrast.

What is the primary source of occupational exposure for radiography workers?

Scatter radiation emitted from the patient.

What three factors affect the production of scatter radiation?

1. kV (beam quality), 2. Part thickness (pathology), 3. Beam Restriction (collimation/field size).

How do grids impact scatter radiation?

Grids clean up the scatter before it reaches the Image Receptor (IR).

What occurs during photoelectric absorption?

A relatively low-energy photon uses all of its energy to eject/remove an inner-shell electron.

What is the name of the electron ejected during photoelectric absorption?

A photoelectron.

What is the typical travel distance of a photoelectron?

$$1-2\,mm$$

What is secondary radiation in the context of photoelectric absorption?

Energy released in the form of a characteristic photon when an inner-shell vacancy is filled.

What is the process called where electron transfers continue from shell to shell until the atom returns to normal?

A characteristic cascade.

Which interaction contributes most significantly to patient dose?

Photoelectric absorption.

What is the mathematical expression for photoelectric absorption according to Carlton/Adler?

$$Ei = Eb + Eke$$

What is the first rule governing photoelectric interactions regarding energy?

The incident x-ray photon energy must be greater than the binding energy ($keV$) of the inner-shell electron.

When is a photoelectric interaction more likely to occur between photon energy and binding energy?

When the x-ray photon energy and the electron binding energy are nearer to one another.

Is photoelectric interaction more likely to occur in bone or soft tissue?

Bone, because it has a higher atomic (Z) number.

As photon energy increases, what happens to the probability of photoelectric interaction?

The chance of photoelectric interaction decreases drastically.

What are the percentages of Compton and Photoelectric interactions at $50\,kVp$?

$50\%$ Compton and $50\%$ Photoelectric.

What are the percentages of Compton and Photoelectric interactions at $130\,kVp$?

$75\%$ Compton and $25\%$ Photoelectric.

According to the 2017 ARRT, what is the definition of the signal difference in the remnant beam?

The result of the different absorption characteristics of the tissues and structures making up the part.

How does fluid in the lungs affect AEC use in the lab?

Fluid increases subject density, causing the AEC to remain on longer and overexposing aerated lungs.

Under what energy range does Coherent, Classical, or Unmodified scattering occur?

Below approximately $10\,keV$.

Does Coherent scattering result in the ionization of the atom?

No, because no electron is removed.

What happens to the excess energy in Coherent scattering?

It is released immediately as a secondary photon with the same energy and wavelength as the incident photon.

What effect does Coherent scattering have on diagnostic radiography images?

It has little effect and is considered insignificant.

What is the definition of attenuation?

The reduction in the number of x-ray photons of the primary beam, and the resulting loss of energy, as the beam passes through matter.

How does an increase in part thickness affect attenuation?

Thicker parts lead to increased attenuation.

What is the definition of mass density?

The quantity of matter per unit of volume, describing how tightly atoms are packed.

If mass density is doubled, what happens to the chance for x-ray interaction?

The chance for x-ray interaction is doubled.

Rank the following from least attenuation to greatest attenuation: Muscle, Air, Bone, Fat, Water.

Air, Fat, Water, Muscle, Bone.

Does the atomic number determine the probability of Compton interactions?

No, Compton scattering is just as likely to occur in bone as in soft tissue.

At what kVp range are photoelectric interactions more likely to occur?

$40-70\,kVp$ (lower-energy ranges of $25-45\,keV$).

What are isotopes?

Atoms that have the same atomic number ($Z$) but a different mass number.

What is the atomic number ($Z$) of Bone?

$$13.8$$

What is the atomic number ($Z$) of Soft Tissue?

$$7.4$$

How much greater is the probability of photoelectric interaction in bone compared to soft tissue?

Approximately seven times greater.

Which two interactions have a significant impact on an x-ray image?

Photoelectric absorption and Compton scattering.

For $10\,cm$ of tissue at $130\,kVp$, what percentage of photons are transmitted?

$$6\%$$

Why is it necessary to reduce mAs when kVp is increased?

To maintain the same exit dose to the IR, because the total number of photons transmitted increases with kVp.

What are the image quality and patient dose results of using High kVp/Low mAs?

Low contrast (due to fog) and reduced patient dose.

What are the image quality and patient dose results of using Low kVp/High mAs?

High contrast and higher patient dose.

What is the SI unit for absorbed dose?

Gray ($Gy$ or $Gy_t$).

What is the SI unit for dose equivalent?

Sievert ($Sv$).

What is the SI unit for Exposure?

Coulomb per kilogram ($C/kg$).

What is the SI unit for effective dose?

Sievert ($Sv$).

What was the British/Customary unit for Exposure?

Roentgen ($R$).

What is the conversion factor between Roentgen ($R$) and Coulomb per kilogram?

$$1\,R = 2.58 \times 10^{-4}\,C/kg$$

Exposure ($C/kg$) applies only to which types of radiation and in what medium?

X-rays and gamma rays and interactions in air.

What does Air kerma measure?

Kinetic Energy Released in Matter; specifically energy absorbed in air.

What unit is used to measure kerma?

The Joule per kilogram ($J/kg$).

Write the relationship between Joule per kilogram and Gray for air.

$$1\,J/kg = 1\,Gy_a$$

What is the customary/British unit for absorbed dose?

Rad (radiation absorbed dose).

How is $Gy_t$ defined compared to air?

$1\,J/kg = 1\,Gy_t$, where tissue is the absorber.

What does absorbed dose describe?

The quantity of any type of ionizing radiation received by a patient and deposited in any material.

What is the customary/British unit for Equivalent Dose ($EqD$) and Effective Dose ($EfD$)?

rem (radiation equivalent man).

What does the Sievert ($Sv$) express in radiation measurement?

The dose-effect relationship or the amount of actual biological harm.

What is the formula to calculate Equivalent Dose ($EqD$)?

$$\text{Absorbed dose} \times W_r = \text{Equivalent Dose}$$

What is the formula to calculate Effective Dose ($EfD$)?

$$\text{Absorbed dose} \times W_r \times W_t = \text{Effective Dose}$$

What does $W_r$ account for in Equivalent Dose?

Various types and energies of ionizing radiations (Radiation weighting factor).

What does $W_t$ account for in Effective Dose?

The radiosensitivity of the various organs and tissues of the body (Tissue weighting factor).

Which does more biological harm: Alpha particles or X-rays?

Alpha particles.

What is the term for the equivalent whole-body dose?

Effective dose ($EfD$).

According to the transcript, how are Quality factors ($QF$) related to weighting factors?

Quality factors ($QF$) are your $W_t$ and $W_r$.

What is the conversion from rad to rem?

$$(\text{rad} \times QF) = \text{rem}$$

At $50\,kVp$, what percentage of photons are attenuated in $10\,cm$ of tissue?

$$> 99\%$$