Radiation Interactions with Matter

Flashcards covering radiation interactions with matter including scattering, photoelectric effect, pair production, dosimetry, and attenuation based on the provided lecture notes.

No interaction

X-ray passes completely through tissue and into the image recording device.

Complete absorption

X-ray energy is completely absorbed by the tissue, resulting in no imaging information.

Partial absorption with scatter

Involves a partial transfer of energy to tissue, with the resulting scattered X-ray having less energy and a different trajectory.

Classical Scattering (Synonyms)

Also known as Coherent, Thompson, or Rayleigh Scattering.

Target atom behavior in Classical Scattering

The incident photon interacts with matter and excites an atom, causing it to vibrate.

Wavelength relationship in Classical Scattering

The target atom releases excess energy as scattered x-ray with equal wavelengths ($\lambda=\lambda'$).

Classical Scattering occurrence (Low energy)

Occurs only in diagnostic radiology or low energy x-rays less than ($<10\,keV$).

Classical Scattering occurrence at $70\,keV$

$5\%$ occurrence.

Classical Scattering occurrence at $30\,keV$

$12\%$ occurrence.

Classical Scattering effect on radiology

Contributes to film fog and has no useful effect.

Classical Scattering ionization

No ionization occurs; only excitation.

Energy in vs. Energy out (Classical)

Energy in equals Energy out with only a change in direction and no loss in energy.

Classical Scattering angle relationship

The scattering angle increases as the incident x-ray energy decreases.

Compton Interaction (Synonyms)

Also known as Compton Scattering, Incoherent, Inelastic, or Non-Classical interaction.

Compton Interaction range

Predominant interaction in the diagnostic range above ($26\,keV$) with soft tissue.

Compton Interaction definition

A type of photon interaction resulting when an incident photon of relatively high energy ejects few electrons.

Outer orbital electrons in Compton

Electrons are deflected by the same electron causing it to travel in a new direction as scattered or secondary radiation.

Compton Ionization

Not only scatters x-ray but also produces enough energy to ionize other atoms as well.

Compton electron target

Interacts with outer (valence) shell electrons.

Radiation hazard (Occupational)

Compton Scattering is the most hazardous to radiation workers due to scatter radiation.

Compton Energy Equation

The energy of the incident photon is equal to the sum of the energy of the scattered photon and the kinetic energy of the ejected electron.

Compton energy retention at $100\,keV$

$90\%$ of energy will be retained to the scattered x-ray.

Ejected electron scattering angle limit

Cannot exceed $90$ degrees.

Backscatter (Compton)

Scattered photons can include a $180$-degree backscatter.

Electron density definition

Number of electrons/g $\times$ density; Compton interaction depends on this.

Compton probability vs. Atomic Number ($Z$)

The probability of occurrence is nearly independent of the atomic number of the absorber.

Compton probability vs. Material density

The probability of occurrence is approximately proportional to the density of the material.

Hydrogenous materials interaction

Have a higher probability of Compton occurrence than non-hydrogenous material of equal mass.

Modified scatter

An x-ray photon deflected with somewhat reduced energy during Compton scattering.

Compton electron

The electron released from the outer shell during a Compton interaction.

Compton scattering and patient hazard

Since the scattered photon exits the body, it does not pose a radiation hazard to the patient.

Fluoroscopic procedures hazard

Scattered photons from Compton contribute to film fog and pose a radiation hazard to personnel.

Predominant interaction below $60\,kVp$

Photoelectric absorption.

Predominant interaction above $60\,kVp$

Compton scatter begins to increase.

Compton effect vs. Tissue type

Compton is just as likely to occur with soft tissue as bone.

Effect of Compton on image contrast

Produces a uniform optical density on the radiograph that reduces image contrast.

Major source of technologist exposure

Scattered radiation from Compton, especially during fluoroscopy.

Photoelectric Effect discovery

First observed by Heinrich Hertz in $1887$.

Photoelectric Effect explanation

Albert Einstein clearly explained the photoelectric effect in detail.

Photoelectric Effect probability

Occurs with the probability of $$75\%$^$.

Photoelectric Effect definition

Occurs when an incident x-ray is totally absorbed during the ionization of an inner shell electron.

Photo-electron

The K-shell electron ejected from the atom during the photoelectric effect.

Photo-electron energy level

Equal to the difference between the incident photon and the electron binding energy.

Photo-electron kinetic energy (Low $Z$ target)

Binding energy of k-shell is low; released with kinetic energy nearly equal to incident x-ray energy.

Photo-electron kinetic energy (High $Z$ target)

Binding energy of k-shell is high; released with lower kinetic energy compared to incident x-ray energy.

K-shell vacancy result

Ejection of a photoelectron results in a vacancy accompanied by the emission of an x-ray.

Photoelectric probability vs. Energy

Inversely proportional to the third power of the x-ray energy.

Energy doubling effect (Photoelectric)

If photon energies are doubled, the probability of occurrence is decreased eight-fold.

Photoelectric effect below $50\,keV$

Plays an important role in imaging soft tissue.

Attenuation amplification (Photoelectric)

Used to amplify differences in attenuation between tissues with slightly different atomic numbers to improve contrast.

Characteristic ray in Photoelectric Effect

Produced when an electron from a shell above drops down to fill an inner shell vacancy.

Absorbers of high atomic number

Materials like bone and positive contrast media where the photoelectric effect is more likely to occur.

Photoelectric effect and patient dose

Contributes significantly to patient dose as all photon energy is absorbed.

Auger electron

An electron ejected from the atom when energy from an inner-shell transition is transferred to it instead of being released as a photon.

Cascade of electrons

The sequential dropping of electrons into lower shell vacancies.

Pair Production energy threshold

Occurs with x-rays that have energies greater than $1.022\,MeV$.

Pair Production target

Interacts with the electric field of the nucleus of the target atom.

Pair Production transformation

Photon energy is transformed into an electron-positron pair.

Negatron

The negative electron part of the pair produced in pair production.

Positron

The positive electron part of the pair produced in pair production.

Pair Production particle energy

Each electron and positron possesses $0.511\,MeV$.

Pair Production application

Important in Positron Emission Tomography (PET).

Photodisintegration energy threshold

Energy of x-ray required is greater than or equal to $7 - 10\,MeV$.

Photodisintegration process

The photon is absorbed by the nucleus, and a nuclear fragment is emitted.

Nucleon

A nuclear fragment (like a proton or neutron) emitted during photodisintegration.

Photodisintegration application

High photon energies are important in megavoltage therapy and high energy accelerator physics.

Attenuation definition

The total reduction in the number of photons remaining in an x-ray beam after penetration through tissue (Absorption + Scattering).

Differential Absorption definition

The difference between those x-rays absorbed and those transmitted to the image receptor (IR).

Primary beam interaction percentage

Approximately $1\%$ of photons reach the IR, and only $0.5\%$ interact to form the image.

Constituents of Differential Absorption

Compton Scattering, Photoelectric absorption, and X-ray transmission.

Light areas on image

Produced by Photoelectric absorption.

Grey/dark areas on image

Produced by transmitted x-rays.

Pathlength

The actual distance a particle travels.

Range (Particle)

The actual depth of penetration of the particle in matter.

Linear Energy Transfer (LET)

The amount of energy deposited per unit path length ($eV/cm$).

LET Formula

$LET = \text{specific ionization (IP/cm)} \times \text{average energy deposited per ion pair (eV/IP)}$.

Radiation Weighting Factor ($W_R$)

A quality factor used in radiation protection determined by LET.

Elastic Scattering

Events wherein the total kinetic energies of the colliding particles remain unchanged.

Inelastic Scattering

Scattering events that occur with a loss of kinetic energy.

Bremsstrahlung Scattering

Inelastic interactions between electrons and atomic nuclei causing deceleration and x-ray production.

Braking radiation

The common name for Bremsstrahlung radiation produced by the deceleration of high-speed electrons.

Bremsstrahlung emission probability

Directly proportional to $Z^2$ of the absorber.

Maximum kinetic energy of electrons

Determined by the x-ray tube voltage (e.g., $100\,kV$ produces up to $100\,keV$ kinetic energy).

Continuous spectrum of x-rays

The result of Bremsstrahlung interactions in an x-ray tube.

Bremsstrahlung percentage at $100\,kVp$

$85\%$ of produced x-rays.

High energy Bremsstrahlung cause

Occurs close to the nucleus with high projectile kinetic energy loss and greater deflection.

Characteristic Radiation

Produced when a projectile electron removes an inner-shell electron of a target atom and an outer-shell electron fills the vacancy.

Tungsten K-characteristic threshold

Requires a tube potential of at least $70\,kVp$.

Neutron Interactions nature

Uncharged particles that indirectly cause excitation and ionization by interacting with atomic nuclei.

Linear Attenuation Coefficient

The fraction of photons removed from a mono-energetic beam per unit thickness ($cm^{-1}$).

Mass Attenuation Coefficient (MAC)

The linear attenuation coefficient normalized to unit density ($\frac{\mu}{\rho}$).

Half Value Layer (HVL)

The thickness of material required to reduce beam intensity to one-half of its initial value.

Tenth Value Layer (TVL)

The thickness of material necessary to reduce beam intensity to one-tenth its original value.

TVL to HVL conversion

$1\,TVL = 3.3\,HVL$.

Beam Hardening

The shift of the x-ray spectrum to higher effective energies as the beam traverses matter.

Kerma

Acronym for Kinetic Energy Released in Matter; kinetic energy transferred to charged particles per mass.

Absorbed Dose ($D$)

The energy deposited by ionizing radiation per unit mass of material, measured in Gray ($Gy$).

Gray to rad conversion

$1\,gray = 100\,rads$.

Exposure definition

The amount of electrical charge produced by ionizing electromagnetic radiation per mass of air.

Roentgen (R) to C/kg conversion

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