X-Ray & γ-Ray Interactions with Matter

Chapter 5

Attenuation Coefficients

• The relative variation of attenuation coefficients with energy and between different materials affects both the absorption of radiation dose in patients and the radiographic images produced.

• As the energy of the photons increase, the probability of interaction drops rapidly.

Photon Energy Dependant Interactions

• Low energy photons interact with whole atom.

• Moderate energy photons interact with orbital electrons.

• High energy photons interact with nucleus.

Coherent Scatter

•

AKA Thomson, Rayleigh, classical, unmodified or simple scatter

•

Energy

: very low energy photons (below 10 keV, ex: light) • •

Interacts with

: outer shell electrons

Incoming photons

: absorbed then released (no overall change) •

Interaction

: excited, not ionized

Coherent Scatter

• • • •

Product

: photon with same energy as incoming photon with different direction

Atomic number:

has no effect

Importance in diagnostic: Importance in therapy:

blurs shadows none due to low probability and the fact that no energy is deposited.

Photoelectric Effect

• • • •

Energy

: high energy photons (40-70 kVp), as energy increases, probability of photoelectric effect decreases.

Interacts with

: tightly bound inner shell electrons

Incoming photons

photoelectron: E ke : absorbed; energy transferred to electron (released as , mass, reabsorbs quickly)  More likely to occur when x-ray photon has just slightly more energy than E b of a K or L shell electron

Interaction

: ionized, ion pair formed, causes characteristic cascade

Photoelectric Effect

• • • •

Product

: characteristic photons with energies equal to the differences in electron shell energies

Atomic number:

number increases.

probability increases as atomic

Importance in diagnostic:

produces shadows of high atomic number material (bone), responsible for contrast (contrast increases as energy decreases)

Importance in therapy:

none

Compton Effect

• • • • •

Energy

: high energy photons, important in orthovoltage/ megavoltage range

Interacts with

: loosely bound outer shell electrons

Incoming photons

some scattered.

: some energy absorbed by electron (released as Compton/recoil electron) &

Interaction

: ionized, ion pair formed, remaining energy released as photon.

Product

: photons with reduced energies related to the angle of scatter, change of direction, will continue to interact until absorbed photoelectrically.

Compton Effect

• • • •

Atomic number:

are available  independent of atomic number (depends on electron density the more “free electrons” higher probability of effect)

Importance in diagnostic:

degrades image by graying film; also looked at in determining shielding requirements.

Importance in therapy:

of varying mass density.

best contrast obtained in areas

Source of occupational exposure photon possesses enough energy to be emitted from patient) and radiation fog (scatter places exposure on film unrelated to anatomy)

Pair Production

• • • •

Energy

: threshold: at least 1.02 MeV, usually > 10 MeV; as energy increases, probability of effect increases

Interacts with

: electric field of nucleus

Incoming photons

: absorbed by nucleus

Interaction

: negatron & positron produced which deposit energy as it interacts with matter  Bremsstrahlung possible but unlikely due to body tissues having low Z

Pair Production

• • • •

Product

: two photons produces in annihilation reaction (0.511 MeV each) traveling in opposite directions

Atomic number:

strength of electric field is a function of the atomic number.

Importance in diagnostic: Importance in therapy:

none

Photonuclear Interaction

• • • • •

AKA photonuclear disintegration, gamma n interaction ( γ,n)

Energy

: energies greater than 15 MeV

Interacts with Incoming photons Interaction

: : nucleus : absorbed by nucleus.

Photonuclear Interaction

•

Product

: neutron •

Atomic number:

•

Importance in diagnostic:

none •

Importance in therapy:

Review: Technical Factors

• Increase kVp:  PE absorption decreases  Compton effect increases  Decreases contrast (absorption)  Increases Scatter  Total number of photons that are transmitted without interaction increases.

• A decrease in kVp will result in higher contrast (more absorption) and increased dose to patient.