Photon Interaction with Matter

orbit the nucleus in a variety of planes and specific energy states

electrons

ionization

addition or removal of electron from atom

when photon interacts with atom what is ejected

electron

higher Z# yields

higher binding energy

the further the electron is from the nucleus

the less energy binding the electron has

electrons in outer shell possess

higher kinetic energy

electrons in inner shell have lower KE but

higher binding energy

as xray beam passes through matter it undergoes

attenuation

the farther from the nucleus an electron is

the higher the total energy of the electron will be

attenuation

reduction in the number of photons in the beam as well as loss of energy (scatter and absorption)

xrays can undergo what processes

transmission through matter, total absorption by matter, interaction with matter and change direction

interactions can occur within

entire atom, orbital electrons, atomic nucleus

most common interaction in diagnostic xray range is with the

orbital electron

low energy photons interact with

whole atom

moderate energy photons interact with

orbital electrons

high energy photons interact with

nucleus

5 basic interactions: low to high photon energy

coherent scattering, photoelectric absorption, compton scatter, pair production, photodisintegration

therapeutic/meV range interaction

pair production and photodisintegration

diagnostic/keV interactions

coherent scatter, PE, compton scatter

coherent scatter

least likely, photon initially absorbed by whole atom, photon energy is ejected as photon in different direction, no ionization

coherent scatter is also known as

classical, thompson, rayleigh, and unmodified

coherent scatter occurs between

very low energy xray range and has little significance in diagnostic imaging

during coherent scattering, when a very low energy photon interact with an electron, the electrons

vibrate at the same frequency, and the photon is scattered in different direction

photoelectric absorption

main interaction at low kVp levels, photon completely absorbed by inner shell electron, ionization occurs by removing electron (photoelectron)

since photoelectric absorption makes atom unstable the

cascade effect occurs

the cascade effect creates

characteristic photons

this is the primary source of patient radiation dose

photoelectric absorption

patient dose comes from

original absorbed photon and more from characteristic photons

characteristic photons

secondary radiation, NOT scatter

characteristic xrays differ because they

have lower energies

the cascade effect lasts until

the atom is stabilized again

photoelectric absorption is more possible when

the xray photon’s energy and inner shell binding energy are close to the same

photoelectric absorption also increases when the atomic number is

higher, i.e. barium, bone, iodine, metallic artifact (joint replacement)

in diagnostic image contrast is mostly because of the

predominance of this interaction

since PE absorption is more likely to occur with higher atomic number then it will occur more in

bone than soft tissue

rules of PE: incident photon must be

greater than the binding energy of the inner shell electron

rules of PE: photoelectric interaction is more likely to occur when

the xray photon energy and electron binding energy are close to one another

rules of PE: PE interaction is more likely to occur with an electron that is

more tightly bound in its orbit (higher atomic numbers and inner shell electrons)

ionization in PE Absorption

ionization occurs, increases PT dose, no effect on bystander, positive effect on image quality

compton scattering

most common interaction, main IX at high energy, photon partially absorbed by outer shell electron, ionization occurs

when ionization occurs during compton scattering it is called

compton (recoil) electron

during compton, the energy of the incident photon is

divided between the scatter photon and ejected electron

during compton, the ejected electron will

lose its KE by excitation and ionization of atoms in the surrounding tissues

forward scatter results in

degradation of image contrast

side scatter results in

occupational exposure

backscatter results in

patient exposure

the higher energy incident photons will travel relatively in the

same direction when scattered

scatter that moves back toward incident photon is called

backscatter

compton scatter has high energy and is a

radiation hazard

ways to decrease scatter production

increased collimation, optimal kVp, compression

ways to decrease scatter reaching the IR

use a grid, higher grid ratio, increase OID

ways to decrease scatter reaching the technologist

increase distance from source and shielding

ionization with compton scattering

ionization occurs, bad for pt dose, bad for bystander dose, bad for image quality

pair production requires a minimum of

1.02 MeV of photon energy (not diagnostic)

pair production

xray photon interacts with the nucleus of a target atom and is converted into 2 electrons (positron and negatron)

both particles in pair production share the

original xray photon’s energy equally

in pair production the negatron is

absorbed

annihilation radiation

positron combined with electron and converted into 2 xrays each containing 0.51 MeV

photodisentegration requires a minimum of

10 MeV of photon energy (not diagnostic)

during photodisintegration the xray hits and is absorbed by the

nucleus

during photodisintegration the target nucleus is raised to an excited state and

emits a nucleon or other nuclear fragments

of the 5 interactions, which 2 have a significant impact on an xray image

photoelectric absorption and compton scatter

within the diagnostic range most of the xrays are

attenuated and a small percentage will exit the patient to create the image

as kVp increases the number of photons reaching the IR will

increase

as kVp increases the number of photoelectric interactions

decreases

as kVp increases the number of compton scatter interactions

increases

as kVp increases, scatter

increases and absorption decreases

in the human body, _______ is the predominant interaction through most of the diagnostic xray range

compton scattering

photoelectric interaction predominate in two circumstances

lower energy ranges (25-45 keV/40-70 kVp) and when high atomic number elements are introduced (iodine and barium)

when compton interactions predominate resulting image will have

lower contrast

when compton interactions predominate high kVp and low mAs produced more compton interaction but less

patient exposure

when PE absorption predominates resulting image will have

higher contrast

when PE absorption predominates low kVp and high mAs results in

increased PT exposure

Which energy level electrons possess the
highest total energy?
a) K
b) L
c) M
d) All atomic shells have the same total energy

M

During photoelectric absorption, a/an ____ shell electron is ejected.
a) inner-
b) middle-
c) outer-
d) all of the above

inner

An interaction that occurs between very low energy x-ray photons and matter is called ____ scatter.
a) coherent
b) classical
c) unmodified
d) all of the above

all of the above

Which of the following best describes the energy distribution during Compton scattering?
a) The incident photon energy is absorbed entirely by the
recoil electron.
b) The incident photon energy is absorbed entirely by the
scattered photon.
c) The incident photon energy is divided between the
recoil electron and the scattered photon.
d) The incident photon energy is transferred completely to
the recoil electron, with no energy remaining in the
scattered photon.

The incident photon energy is divided between the
recoil electron and the scattered photon

Which of the tissue interactions does NOT result in ionization?
a) photoelectric interaction
b) pair production interaction
c) compton interaction
d) coherent interaction

coherent interaction

Which interaction is responsible for the contrast in UGI studies?
a) Compton
b) Coherent
c) Photoelectric
d) Photodisintegration

Photoelectric

Which of the following interactions with matter will impact image contrast if excessive?
a) Compton
b) Coherent
c) Photoelectric
d) Photodisintegration

Compton