Final Review : Xray interactions Flashcards
-Reduction in number of xray photons in beam
- results of xray photons interacting with matter (interacts happen at the atomic level)
Attenuation
photons that interact with the whole atom
low energy photons
photons that interact with orbital electrons
moderate electrons
photons that interact with nucleus
high energy photons
Xrays can undergo any of three processes
-transmission without any energy
-total absorption
-penetration with loss of energy
-interactions can occur with: entire atom, orbital electrons, atomic nucleus
-anatomic tissue absorbs and transmits xrays differently based on their composition (anatomic number and tissue density)
-bone absorbs more xrays than muscle
-attenuation:the primary xray beam loses some of its energy (number of photons) as it interacts with anatomic tissue (absorption, scattering)
Differential Absorption
Three essential aspects of tissues will determine their attenuation properties and the resulting subject contrast
tissue thickness
tissue density
tissue atomic number
Electrons close to nucleus are tightly bound to nucleus due to location and positive charge of nucleus
orbital electrons
electrons further away are weakly attracted to nucleus and easily removed
creating ionization
when the primary xray beam interacts with anatomic tissues. Three processes occur during attenuation of the xray beam
absorption
scattering
transmission
If the incoming xray photon passes through the atomic part without any interaction with the atomic structure, what is it called?
Transmission
-a static energy from positive charge of the nucleus
-dependent upon atomic number of element
Electron binding energy
(higher atomic number yields higher binding energies and inner shell electrons have higher binding energy due to proximity to positively charged nucleus)
Total energy of electron is a function of which two combined energies
binding and kinetic
Dynamic energy of electron due to its relative position in the electron orbital cloud
electron Kinetic energy
-electrons in outer shell posses higher kinetic energy
-electrons in inner shells (K, L, M) have lower kinetic energy but higher binding energies
Interactions between Xray and Matter
coherent scattering
photoelectric absorption
compton scattering
Complete absorption of the incoming photon :
-xray ionizes atom
-low energy secondary xray photon created
Photoelectric Effect
what is the probability of photoelectric effect dependent on
the energy of the incoming xray photon and tissue atomic number
During what interaction is the energy of the primary beam deposited within the atoms comprising the tissue
During absorption
what interaction occurs when an incoming photon loses some but not all of its energy then changes its direction
Compton effect
It can occur within all diagnostic xray energies and is dependent only on the energy of the incoming photon, not the atomic number of the tissue
Compton Effect
What happens to the number of compton interactions when there is higher kvp
Higher kvp reduces the number of interactions overall, but the number of compton interactions increases in comparison to the number of photoelectric interaction
What does an increase in atomic number and absorption do to transmission
decreased transmission
Steps for photoelectric absorption:
- Incident electron passes near K or L shell electron of a patients atom
2 The incident photon is completely absorbed and ejects the inner shell electron (photoelectron)
- A vacancy is created in the orbital shell
- The ionized atom attempts to return to the normal state by filling the vacancy with the outer shell electrons
- The movement of the outer shell electrons creates a characteristic cascade
- Potential energy of each orbital electrons movement is converted to a low energy characteristic photon
What four words are associated with Photoelectric absorption
**diagnostic value
**absorption
**Inner shell
**characteristic cascade
what two interactions have a characteristic casecade
characteristic in the tube and photoelectric in person
photoelectron energy can be determined with the following equation:
Ei=Eb + Eke
Photoelectron characteristics
kinetic energy
mass
reabsorbs quickly
-vacancy filled by outer shell electron
-electron undergoes change in energy level (emits characteristic photon and energy of photon determined by difference in binding levels)
Characteristic Cascade
-Radiation that originated from irradiated material outside xray tube
-production similar to characteristic xray production within target
-Characteristic photons emitted from atoms of patient after PE absorption interaction
Secondary Radiation Energy
what type of secondary radiation results with low atomic number in tissue
low energy secondary radiation
what type of secondary radiation results in higher atomic number with contrast agents
Higher energy secondary radiation
incident electron energy (Ei) must be greater than or equal to binding energy (Eb) of inner shell electron
Example:
Ei = 40 keV
Eb= 45 keV
No photoelectric interaction will take place here
Incident photon energy (Ei) and inner shell electron binding energy (Eb) are close to eachother
-as photon energy increases, chance of PE interaction decreases dramatically (inverse relationship)
example:
Ei=40 keV
Eb=36 keV
PE interaction likely to occur
Given the follow which interaction has a probability of happening :
Ei=120 kev
Eb=36 kev
Probability of transmission higher than PE absorption interaction
PE absorption interaction more likely to occur in elements with higher atomic number and what does that do to the binding energy
Therefore higher binding energy of inner shell electrons
what is the relationship when the increased atomic number has a dramatic impact on amount of PE absorption
direct cubed relationship
what is the relationship betwen PE absorption when the atomic number is doubled
increases change of PE absorption interaction by factor of 8
If the atomic number is high or low what shell is being interacted with
-low atomic number atoms experience PE absorption interaction with K shell
-Higher atomic number atoms experience PE absorption interaction in K, L, or M shell
this interaction is crucial to the formation of the of the radiographic image and is responsible for the production of contrast on the radiographic image
Photoelectric effect
Photons can go through, transmit
Radiolucent
All absorbed, no transmission, cant go through
Radiopaque
3 words associated with coherent
-low energy
-no clinical value
-scatter radiation
-incident and scattered same wavelength
-same energy incoming, same outgoing
-non-ionizing, no removal of electrons
Coherent Scatter
During attenuation of the beam what is photoelectric effect responsible for?
total absorption of the incoming xray photon
This term refers to any xray photon which has changed direction from the direction of the primary beam
Scatter
how much percentage of scatter xray photons orginate from Compton interactions in the patient
99%
Where do Compton and photoelectric interactions occur
-Compton occurs only in outer shells of an atom
-photoelectric interactions occur only in the inner most shell of an atom
Factor affecting beam attenuation
Tissue thickness- xrays attenuated reduced by 50% for each 4 to 5 cm of tissue thickness
Type of tissue- tissues composed of higher atomic number will increase beam attenuation
Tissue density- increasing the compactness of the atomic particle will increase beam attenuation
xray beam quality- higher kvp increases the energy of the xray beam and will decrease beam attenuation
what type of radiation is composed of transmitted and scattered radiation
remnant or exit
what is it called when scatter radiation reaching the image receptor creates unwanted exposure
Fog
True or false
most secondary radiation is scattered
true
Refers to that secondary radiation which has been emitted in a diraction different than the orginal xray beam
scatter radiation
refers to any radiation resulting from interactions within the patient
secondary radiation
-No clinical value
-involves low energy photons below 10 kev
-Thompson-single outer shell electron
-Rayleigh- all electrons of atom
Coherent Scatter
-electrons excited and vibrate at photon frequency
-no electrons ejected
-no ionization taken place
-atom stabilizes itself by releasing photon equal in energy to incident photon but in a different direction
Coherent scatter
what is your patient dose
photoelectric
what is your occupational dose
comptom
bi products for Compton
recoil electron and scatter photon
-incident electron interacts with outershell, loosely bound electron and ejects it (recoil electron)
-ion pair formed
- scattered photon redirected at some angle with a loss of kinetic energy
-higher kev photons, scattering more forward direction
-outershell
-form of scatter
what interaction has one incoming and two leaving
compton
-Photon transfers some of its kinetic energy to recoil (compton) electron and continues on in a different direction
-incident photon energy is distributed between recoidl electron and scattered photon
Compton Scatter
-Recoil electron travels until it fills vacancy in another atom
-scattered photon continues to interact until absorbed photoelectrically
Compton Scatter
Source of occupational exposure and radiation fog
most scatter travels in foward direction
backscatter
Compton Scatter
which interaction is predominant in higher Z# materials
Photoelectric
Which interaction is predominant in soft tissues
Compton
PE Versus comptom highly dependent on two factors
-incident photon energy
-atomic number of target atom
with attenuated beam:
as kvp increases what happens to PE and compton
PE absorption decreases
Compton effect increases
increases percentage of scatter and decreases percentage of absorption
which interaction predominates in two circumstances:
low energy ranges
in elements with higher atomic numbers
PE
This interaction typically predominates within diagnostic xray energy range
Compton
When PE absorption predominates:
-resulting imaging will have higher contrast
-clearly seen with film/screen imaging
-not obvious with digital processing
low kvp and high mAs
When compton predominates:
-resulting image will have lower contrast
-clearly seen with film/screen imaging
-not obvious with digital processing
- high kvp and low mas produce more Compton interactions but less patient exposure
What happens to
beam attenuation
absortion
transmission
when tissue thickness increases
and decreases
increase:
increase attenuation
increase absorption
decrease transmission
decrease
decrease attenuation
decrease absorption
increase transmission
What happens to
beam attenuation
absortion
transmission
when tissue atomic number increases
and decreases
increase :
increase attenuation
increase absorption
decrease transmission
decrease
decrease attenuation
decrease absorption
increase transmission
What happens to
beam attenuation
absorption
transmission
when tissue density increases
and decreases
increase :
increase attenuation
increase absorption
decrease transmission
decrease
decrease attenuation
decrease absorption
increase transmission
What happens to
beam attenuation
absorption
transmission
when xray beam quality increases
and decreases
increase:
decrease attenuation
decrease absorption
increase transmission
decrease:
increase attenuation
increase absorption
decrease transmission
Steps of Coherent interaction:
- A low energy photon approached an atom inside the patient
- the electron is momentarily excited
- the unstable atom re-emits the xray photon to regain stability
- the re-emitted photon travels in a new direction with no loss of energy
