Attenuation of X-rays Flashcards
what happens to X-rays when they hit the patient
- Scattered and contribute to fogging
- absorbed completely in patient
- pass through without interacting with the tissue atoms
what is light and dark on an X-ray
Light: photons absorbed
Dark: photons pass through
In the diagnostic X-ray energy range, the interactions are what types
Coherent scattering
Photoelectric absorption
Compton scattering
what happens to X-rays in a beam path
Either absorbed or scattered
what do photons do in absorption interactions
Photons ionize absorber aoms
what do photons do in scattering interactiosn
interact with absorber atoms but then are scattered in another direction
what allows us to see differences in a radiograph
The differential exposure due to scattering and absorption
what are the means of beam attenuation
Photoelectric absorption
coherent scattering
Compton scattering
what percent of primary photons do what when in a patient
No interaction: 9%
Coherent scattering: 7%
Photoelectric absorption: 27%
Compton scattering:57%
what is the interaction in coherent scattering
- Low energy photons interact with outer electons causing the outer shell to vibrate momentarily at the same freqeuncy of incoming photon
- incident photon ceases to exist
- entire energy given off as scattered photon to be absorbed by tissue
what does the Coherent scattering ultimately do to the incident X-ray photon
Direction is altered
what happens in Photoelectric Absorption
- Incident photon interacts with an inner electrons (K)
- overcomes binding energy of electron and electron is is ejected (ionization)
- ejected electron acquires the remainder KE from the photon
what is the elctron called after it is ejected
Photoelectron or recoil electron
what happens to the elctron deficiency from photoelectric absoprtion
K shell electron deficiency is instantly filled with an L or M shell electron with release of charcteristic radition
what is the strength of the radtion due to filling of K shel electrons from L and M shell electrons
Low energy that does not fog image
what is the frequency of the photoelectric related to concerning to the photon energy
inversely proportional to the cube of photon energy (higher energy photons are less likely to undergo absorption
how does the frequency of photoelectric interactions relate to atomic number
directly with the thrid power of the atomic number of the absorber (as z increases, more photoelectric absorption)
how much more likely is bone to absorb by photoelectric interaction compared to soft tissue
6.5 times greater
what is the importance of different photoelectric interactions
different density in radiographic images
what happens to recoil electrons released via the photoelectric effect
travel short distace in the absorbed before they give up energy (absorbed by patient)
how does the absorption of recoil electrons due to the photoelectric effect by the absorber help produce high-quality radiographs
No scattered raditation fog in the recepor
how does the absorption of recoil electrons from the photoelectric affect affect the patietn
Bad for patients due to increased radition absorption
How does Compton’s Scattering occur
- Incident photons interact with an outer electron
- electronic is ejected (compton’s electron) aquiring part of KE
- remaineder of energy given off as a scattered photon
what does the compton’s electron do
Causes secondary and tertiary ionization
which type of radiation leads to scatter raditiation
Compton’s scattering
what is the Probability of a compton interaction related to
Directly proportional to the number of outer-shell electrons available in the absorbing medium (electron density)
does bone or soft tissue have a higher probability of compton’s scattering
greater in bone
how does compton interactions relate to photon energy
Inversely proportional to photon energy
what direct do scattered photons move in compton interactions
in any direction (including 180 degrees, backscattered)
how does increasing the energy of an incident photon affectthe direction of a compton photon
greater probability to angle of scatter of the secondary photon will be small and its direction will be forward
how does the angle of deflection affect the energy of scattered X-ray
angle of deflection decreases leading to an increase in the scattered x-ray increased
what direction maximizes the energy of a secondary photon
When the photon is backscattered
what percent of the scattered photons formed during a dental x-ray exposure exit patients head
30%
pros and cons of compton scattering leaving the patients head
Good: energy of raditation escapes tissue
Bad: causes nonspecific radiograph darkening (no useful info
how do secondary electrons give of energy
Collisional interaction
Radiative interactions
eventuallly all dissipate as heat
Colisional interactions lead to
Result in ionization or exciation of the atom
what does Radiative intearaction lead to
Production of bremsstrahlung raditation
Dental differential absorption
Enamel and Lamina dura absorb most
Dentin absorbs some
Pulp and PDL transmit most
what is the Half Value LAyer
thickness of an absorbed required to reduce by one half the number of x-ray photons passing through it
what happens to a monochromatic beam of radiation as it passes through a unit thickness of an absorber
a constant fraction of the beam is attenuated as the beam passes through
what does Half Value Layer do
measures beam energy by describing the amount of absorber that reduces beam intesnity by half
what does the absorption of the beam depend on
Thickness and mass of absorber
Energy of beam
how does BEam Attenuation occure in a polychromatic beam
absorption depends on energy with low energy more likely to be absorbed
what is beam hardening
when a polychromatic beam passes through mattering leading to a beam decrease but an increase in beam energy
How is a polychromatic beam absorbed at each unit thickness
Absorbed less and less at each beam thickness
