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radiology > Attenuation of X-rays > Flashcards

Attenuation of X-rays Flashcards

1
Q

what happens to X-rays when they hit the patient

A
  • Scattered and contribute to fogging
  • absorbed completely in patient
  • pass through without interacting with the tissue atoms
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2
Q

what is light and dark on an X-ray

A

Light: photons absorbed
Dark: photons pass through

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3
Q

In the diagnostic X-ray energy range, the interactions are what types

A

Coherent scattering
Photoelectric absorption
Compton scattering

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4
Q

what happens to X-rays in a beam path

A

Either absorbed or scattered

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5
Q

what do photons do in absorption interactions

A

Photons ionize absorber aoms

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6
Q

what do photons do in scattering interactiosn

A

interact with absorber atoms but then are scattered in another direction

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7
Q

what allows us to see differences in a radiograph

A

The differential exposure due to scattering and absorption

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8
Q

what are the means of beam attenuation

A

Photoelectric absorption
coherent scattering
Compton scattering

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9
Q

what percent of primary photons do what when in a patient

A

No interaction: 9%
Coherent scattering: 7%
Photoelectric absorption: 27%
Compton scattering:57%

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10
Q

what is the interaction in coherent scattering

A
  • 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
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11
Q

what does the Coherent scattering ultimately do to the incident X-ray photon

A

Direction is altered

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12
Q

what happens in Photoelectric Absorption

A
  • 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
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13
Q

what is the elctron called after it is ejected

A

Photoelectron or recoil electron

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14
Q

what happens to the elctron deficiency from photoelectric absoprtion

A

K shell electron deficiency is instantly filled with an L or M shell electron with release of charcteristic radition

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15
Q

what is the strength of the radtion due to filling of K shel electrons from L and M shell electrons

A

Low energy that does not fog image

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16
Q

what is the frequency of the photoelectric related to concerning to the photon energy

A

inversely proportional to the cube of photon energy (higher energy photons are less likely to undergo absorption

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17
Q

how does the frequency of photoelectric interactions relate to atomic number

A

directly with the thrid power of the atomic number of the absorber (as z increases, more photoelectric absorption)

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18
Q

how much more likely is bone to absorb by photoelectric interaction compared to soft tissue

A

6.5 times greater

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19
Q

what is the importance of different photoelectric interactions

A

different density in radiographic images

20
Q

what happens to recoil electrons released via the photoelectric effect

A

travel short distace in the absorbed before they give up energy (absorbed by patient)

21
Q

how does the absorption of recoil electrons due to the photoelectric effect by the absorber help produce high-quality radiographs

A

No scattered raditation fog in the recepor

22
Q

how does the absorption of recoil electrons from the photoelectric affect affect the patietn

A

Bad for patients due to increased radition absorption

23
Q

How does Compton’s Scattering occur

A
  • 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
24
Q

what does the compton’s electron do

A

Causes secondary and tertiary ionization

25
Q

which type of radiation leads to scatter raditiation

A

Compton’s scattering

26
Q

what is the Probability of a compton interaction related to

A

Directly proportional to the number of outer-shell electrons available in the absorbing medium (electron density)

27
Q

does bone or soft tissue have a higher probability of compton’s scattering

A

greater in bone

28
Q

how does compton interactions relate to photon energy

A

Inversely proportional to photon energy

29
Q

what direct do scattered photons move in compton interactions

A

in any direction (including 180 degrees, backscattered)

30
Q

how does increasing the energy of an incident photon affectthe direction of a compton photon

A

greater probability to angle of scatter of the secondary photon will be small and its direction will be forward

31
Q

how does the angle of deflection affect the energy of scattered X-ray

A

angle of deflection decreases leading to an increase in the scattered x-ray increased

32
Q

what direction maximizes the energy of a secondary photon

A

When the photon is backscattered

33
Q

what percent of the scattered photons formed during a dental x-ray exposure exit patients head

A

30%

34
Q

pros and cons of compton scattering leaving the patients head

A

Good: energy of raditation escapes tissue
Bad: causes nonspecific radiograph darkening (no useful info

35
Q

how do secondary electrons give of energy

A

Collisional interaction
Radiative interactions
eventuallly all dissipate as heat

36
Q

Colisional interactions lead to

A

Result in ionization or exciation of the atom

37
Q

what does Radiative intearaction lead to

A

Production of bremsstrahlung raditation

38
Q

Dental differential absorption

A

Enamel and Lamina dura absorb most
Dentin absorbs some
Pulp and PDL transmit most

39
Q

what is the Half Value LAyer

A

thickness of an absorbed required to reduce by one half the number of x-ray photons passing through it

40
Q

what happens to a monochromatic beam of radiation as it passes through a unit thickness of an absorber

A

a constant fraction of the beam is attenuated as the beam passes through

41
Q

what does Half Value Layer do

A

measures beam energy by describing the amount of absorber that reduces beam intesnity by half

42
Q

what does the absorption of the beam depend on

A

Thickness and mass of absorber

Energy of beam

43
Q

how does BEam Attenuation occure in a polychromatic beam

A

absorption depends on energy with low energy more likely to be absorbed

44
Q

what is beam hardening

A

when a polychromatic beam passes through mattering leading to a beam decrease but an increase in beam energy

45
Q

How is a polychromatic beam absorbed at each unit thickness

A

Absorbed less and less at each beam thickness

radiology (10 decks)

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