2. XR Interactions with People Flashcards

1
Q

Types of x-ray interactions

A

classical/coherent, compton, photoelectric, pair production, photodisintigration

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2
Q

What energy level does compton scatter occur compared to classical/coherent interaction?

A

higher energy

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3
Q

Describe the compton scatter interaction

A
  1. incident X-ray frees an electron (compton e-)
  2. atom is ionized (missing e-)
  3. incoming x-ray is deflected (scattered photon)
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4
Q

dominant force contributing to scatter/fog?

A

compton interactions, >100 keV

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5
Q

major source of occupational exposure?

A

compton

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6
Q

probability of compton depends on?

A

density of material or hydrogenous material/people (materials rich in hydrogen have increased probability)

decreased with increased x-ray energy

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7
Q

mechanism of photoelectric interactions

A

x-ray strikes inner shell electron, displacing it and energy is absorbed; Auger electron is released

  1. characteristic radiation
  2. negative ion (photoelectron)
  3. positive ion (atom missing an electron)
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8
Q

what energy level does photoelectric interactions take place?

A

low energy; although present throughout diagnostic range (20-120)

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9
Q

probability of PE?

A
  • inversely proportional to incident photo energy 1/E^3
  • must have minimum incident photon energy to free K shell electron
  • probability increases as binding energy/incident photon energy are similiar
  • proportional to the atomic number cubed (Z^3)
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10
Q

how does Z and PE affect contrast?

A

small differences in Z (bone > soft tissue) are amplified (cubed); better soft tissue delineation

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11
Q

why do xrays appear overexposed?

A

kVP is too high; not enough photoelectric effect/contrast

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12
Q

lead and the photoelectric effect

A

lead has high Z; more absorption

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13
Q

k shell of calcium

A

4 keV

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14
Q

role of barium/iodine in secondary radiation

A

higher Z elements; generate secondary x-rays that can leave patient/fog radiation

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15
Q

k-edge

A

photoelectric energy peaks around binding energy for the inner shell electron

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16
Q

optimal kvP for contrast agents

A

65-90 kVp as barium (k edge 47), iodine (k edge 33)

17
Q

downside of PE interactions?

A

generate more dose than other types of radiation because ALL the energy of incident x-ray photon is absorbed

18
Q

x-ray production increases how with mA vs kVP

A

linearly with mA; increase kVp will double the intensity

19
Q

tactic to visualize low contrast objects

A

keep kVp constant, increase mAs; optimize PE

20
Q

lower dose but maintain exposure?

A

raise kVp 15%, lower mAs 50%

21
Q

optimal kVp for contrast agents

A

double the binding energy of contrast agent; if K edge of Iodine is 33, you want kVp of 66; barium kVp is 74 (k edge is 37 kev)

22
Q

what energy photon is needed to create pair production?

A

1.022 MeV, high energy photon on something with high Z

23
Q

mechanism of pair production

A

high energy photon interacts with nucleus and is absorbed; production of 1 e/positron. Positron will interact with an electron and annihilate giving off 2 511 keV photons 180 degrees apart

24
Q

what is the mechanism of photodisintegration

A

high energy photons (>10 MeV) strke nucleus and cause ejection of alpha particles

25
dominant force below 30KeV
PE; low scatter
26
30 keV dominant interaction
probability of compton and PE are equal; increased probability of penetration and reduction in total attenuation
27
>30 keV dominant interaction
reduced PE interaction (1/E^3) and compton scatter
28
Increased tissue mass density results in?
decreased penetration, increased compton interactions, increased photoelectric interactions
29
increase in atomic number
decreased penetration, no change in compton, increase in PE
30
attenuation definition
xrays that interact with matter (compton/PE) and do not travel through an object
31
linear attenuation definition
fraction of photons interacting per unit thickness; linear attenuating coefficient
32
Factors that influence attenuation
more: denser objects, higher Z, at K-edge less: higher kVp
33
applied absorption theory
low keV the contrast is greatest, but photoelectric effects predominate at lower keV and increases absorption
34
what is entrance skin dose
radiation absorbed by skin as beam strikes patient
35
factors influencing entrance skin dose
tube current (mA), time of exposure (s), peak kilovoltage (squared), distance (inverse squared)