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XR Physics > 2. XR Interactions with People > Flashcards

2. XR Interactions with People Flashcards

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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
Q

dominant force below 30KeV

A

PE; low scatter

26
Q

30 keV dominant interaction

A

probability of compton and PE are equal; increased probability of penetration and reduction in total attenuation

27
Q

> 30 keV dominant interaction

A

reduced PE interaction (1/E^3) and compton scatter

28
Q

Increased tissue mass density results in?

A

decreased penetration, increased compton interactions, increased photoelectric interactions

29
Q

increase in atomic number

A

decreased penetration, no change in compton, increase in PE

30
Q

attenuation definition

A

xrays that interact with matter (compton/PE) and do not travel through an object

31
Q

linear attenuation definition

A

fraction of photons interacting per unit thickness; linear attenuating coefficient

32
Q

Factors that influence attenuation

A

more: denser objects, higher Z, at K-edge
less: higher kVp

33
Q

applied absorption theory

A

low keV the contrast is greatest, but photoelectric effects predominate at lower keV and increases absorption

34
Q

what is entrance skin dose

A

radiation absorbed by skin as beam strikes patient

35
Q

factors influencing entrance skin dose

A

tube current (mA), time of exposure (s), peak kilovoltage (squared), distance (inverse squared)

XR Physics (6 decks)

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