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MRAD 251 - Computed Tomography > Week 1 Lecture 1 > Flashcards

Week 1 Lecture 1 Flashcards

1
Q

what are the three components of x-ray imaging systems that influence the flow of electrons?

A
  1. operating console
  2. generator
  3. x-ray tube
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2
Q

what does the operating console consist of?

A

specialized keyboards that control the intensity and quality of the radiation used during imaging

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3
Q

why are high voltage generators preferred?

A
  • production of near constant-potential
  • produces greater x-ray quantity and quality
  • compact in size
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4
Q

where do you find the generators on new CT equipments?

A

built into the scanners because they are so small

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5
Q

why is the production of greater x-ray quantity and quality important?

A

it improves the image quality and decreases patient dose

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6
Q

x-ray tube

A
  • electronic vacuum that produces x-ray photons
  • focuses useful beams of radiation through a window
  • help with heat dissipation
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7
Q

what are the 2 components that make up the x-ray tube?

A
  1. anode
  2. cathode
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8
Q

protective housing

A

contains the x-ray tube

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9
Q

what are the 3 functions of the protective housing?

A

helps to prevent:
1. electric shock
2. exposure to leaking radiation
3. damage to internal components

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10
Q

what is the role of metal and glass envelopes?

A

to maintain the vacuums inside the tube as well as providing structural support

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11
Q

why is the vacuum important for the tube?

A
  • maintains the tube’s current
  • allows for more efficient x-ray production and longer tube life
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12
Q

in modern ct tubes, which is preferred? metal or glass envelopes?

A

metal

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13
Q

why is oil found between the housing?

A

to dissipate heat

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14
Q

what causes arcing?

A

deposits from tungsten vaporization

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15
Q

why are metal envelopes preferred over glass ones?

A

metal envelopes:
- prevent arcing
- enables higher tube currents
- improves heat dissipation

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16
Q

window on the envelope

A
  • emits useful x-ray photos from the tube
  • enables maximum emission of x-ray photons with the least amount of of absorption
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17
Q

where do you find the window?

A

near the target – on the anode side

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18
Q

cathode

A

negatively charged portion of the x-ray tube

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19
Q

what is the purpose of the cathode?

A
  • produces electrons
  • directs the electrons to a specific location on the anode
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20
Q

how does the cathode produce electrons?

A

thermionic emission

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21
Q

what are the two primary parts to the cathode?

A
  1. filament
  2. focusing cup
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22
Q

filament

A

thoriated tungsten coils that boils off electrons when it is heated

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23
Q

why are there two filaments in an x-ray tube?

A

difference in the filament size used affects spatial resolution and impacts the heat capacity of the anode

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24
Q

coil size of the filament correlates to what?

A

focal spot sizes

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25
Q

both filaments can be used at once. t/f

A

false

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26
Q

larger filaments are _____ and _____.

A

longer and wider

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27
Q

focusing cup

A
  • metal canal that surrounds the filament to electrostatically confine electrons
  • ensures the electrons will strike the anode target at the right spot
28
Q

why is the focusing cup negatively charged?

A

to prevent the electrons from spreading out

29
Q

anode

A

positively charged portion of the x-ray tube

30
Q

what are the 3 purposes of the anode?

A
  1. produces x-rays
  2. conducts electricity
  3. provides mechanical support for the target
31
Q

what are the 3 components of the anode?

A
  1. bearing assembly
  2. rotor
  3. target/disk
32
Q

where is the bearing assembly found?

A

it is covered by the rotor + covered in oil

33
Q

what is the purpose of the bearing assembly?

A
  • to provide and ensure smooth rotation of the anode disk
  • conducts heat away from the x-ray tubes
34
Q

liquid bearing method vs. silver ball-bearing technology

A

liquid bearing method improves anode disk rotation
reduces the vibration and noise

35
Q

what is the purpose of the rotor?

A

prevents transmission of heat from disk to the bearings

36
Q

what is the stator?

A
  • tube-like structure that surrounds the rotor
  • makes the target rotate by electromagnetic induction
37
Q

where do you find the stator?

A

outside the metal envelope

38
Q

what is the purpose of the target?

A

collides with the electrons from the cathode

39
Q

how are x-rays created?

A

when fast moving electrons strike the metal target

40
Q

targets are designed to improve heat capacity and spatial resolution. how?

A

by the angles that are found on the targets

41
Q

how does the target move?

A

by the stator due to electromagnetic induction

42
Q

focal spot(s)

A

area of electron collision therefore site of x-ray emission

43
Q

where do you find focal spots?

A

on the angled portion of the targets

44
Q

larger or steeper angles = ?

A

larger or steeper angles of target will result in a larger surface areas

larger surface areas will mean larger area of interaction which reduces the concentration of heat

steeper angles also = ability to irradiate wider sections of anatomy = decrease spatial resolution

45
Q

does CT imaging or general radiography generate more heat?

A

CT imaging

46
Q

to manage heat production, what do CT tubes have in terms of FSS, target angles + size, and rotation speed?

A

larger fss
larger angles on larger and thicker targets
faster target rotation speeds

47
Q

what are the three designs for CT targets?

A
  1. all-metal disk
  2. brazed graphite
  3. chemical vapour deposition (CVD) graphite disk
48
Q

what are limitations for all-metal disks?

A
  • heavy
  • fixed targets
  • poor heat dissipation
  • short tube life
49
Q

*which target design was used in old/conventional scanners?

A

all-metal disks

50
Q

*which two target designs are more commonly found in helical/spiral ct scanners?

A

brazed graphite and chemical vapour deposition graphite disks

51
Q

*what is the difference between the brazed graphite targets and the CVD targets?

A

both have graphite bases, but the difference is brazed graphite’s focal track is made of tungsten rhenium while the CVD’s focal track just has the layer of tungsten r. over it

52
Q

what are the two advantages to brazed graphite?

A
  1. capacity to withstand more heat (higher thermal capacity)
  2. longer tube life
53
Q

what are the three advantages to CVD?

A
  • allows for disks to be large and lightweight
  • large heat-storage capacity
  • fast cooling rates
54
Q

what are the steps to x-ray production?

A

operating console to engage generator
generator sends current of electricity to x-ray tube
cathode’s filament is being heated which creates electrons
potential difference develops (e- from cathode to anode)
target on the anode gets bombarded with electrons
production of heat (99%) and x-ray photons (1%)

55
Q

steps to getting a CT image.

A
  1. perform scout
  2. scan sequence
  3. check for quality/accuracy - repeat scans if necessary
  4. post-procedural care + dismiss patient
  5. perform image post-processing
56
Q

protocols =

A

“how to scan” guide

57
Q

what are the four key considerations to achieve a ct image?

A
  1. scan type (axial or helical)
  2. patient positioning
  3. use of contrast media
  4. scan paramaters (technique selections - kVps, mAs)
58
Q

smaller filaments

A

increases spatial resolution
decreases heat capacity

59
Q

larger filaments

A

used when imaging longer lengths of anatomy or hypersthenic patients

60
Q

hypersthenic

A

obese

61
Q

sthenic

A

“average” body

62
Q

hyposthenic

A

thin - below average

63
Q

asthenic

A

frail - “very thin”

64
Q

why does CT use a thicker target than GR x-ray tubes?

A

exposures are longer and produce more heat

65
Q

what is an important consideration in CT imaging?

A

overheating reduces maximum exposure times

66
Q

when would a large focal spot be preferred?

A

imaging larger body parts

MRAD 251 - Computed Tomography (13 decks)

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