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Pre-BDS3 Radiology > X-Ray Production > Flashcards

X-Ray Production Flashcards

1
Q

what are the main components of the dental x-ray unit

A

tubehead, collimator, positioning arm, control panel, circuitry

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2
Q

what are the components of the x-ray tube

A

glass envelope, cathode, anode

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3
Q

what is the cathode filament

A

coiled metal wire

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4
Q

what gets passed through the cathode filament

A

low voltage, high current electricity

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5
Q

how are electrons released from the cathode wire

A

by thermionic emission

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6
Q

what happens if you increase the current in the cathode filament

A

increased heat and increased electrons

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7
Q

what is the cathode filament made of

A

tungsten

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8
Q

what are the properties of tungsten

A

high melting point, high atomic number (lots of electrons), malleable

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9
Q

what is the cathode focusing cup

A

metal plate shaped around filament

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10
Q

what is the cathode focusing cup made of

A

molybdenum

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11
Q

what are the properties of molybdenum

A

high melting point, relatively poor thermionic emitter

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12
Q

what is the relationship between the cathode and anode

A

electrons released at filament are repelled away from the cathode and attracted to the anode

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13
Q

how is kinetic energy generated between cathode and anode

A

when the electrons collide with the anode target they have a high kinetic energy

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14
Q

what do transformers do

A

take mains electrical supply and convert it by changing the voltage and current

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15
Q

what is the mains electrical supply

A

220-240v

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16
Q

what are the 2 transformers present in the tubehead

A

step up and step down

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17
Q

what does the step up transformer do

A

increase potential difference across x-ray tube to 60,000-70,000V

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18
Q

what does the step down transformer do

A

decrease the potential difference across filament to about 10V

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19
Q

what do electron volts measure

A

the kinetic energy gained by electrons as they accelerate from cathode to anode

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20
Q

what is an anode target

A

metal block bombarded by electrons

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21
Q

what is the anode target made of

A

tungsten

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22
Q

what is the focal spot on the anode target

A

precise area on target where electrons collide and x-rays are produced

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23
Q

what is the purpose of the heat dissipating block on the anode

A

heat produced in target dissipates into the block by thermal conduction to reduce risk of overheating

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24
Q

what is the heat dissipating block made of

A

copper

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25
Q

what are the properties of copper

A

high melting point and high thermal conductivity

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26
Q

what is the penumbra effect

A

blurring of a radiographic image due to focal spot not being a single point

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27
Q

how do you minimise the risk of the penumbra effect

A

shrinking size of focal spot

28
Q

what is the solution to the need to have a small focal spot

A

have an angled target

29
Q

what is the effect of having an angled target

A

increases the actual surface area where electrons impact and reduces the apparent surface area from where the x-ray beam is emitted

30
Q

what does the glass envelope do

A

supports cathode and anode, maintains a vacuum

31
Q

what is the purpose of the glass envelope to be leaded

A

to absorb x-ray photons

32
Q

what are the main components of the tubehad

A

x-ray tube, metal shielding, aluminium filtration, oil, spacer cone

33
Q

what is the purpose of the oil in the tubhead

A

dissipates heat produced by x-ray tube by thermal conduction

34
Q

what does the filtration in the tubehead do

A

removes lower energy x-rays from beam to decrease patient dose

35
Q

why is the filtration made of aluminium

A

it can absorb those photons so that the resulting x-ray beam contains mostly diagnostic x-ray photons

36
Q

what is the minimum thickness of aluminium required for a filtration

A

for less than 70kV = 1.5mm
for greater than 70kV = 2.5mm

37
Q

what does the spacer cone do

A

dictates distance between focal spot and patient and indicates direction of the beam

38
Q

what is the distance between the focal spot and patient called

A

focus to skin distance

39
Q

what will altering the focus to skin distance do

A

affect degree of divergence of x-ray photons in x-ray beam

40
Q

what does increasing the focus to skin distance do

A

reduced divergence of x-ray beam and therefore reduces magnification of image

41
Q

what does a set focus to skin distance allow

A

consistent radiographic technique

42
Q

what is the focus to skin distance required for modern equipment (>60kv)

A

200mm

43
Q

where is the focus to skin distance measured from

A

the focal spot which is marked on tubehead

44
Q

what are the 3 possibilities for the fate of x-ray photons emitted from the focal spot

A

attenuated by lead shielding
attenuated by aluminium filtration
exit tubehead to form x-ray beam

45
Q

what is the collimator

A

lead diaphragm attached to end of spacer cone

46
Q

what is the purpose of the collimator

A

reduces patient dose and crops x-ray beam to match size and shape of x-ray receptor

47
Q

when using size 2 receptors what should rectangular collimators reduce the beam area at the patient end of spacer cone to

A

50mm x 40mm

48
Q

what can rectangular collimation reduce the surface area irradiated to

A

by almost 50%

49
Q

what is the disadvantage to using collimation

A

can increase risk of collimation errors

50
Q

what are the consequences of electrons bombarding the target

A

heat production and x-ray production

51
Q

what kind of electrons are involved in heat production at the target

A

outer shell electrons of tungsten atoms

52
Q

what kind of electrons are involved in x-ray production at the target

A

inner shell electrons and nuclei of tungsten atoms

53
Q

how common is it for x-rays to be produced at the target

A

<1%

54
Q

what happens in a heat producing interaction

A

bombarding electron reaches tungsten outer shell electron which either collides or deflects
bombarding electron loses kinetic energy which is converted to heat
heat energy dissipated

55
Q

what are the 2 x-ray producing interactions

A

continuous radiation interactions
characteristic radiation interactions

56
Q

what is the continuous radiation interaction

A

bombarding electron passes close to target nucleus causing it to be rapidly decelerated and deflected meaning lost kinetic energy released as x-ray photons

57
Q

when is the maximum energy achieved in the continuous radiation spectrum

A

when the electron collides directly with the nucleus and stops completely

58
Q

what does continuous radiation spectrum mean

A

photons produced over wide range of energies

59
Q

what does filtration of the continuous spectrum do

A

remove lower energy non-diagnostic x-ray photons from beam

60
Q

what is the characteristic radiation interaction

A

bombarding electron collides with an inner shell electron and either displaces it into a more peripheral shell or removes it completely
remaining orbiting electrons rearrange themselves to re-fill the innermost shells

61
Q

what happens when electron drops to a lower shell

A

loses energy which is emitted as a photon of specific energy

62
Q

what is photon energy

A

difference in the binding energies of the 2 shells involved

63
Q

what is the binding energy of a K shell

A

69.5keV

64
Q

why do dental x-ray tubes operate at 70kV

A

so that bombarding electrons have sufficient energy to displace K shell electrons

65
Q

what is the dental x-ray beam spectrum

A

continuous radiation + characteristic radiation - filtered photons

Pre-BDS3 Radiology (17 decks)

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