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Radiology > X-ray Production > Flashcards

X-ray Production Flashcards

1
Q
A
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2
Q

What are the basic principles of x-ray production?

A
  1. Electrons accelerated towards atoms at very high speed
  2. On collision, the kinetic energy of these electrons is converted to heat & electromagnetic radiation (ideally X-ray photons)
  3. The X-ray photons are aimed at a subject
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3
Q

What are the main components of the dental x-ray unit?

A
  • Tubehead (with X-ray tube)
  • Collimator
  • Positioning arm
  • Control panel
  • Circuitry
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4
Q

What is in the x-ray tube?

A

Glass envelope
* Vacuum inside

Cathode (-ve)
* Filament
* Focusing cup

Anode (+ve)
* Target
* Heat-dissipating block

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5
Q

What is a cathode filament and what does it do?

A

a coil of wire that is heated to produce electrons in an x-ray tube

Low-voltage, high-current electricity passed through wire
* Heats up until incandescent (~2200 °C)
* Electrons released from atoms in wire by thermionic emission
* “Cloud” of electrons forms around cathode

incandescent: emitting light as a result of being heated.

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6
Q

How are the electrons released from atoms?

A

thermionic emission

(the process of releasing charged particles, also known as thermions, from a hot metal surface)

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7
Q

What does an increase in current in the filament mean?

A

increase in heat and electrons

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8
Q

What is the filament made out?

A

tungsten

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9
Q

What are the qualities of tungsten?

A
  • High melting point (3422°C)
  • High atomic number (Z = 74) - lots of electrons per atom
  • Malleable
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10
Q

What is the focusing cup, what is it made of and what does it do?

A

Metal plate shaped around filament
* Negatively charged → repels electrons released at filament
* Shaped to focus the electrons at a small point on the anode target

Made of molybdenum
* High melting point (2623°C)
* Relatively poor thermionic emitter

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11
Q

What does an increase in potential difference increase?

A

increase in acceleration
increase in kinetic energy

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12
Q

How do the electrons travel?

A

Electrons released at filament are repelled away from the cathode (filament & focusing cup) & attracted to the anode (target)
* Accelerate to a very high speed over a very short distance (up to half the speed of light)

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13
Q

What do transformers do?

A

ransformers take mains electrical supply (220-240V) & convert it by changing the voltage & current

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14
Q

What are the two transformers present in the tubehead?

what do they each do?

A
  • Step-up transformer
    increase potential difference across X-ray tube to 60,000-70,000V
  • Step-down transformer
    decrease potential difference across filament to ~10V
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15
Q

What is a electron volt?

A

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

1 eV = kinetic energy gained by 1 electron moving across a potential difference of 1 volt

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16
Q

What is the anode made of?

A

tungsten

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17
Q

What is the anode and what does is produce?

A

Metal block bombarded by electrons
* Produces photons (& lots of heat)

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18
Q

What is the focal spot?

A

prescise area on target where electrons collide & X-rays are produced (ie. the X-ray “source”)

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19
Q

What is the role of the heat disspitating block that the target is embedded in?

what is it made of?

A

Heat produced in target (by electron collisions) dissipates into this block by thermal conduction

  • Reduces risk of overheating which may damage target

Made of copper
* High melting point (1085°C)
* High thermal conductivity

20
Q

What is the penumbra effect?

A

Blurring of radiographic image due to focal spot not being a single point (but rather a small area)
* Minimised by shrinking size of focal spot

21
Q

What is the problem with a small focal spot?

A
  • <1% of kinetic energy from the electrons is converted to X-ray
    photons whereas ~99% is converted to heat
  • ↓ focal spot size = ↑ image quality but ↑ heat concentration
22
Q

How can the heat problem be solved in small focal spots?

A

Angled target
* Increases the actual surface area where electrons impact
* ↑ better heat tolerance
* Reduces the apparent surface area from where the X-ray beam is emitted
* ↓ penumbra effect

23
Q

What is the glass envelope and what is it’s role?

A

Air-tight enclosure
* Supports cathode & anode
* Maintains a vacuum
* Electrons able to travel from cathode to anode unhindered by gas molecules

24
Q

What is the glass envelope made of?

A

Leaded glass to absorb X-ray photons
* Except for an un-leaded window
* Only the X-ray photons traveling in the desired direction can escape from the X-ray tube

25
Q

What are the main components of the tubehead?

A
  • x-ray tube
  • metal shielding
  • alumination filtration
  • oil
  • spacer cone
26
Q

What does the metal shielding do?

A

usually lead
absorbs x-rays
window where x-ray beam exits

27
Q

What is the role of the oil?

A

dissipates heat produced by x-ray tube by thermal convection

28
Q

What does filtration do and how?

A

Removes lower energy (non-diagnostic) X-rays from beam
* Low energy photons would all be fully absorbed by patient’s tissues
& increase patient dose but not contribute to image

Aluminium (of adequate thickness) is able to absorb these photons so that the resulting X-ray beam contains mostly diagnostic X-ray photons

29
Q

What is the minimum thickness of aluminium required for <70kV and >70kV?

A
  • <70kV→1.5mm
  • ≥ 70 kV → 2.5mm
    (modern equipment operates in the range of 60-70 kV)
30
Q

What is the role of the spacer cone?

A

Dictates distance between focal spot (of target) & patient
* “Focus to skin distance” (fsd)
* Altering fsd will affect degree of divergence of X-ray photons in X-
ray beam (& affect intensity of X-ray beam)
* Increasing fsd reduces divergence of X-ray beam & therefore reduces magnification of image (but also reduces intensity of X-ray beam)
* A set distance helps ensure a consistent radiographic technique

31
Q

What are the specific sizes required for fsd?

A
  • <60kV→100mm
  • ≥ 60 kV → 200mm (ie. modern equipment)
32
Q

What is the fate of the x-ray photons emitted from the focal spot?

A

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

33
Q

What is a collimator, what is it’s role and what does it do?

A

Lead diaphragm attached to end of spacer cone
* Reduces patient dose

Crops X-ray beam to match size & shape of X-ray receptor
* Tubeheads inherently create a circular beam
* Collimators typically change circular cross-section to a rectangular cross-section

34
Q

What is the beam area reduction when using size 2 receptors with collimators?

A

50mm x 40mm

35
Q

What are the advantages/disadvantages of rectangular collimation?

A

advantage - can reduce effective dose to patient by approximately 50%

disadvantage - increases risk of collimation errors (but this can be minimised using good radiographic technique)

36
Q

What does the control panel have?

A
  • On/off switch & light
  • Electronic timer
  • Exposure time selector & presets
  • Warning light & noise (for when X-rays are being generated)
  • (Kilovoltage selector)
37
Q

What are the consequences of electrons bombarding target?

A
  • heat production (involve outer shell electrions of tungsten atoms at target)
  • x-ray production (involve inner shell electrons and nuclei of tungsten atoms at target, less than 1% of interations)
38
Q

How do the heat producing interactions occur?

A

Bombarding electron reaches tungsten outer shell electron & either:

  • Comes into close proximity & is then decelerated & deflected - both negatively charged
  • Collides & is deflected

Bombarding electron loses kinetic energy which is converted to heat

Heat energy dissipated
Tungsten target > copper block >oil in tubehead > air

39
Q

How do the x-ray producing interactions occur?

2 ways

A

Continuous radiation interactions (majority)

Characteristic radiation interactions

40
Q

What is continuous radiation?

A

Bombarding electron passes close to target nucleus, causing it to be rapidly decelerated & deflected → lost kinetic energy released as X-ray photons
* aka. Bremsstrahlung (ie. German for “breaking radiation”)

41
Q

What is the continous radiation spectrum?

A

Photons produced over wide range of energies
* increase in proximity of electron to nucleus → increase in deceleration + deflection → increase in energy released

  • Greater proportion of lower energy photons
42
Q

When is the maximum energy achieved?

A

when electron collides directly with nucleus & stops completely
* Rare
* Numerically identical to the potential difference (ie. voltage) across the X-ray tube
* eg. 70 kV → 70 keV photon produced

43
Q

What is characteristic radiation?

A

Bombarding electron collides with an inner-shell electron & either displaces it into a more peripheral shell (excitation) or removes it completely (ionisation)

The remaining orbiting electrons rearrange themselves to re-fill the innermost shells
* When an electron “drops” to a lower shell it loses energy which is emitted as a photon of specific energy
* Values depend on the element involved (eg.tungsten)

44
Q

What is the characteristic radiation spectrum?

A

Photon energy equals the difference in the binding energies of the 2 shells involved (which are specific to that element)

Target made up of tungsten atoms
* K shell binding energy = 69.5 keV
* L shell binding energy = 10.2 keV
* M shell binding energy = 2.5 keV

45
Q

Can dental x-ray tubes displace the k shell electrons?

A

dental X-ray tubes often operate at 70 kV so that bombarding electrons have sufficient energy (70 keV) to displace K shell electrons

46
Q

Continous vs Characteristic

A

Continuous
* Produces a continuous range of X-ray photon energies
* Maximum photon energy matches the peak voltage
* Bombarding electron interacts with nucleus of target atom

Characeristic
* Produces specific energies of X-ray photon, characteristic to the element used for the target
* Photon energies depend on the binding energies of electron shells
* Bombarding electron interacts with inner- shell electrons of target atom

47
Q

What is the x-ray beam made of?

A

continuous radiation + characteristic radiation - filtered photons

Radiology (15 decks)

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