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AA Clinical Imaging > X-Ray Tube > Flashcards

X-Ray Tube Flashcards

1
Q

The two types of x-ray production

A

Characteristic

Bremsstrahlung

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2
Q

Parts of an x-ray tube

A

Cathode

Anode

Evacuated glass tube

Vacuum Housing

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3
Q

Requirements of x-ray tube

A

Controllable

Safe

Point source

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4
Q

Thermionic emission

A

Electric current causes heating

Above 4 amps electrons boiled off

Cloud of electrons forms around filament

Contained by space charge effect

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5
Q

Why tungsten?

A

Good thermionic emitter

High melting point (3410 degrees C)

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6
Q

How does the focusing cup work?

A

Controls stream of electrons hitting anode

Negative charges repel each other

>3kV relative to filament will alter electron flow

Charge can be altered to change focal spot size

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7
Q

What are the functions of the anode and the two types ?

A

Support target material: source of x-ray photons

Electrical conductor

Thermal conductor

  • Stationary and rotating
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8
Q

When would a stationary anode be used?

A

Dental- low output needed

99.5% of electron energy converted to heat

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9
Q

Benefits of a rotating anode

A

Increase heat dissipation

Increase anode load

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10
Q

How efficient is the x-ray process?

A

99.5% of energy lost as heat

Only a small percentage of photons produced exit the window

Of those photons that make up the beam, most are attenuated by the patient

Total energy put into process- 1.3x10-12% creates the image

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11
Q

Structure of the envelope and vacuum of the anode?

A

Must be strong, rigid and withstand heating and high voltage environment

Glass, metal or ceramic

Contains anode and cathode within vacuum

Vacuum improves efficiency as electrons are not impeded by molecules

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12
Q

Functions of tube housing (protections)

A
  • Physical protection
  • Radiation protection
  • Electrical protection
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13
Q

Properties of tube housing

A
  • Steel/lead lining
  • Exit window
  • Oil filled- heat dissipation and electrical insulation
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14
Q

What happens at prep and exposure?

A

Thermionic cloud developed by current

Rotating anode begins to spin

A large voltage is applied across the c- and a+

Potential energy of electrons becomes kinetic

Cathode repels electron cloud to anode

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15
Q

Problems with pressing the button to expose

A

If the anode is not up to speed and fully prepped, exposure won’t trigger on full press

Prepping too long can prematurely age the filament as thermionic cloud is maintained

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16
Q

When would Fine focus be used?

A

Used when image quality is limited by geometric factors

Energy focussed on small area

Need to be aware of tube loading to prevent damage

17
Q

When would Broad focus be used?

A

Used when image quality is limited by patient attenuation

Increase heat dissipation

Permits higher tube loading

18
Q

Anode angle- real and effective

A
  • Effective spot determined by real focus size and anode angle
  • Anode angle 7-15 degrees
  • Effective spot ~3 times smaller than real focus spot
19
Q

What does a ‘smaller real focus’ mean?

A
  • Finer effective focus
  • Less heat dissipation
20
Q

What does a ‘smaller anode angle’ mean?

A

Finer effective focus

Smaller beam field size possible

21
Q

Explain the ‘anode heel effect’

A

Emitted X-rays pass through different thickness of tungsten depending on anode angle

X-ray B travels greater distance through target than X-ray A

Results in greater image intensity on cathode end of image

Increases as anode angle is reduced

22
Q

Light beam diaphragm and collimator benefits

A

Control dimensions of primary beam

Uses light and mirror to centre rays

Lead shutters

May auto cone to receptor when inserted in bucky

23
Q

What does filtration do?

A

Enriches beam with high energy photons by absorbing lower energy x-rays

Usually preset into protocols

‘Beam hardening’

Higher average energy- greater average energy, reduce skin dose

24
Q

Inherent filtration examples

A

Envelope

Tube housing

LBD

Mirror

etc

25
Q

Added filtration examples

A

For normal x-ray work ~2.5mm aluminium or 0.3mm copper

26
Q

Compensation filtration examples

A

Wedge filter

Bow tie filter

27
Q

What are Ionisation chambers?

A

Gas-filled ionisation chamber

A proportion of incident photons cause ionisations which are collected at the charged anode

Measures dose area product cGy.cm2

28
Q

High voltage generator

A
  • High voltage transformer- step up 240V-100kV
  • Voltage rectifier- converts AC to DC
  • High frequency generator- 50Hz to 500-5,000Hz

AA Clinical Imaging (7 decks)

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