X-ray Production

Question 1

What are the components of an X-ray tube?

Answer 1

• glass envelope
  
  • vacuum inside
• cathode (-ve)
  
  • filament
  • focussing cup
• anode (+ve)
  
  • target
  • heat dissipating block

Question 2

What is a filament?

Answer 2

• coiled metal wire
• low voltage, high current electricity passed through wire
  
  • heats up until incandescent
    - ~2200 degrees
  • electrons released from atoms in wire
    - thermionic emission
  • cloud of electrons forms around cathode
• increasing current increases heat and number of electrons

Question 3

What are filaments made of?

Answer 3

• tungsten
  
  • high melting point
    - 3422 degrees
    - able to withstand high temperatures
  • high atomic number
    - 74
    - lots of electrons per atom
  • malleable

Question 4

What is a focussing cup?

Answer 4

• metal plate shaped around filament
  
  • negatively charged
    - repels electrons released at filament
  • shaped to focus the electrons at small point on anode target

Question 5

What are focussing cups made of?

Answer 5

• molybdenum
  
  • high melting point
    - 2623 degrees
  • relatively poor thermionic emitter

Question 6

What is the cathode-anode relationship?

Answer 6

• high voltage electricity passed through X-ray tube
• electrons released at filament are repelled away from cathode
  
  • attracted to anode (target)
  • accelerate at high speeds over short distance
    - up to half the speed of light
  • increased voltage increases acceleration and kinetic energy
• electrons have high kinetic energy upon collision with anode target

Question 7

What is the target in the anode?

Answer 7

• metal block bombarded by electrons
  
  • produces photons (+ heat)
• off-angle in relation to filament
• focal spot
  
  • precise area on target where electrons collide
  • x-rays are produced

Question 8

What are targets made of?

Answer 8

• tungsten
  
  • high melting point
    - 3422 degrees
    - produces x-ray photons of useful energies

Question 9

What is a heat-dissipating block?

Answer 9

• block into which target is embedded
  
  • heat produced dissipates into block
    - thermal conduction
    - reduces risk of overheating which may damage target

Question 10

What are heat-dissipating blocks made of?

Answer 10

• copper
  
  • high melting point
    - 1085 degrees
  • high thermal conductivity

Question 11

What is the penumbra effect?

Answer 11

• blurring of radiographic image due to focal spot not being a single point
  
  • minimised by shrinking size of the focal spot

Question 12

What is focal spot angulation?

Answer 12

• used to achieve a small focal spot
  
  • decreasing focal spot side increases image quality
    - also increases heat concentration
• angled target used to increase surface area where electrons impact
  
  • better heat tolerance
  • reduces apparent surface area from where x-ray beam is emitted
  • reduced penumbra effect

Question 13

What is a glass envelope?

Answer 13

• air tight enclosure
  
  • supports cathode and anode
  • maintains a vacuum
    - electrons able to travel from cathode to anode
    - unhindered by gas molecules
• leaded glass to absorb x-ray photons
  
  • except for an un-leaded window
  • only x-ray photons travelled in desired direction can escape

Question 14

What are the main points of an X-ray tube head?

Answer 14

• X-ray tube
• metal shielding
  
  • usually lead
  • absorbs x-rays
  • window through which x-rays escape
• aluminium filtration
• oil
  
  • dissipates heat produced by X-ray tube by thermal convection
• spacer cone

Question 15

What is aluminium filtration?

Answer 15

• removes lower energy (non-diagnostic) x-rays from beam
  
  • low energy photons fully absorbed by tissue
  • would increase dose but not contribute to image
• aluminium able to absorb low energy photons
  
  • minimum thickness
    - <70kV = 1.5mm (modern operates at 60-70kV)
    - >70kV - 2.5mm

Question 16

What is a spacer cone?

Answer 16

• dictated the distance between the focal spot and patient
  
  • focus to skin distance (fsd)
  • set distance helps ensure a consistent radiographic technique
• indicates the direction of the beam
• may be detachable

Question 17

What is focus to skin difference?

Answer 17

• distance between focal spot of target and patient
  
  • altering affects degree of divergence of beam
• increasing fsd reduces divergence, magnification and intensity
• 200mm
  
  • > 60kV
• measurement taken from focal spot, marked on tube head

Question 18

What happens to x-ray photons emitted from the focal spot?

Answer 18

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

Question 19

What is a collimator?

Answer 19

• lead diaphragm attached to end of spacer cone
  
  • reduces patient dose
• crops x-ray beam to match size and shape of x-ray receptor
  
  • tube heads create a circular beam
  • collimators chance cross section to rectangular
• should come standard on new equipment
  
  • can be retrofitted to older equipment

Question 20

what is rectangular collimation used for?

Answer 20

• size 2 receptors
  
  • 50-40mm collimation
  • 45-35mm collimation preferable
• reduces surface area irradiated by almost 50%
• improves image contract by reducing scatter

Question 21

What are the disadvantages of collimation?

Answer 21

• increased risk of collimation errors
  
  • minimised using good radiographic technique

Question 22

What does the control panel of an x-ray unit contain?

Answer 22

• on/off switch and light
  
  • light indicates when operational
• electronic timer
  
  • duration of exposure
• exposure time selector and presents
  
  • manual change also possible
  • select for teeth being irradiated
  • some teeth require longer exposure
    - bone teeth are contained in
    - width of teeth
• warning light and noise
  
  • for when x-rays are being generated
  • alerted to accidental exposure
  • indicates when patient can move again
• kilovoltage selector
  
  • usually at set number
  • 70kV in dental hospital

Question 23

What are the consequences of electrons bombarding the target?

Answer 23

• heat production
  
  • involves outer-shell electrons of tungsten atoms at target
  • 99% of interactions
    - more common due to plentiful outer shell electrons
• x-ray production
  
  • involves inner shell electrons and nuclei of tungsten atoms
  • 1% of interactions

Question 24

What is involved in heat-producing interactions?

Answer 24

• bombarding electrons reach tungsten outer shell electrons
  
  • comes into close proximity and is decelerated and deflected
    OR
  • collides and is deflected
• bombarding electron loses kinetic energy which is converted to heat
• heat energy dissipated
  
  • tungsten target -> copper block -> oil in tube head -> air

Question 25

What is involved in x-ray producing reactions?

Answer 25

• continuous radiation interactions
  
  • majority
  • bombarding electron passes close to target nucleus
  • electron is rapidly decelerated and deflected
  • kinetic energy lost released as x-ray photons
• characteristic radiation interactions
  
  • bombarding electron collides with an inner shell electron
  • displaces to a more peripheral shell (excitation)
    OR
  • removes it completely (ionisation)
  • remaining orbiting electrons rearrange themselves
  • refill innermost shells
    - energy released when drops to lower shell
    - emitted as photon