X Ray Production

Question 1

x ray tube components

Answer 1

1. glass envelope - vacuum inside
2. cathode (-ve) - filament & focusing cup
3. anode (+ve) - target, heat dissipating block

Question 2

cathode filament

Answer 2

made of tungsten -
high atomic no - lots of electrons per atom
high mpt - able to withstand high temp
malleable

Question 3

cathode focusing cup

Answer 3

made of molybdenum
negatively charged to repel electrons released at filament & focus them to anode target

Question 4

anode target

Answer 4

made of tungsten (high mpt)
metal block bombarded by electrons, produces photons (x ray ones of useful energies)
off angle in relation to filament - important

Question 5

anode heat dissipating block

Answer 5

target embedded in block of copper - high mpt and high thermal conductivity
reduces risk of over heating

Question 6

focal spot

Answer 6

precise area on anode target where electrons collide & x rays are produced i.e. x ray source

Question 7

omnidirectional emission of divergent x ray photons

Answer 7

multi directional imaging of divergent x ray photons means when you have electrons being produced at filament they accelerate out towards the target, colliding at the focal spot

Question 8

penumbra effect

Answer 8

blurring of radiographic image due to focal spot not being a single point but a small area
minimised by shrinking size of focal spot
shrinking it will increase image quality but also heat concentration therefore must angle it

Question 9

why angle anode target

Answer 9

• increases actual SA where electrons impact so better heat tolerance
• reduces apparent SA from where x ray beam is emitted so decreases penumbra effect

Question 10

purpose of glass envelope

Answer 10

1. air tight enclosure - supports cathode & anode, maintains vacuum so electrons can travel from cathode to anode unhindered by gas molecules
2. leaded - to absorb x ray photos, except for unleaded window, ensures only x ray photons travelling in desired direction can escape x ray tube

Question 11

main components of tube head

Answer 11

x ray tube
metal shielding - usually lead
aluminium filtration
oil - dissipates heat by thermal convection
spacer cone

Question 12

aluminium filtration

Answer 12

removes lower energy (non diagnostic) x rays from beam as they would be absorbed by tissues but not contribute to image
aluminium thickness required -
<70kV = 1.5mm
>70kV = 2.5mm

Question 13

spacer cone

Answer 13

dictates distance between focal spot & ptx i.e. FSD - focus to skin distance
increasing FSD reduces divergence of x ray beam but reduces magnification & intensity
indicates direction of beam
may be detachable
>60kV FSD must be 200mm

Question 14

fate of x ray photons emitted from focal spot

Answer 14

1. attenuated by lead shielding
2. attenuated by aluminium filtration
3. exit tube head to form x ray beam

Question 15

collimator

Answer 15

lead diaphragm attached to end of spacer cone to reduce ptx dose
crops x ray beam to match size & shape of x ray receptor from circular to rectangular

Question 16

rectangular collimation

Answer 16

can reduce effective dose to the patient by approx 50%

Question 17

consequences of electron bombarding target

Answer 17

heat production - loses kinetic energy which is converted to heat which is dissipated
x ray production - <1% interactions - deflected from nucleus & loses kinetic energy in from of x ray photons

Question 18

continuous radiation spectrum

Answer 18

increase proximity of electron to nucleus -> increase deflection -> increase energy release so greater proportion of lower energy photons. max energy achieved when electron collides with nucleus and stops completely
have to remove non diagnostic lower energy x ray photons from beam

Question 19

characteristic radiation

Answer 19

bombarding electron collides with an inner shell one to displace it (excitation) or completely remove it (ionisation) remaining electrons drop to lower shell to fill space and x ray photons begin to get released as they lose binding energy

Question 20

continuous v characteristic radiation

Answer 20

continuous - produces continuous range of x ray photon energies, max photon energy matches peak voltage, bombarding electron interacts with nucleus of target atom

characteristic - produces specific energies of x ray photon characteristic to element used for the target, photon energies depend on binding energies of electron shells, bombarding electron interacts with inner shell electrons of target atom

Question 21

dental x ray beam spectrum

Answer 21

x ray beam = continuous radiation + characteristic radiation - filtered photons