X-Ray Production Flashcards
where are the x-rays produced in a dental x-ray unit
tubehead
what is the function of the collimator
helps to direct the x-ray beam
what are the components of the x-ray tube
glass envelop
cathode (-ve cahrged)
anode (+ve charge)
what is the filament
coiled metal wire that has high current electricity passed through it and heats up and electrons are released from atoms in the wire by thermionic emission so a cloud of electrons forms around the cathode
what is the cathode filament made of
tungsten - has a high melting point, atomic number and is malleable
what is the cathode focusing cup
metal cup shaped around filament which is negatively charged and repels electrons released from the filament
what is the focusing cup made of
molybdenum
what occurs between the cathode and anode
the filament sits nearer the cathode and when electrons are produced they are repelled away from the negative cathode and are pushed very quickly to the positive anode
what is the measurement used when measuring electrons
electron volts (eV)
what is the most important part of the anode
the target
what is the function of the anode target
produces photons when bombarded with electrons
what is the important point on the anode target
the focal spot - which is a precise area on the target where electrons collide and x-rays are produced
what is situated around the target
heat dissipating block
what is the function of the heat dissipating block
when electrons collide with the target lots of heat is given off - it takes it away from the target to prevent damage
what is the penumbra effect
blurring of the radiographic image due to focal spot not being a single point
how is the penumbra effect minimised
shrinking the size of the focal spot
how can you shrink the size of the focal spot
using focal spot angulation
what is a problem with focal spot angulation
lots of heat concentrated on small points - can overheat target and it will break
what is the solution to the heat problem of focal spot angulation
use angled target - better heat tolerance as the electrons are spread over a larger area
what holds the cathode and anode in place
glass envelop - helps to keep air out of the cathode and anode vacuum
what is incorporated in the glass envelop
lead all over apart from where you want the photons to go
what is filtration
removes lower energy (non-diagnostic) x-rays from the beam
what is used for filtration
aluminium of adequate thickness
what is the spacer cone
dictates the distance between focal spot of target and patient
what size should the spacer cone be if the energy is less than 60kV
100mm
what size should the spacer cone be if the energy is more than 60kV
200mm
what are the three fates of x-ray photons that are emitted from the focal spot
stopped by the attenuation of lead shielding (in the glass envelop or from the tubehead itself)
stopped by the aluminium infiltration
exit tubehead to form x-ray beam
what is the collimator function and structure
lead diaphragm attached to spacer cone which reduces patient dose by cropping x-ray beam to size and shape of x-ray receptor
what size should the collimator be
50 mm x 40 mm
what parts are incorporated in the control panel of an x-ray unit
on/ off switch
electronic timer
exposure time selector and presets
warning light and noise
kV selector
what are the two consequences of electrons bombarding the target
heat production
x-ray production
when do heat producing interactions occur
when bombarding electrons reach outer shell electrons and loses kinetic energy which is turned into heat energy
what are the two x-ray producing interactions
continuous radiation interactions (majority)
characteristic radiation interactions
what is continuous radiation
the bombarding electron passes close to the nucleus but avoids the electrons the nucleus slows down the electron and changes its direction - this lost kinetic energy is released as x-ray photons
what is characteristic radiation
electron comes in and reacts with the inner shell electrons in tungsten atom and displaces it to a more peripheral shell or removes it completely
the remaining electrons reorganise themselves to fill innermost shells and this causes loss of energy which is emitted as photons specific energy
what is photon energy equal to in characteristic radiation
the difference in the binding energies of the 2 shells involved
