X-Ray Production Flashcards
what are the main components of the dental x-ray unit
tubehead, collimator, positioning arm, control panel, circuitry
what are the components of the x-ray tube
glass envelope, cathode, anode
what is the cathode filament
coiled metal wire
what gets passed through the cathode filament
low voltage, high current electricity
how are electrons released from the cathode wire
by thermionic emission
what happens if you increase the current in the cathode filament
increased heat and increased electrons
what is the cathode filament made of
tungsten
what are the properties of tungsten
high melting point, high atomic number (lots of electrons), malleable
what is the cathode focusing cup
metal plate shaped around filament
what is the cathode focusing cup made of
molybdenum
what are the properties of molybdenum
high melting point, relatively poor thermionic emitter
what is the relationship between the cathode and anode
electrons released at filament are repelled away from the cathode and attracted to the anode
how is kinetic energy generated between cathode and anode
when the electrons collide with the anode target they have a high kinetic energy
what do transformers do
take mains electrical supply and convert it by changing the voltage and current
what is the mains electrical supply
220-240v
what are the 2 transformers present in the tubehead
step up and step down
what does the step up transformer do
increase potential difference across x-ray tube to 60,000-70,000V
what does the step down transformer do
decrease the potential difference across filament to about 10V
what do electron volts measure
the kinetic energy gained by electrons as they accelerate from cathode to anode
what is an anode target
metal block bombarded by electrons
what is the anode target made of
tungsten
what is the focal spot on the anode target
precise area on target where electrons collide and x-rays are produced
what is the purpose of the heat dissipating block on the anode
heat produced in target dissipates into the block by thermal conduction to reduce risk of overheating
what is the heat dissipating block made of
copper
what are the properties of copper
high melting point and high thermal conductivity
what is the penumbra effect
blurring of a radiographic image due to focal spot not being a single point
how do you minimise the risk of the penumbra effect
shrinking size of focal spot
what is the solution to the need to have a small focal spot
have an angled target
what is the effect of having an angled target
increases the actual surface area where electrons impact and reduces the apparent surface area from where the x-ray beam is emitted
what does the glass envelope do
supports cathode and anode, maintains a vacuum
what is the purpose of the glass envelope to be leaded
to absorb x-ray photons
what are the main components of the tubehad
x-ray tube, metal shielding, aluminium filtration, oil, spacer cone
what is the purpose of the oil in the tubhead
dissipates heat produced by x-ray tube by thermal conduction
what does the filtration in the tubehead do
removes lower energy x-rays from beam to decrease patient dose
why is the filtration made of aluminium
it can absorb those photons so that the resulting x-ray beam contains mostly diagnostic x-ray photons
what is the minimum thickness of aluminium required for a filtration
for less than 70kV = 1.5mm
for greater than 70kV = 2.5mm
what does the spacer cone do
dictates distance between focal spot and patient and indicates direction of the beam
what is the distance between the focal spot and patient called
focus to skin distance
what will altering the focus to skin distance do
affect degree of divergence of x-ray photons in x-ray beam
what does increasing the focus to skin distance do
reduced divergence of x-ray beam and therefore reduces magnification of image
what does a set focus to skin distance allow
consistent radiographic technique
what is the focus to skin distance required for modern equipment (>60kv)
200mm
where is the focus to skin distance measured from
the focal spot which is marked on tubehead
what are the 3 possibilities for the fate of x-ray photons emitted from the focal spot
attenuated by lead shielding
attenuated by aluminium filtration
exit tubehead to form x-ray beam
what is the collimator
lead diaphragm attached to end of spacer cone
what is the purpose of the collimator
reduces patient dose and crops x-ray beam to match size and shape of x-ray receptor
when using size 2 receptors what should rectangular collimators reduce the beam area at the patient end of spacer cone to
50mm x 40mm
what can rectangular collimation reduce the surface area irradiated to
by almost 50%
what is the disadvantage to using collimation
can increase risk of collimation errors
what are the consequences of electrons bombarding the target
heat production and x-ray production
what kind of electrons are involved in heat production at the target
outer shell electrons of tungsten atoms
what kind of electrons are involved in x-ray production at the target
inner shell electrons and nuclei of tungsten atoms
how common is it for x-rays to be produced at the target
<1%
what happens in a heat producing interaction
bombarding electron reaches tungsten outer shell electron which either collides or deflects
bombarding electron loses kinetic energy which is converted to heat
heat energy dissipated
what are the 2 x-ray producing interactions
continuous radiation interactions
characteristic radiation interactions
what is the continuous radiation interaction
bombarding electron passes close to target nucleus causing it to be rapidly decelerated and deflected meaning lost kinetic energy released as x-ray photons
when is the maximum energy achieved in the continuous radiation spectrum
when the electron collides directly with the nucleus and stops completely
what does continuous radiation spectrum mean
photons produced over wide range of energies
what does filtration of the continuous spectrum do
remove lower energy non-diagnostic x-ray photons from beam
what is the characteristic radiation interaction
bombarding electron collides with an inner shell electron and either displaces it into a more peripheral shell or removes it completely
remaining orbiting electrons rearrange themselves to re-fill the innermost shells
what happens when electron drops to a lower shell
loses energy which is emitted as a photon of specific energy
what is photon energy
difference in the binding energies of the 2 shells involved
what is the binding energy of a K shell
69.5keV
why do dental x-ray tubes operate at 70kV
so that bombarding electrons have sufficient energy to displace K shell electrons
what is the dental x-ray beam spectrum
continuous radiation + characteristic radiation - filtered photons
