What is the ALARP principle?
as low as reasonably practible
What does the ALARP principle include?
choice of technique to optimise dose
selection criteria to justify exposure
production of optimal quality images
What type of energy photon will pass through patient?
high energy photon
these don’t cause damage and are more useful
What type of energy photon builds contrast in image?
lower energy
these do cause damage to pt and don’t reach image receptor to create image
What does constant potential ensure?
keeps voltage steady
produces more useful high energy x-rays
reduces harmful low energy x-rays
What type are majority of modern x-ray sets?
DC
direct current
What type of x-ray sets are the older generation?
alternating current - AC
x-ray beam has more low energy photons and gives higher dose to pt
How is DC different?
eliminates filament warm up time as it’s already warm , this reduces exposure time
more high energy photons which reach detector and help image
more useful for digital equipment
What kV should new equipment operate at?
60-70 kV
What does lower kV mean?
lower energy photons are absorbed in pt through photoelectric effect
more photons absorbed
cellular damage
higher risk of cancer
What happens when the kV increases?
lower pt dose
more high energy x-ray photons pass through pt and reach receptor
lower contrast within the image
Why is 60-70kV recommended?
if kV is too low then there is a high effective dose
as kV increases, effective dose tails off
when too high , this does not result in dose reduction to pt
What is filtration?
removes low energy photons which contribute most to the dose
Why is a rectangular collimator used?
limits the beam size to be same as image receptor that we use
reduce dose by 50% compared to circular collimation as there is less overlapping and lower pt area being exposed
Where is the focal spot?
where x-ray beam is generated
within the x-ray machine , 20cms away to maintain skin distance
most equipment has little dot on outside to show where focal spot is
What is the inverse square law?
beam intensity reduces by the increase in distance squared
eg if distance doubles , intensity reduces by 2 squared (drops to 1/4)
What is the difference between a phosphor plate and a solid state (CCD/CMOS)?
phosphor plate has to be put through a reader for image
Solid state can have immediate image on screen
Which gland can unavoidably be in primary beam of exposure?
thyroid beam
Discuss with MPE about cephalometric, CBCT/panoramics for different options
Why is a protective lead apron never worn for patients?
primary beam should never be directed towards pt
What are some technique errors?
bent plate
cone cutting
incorrect positioning
From what 3 sources does staff exposure come from?
primary beam
radiation leakage from tube head
scatter from patient
What should we never hold when exposing?
image receptor in pt mouth’s during exposure
x-ray tubehead
