Secondary radiation grids Flashcards
What 2 types of incident radiation are part of secondary radiation
photo-electric effect
Compton scatter
when is the photoelectric effect at a maximum
when energy of incident photon is equal to or just greater than binding energy of electron in k shell
what isa the removed electron from photoelectric effect called
photoelectron
why does the photoelectric effect contribute to attenuation of x-ray beam
it passes through matter
describe Compton scatter
photon scatters off bound electron
results in change in direction
results in change in photon energy
what is the probability of the Compton effect dependant on
number of electrons per gram in absorbing material
describe Rayleigh/coherent scatter
photon scatters off bound electron
results in change of direction
no change in photon energy
why is there no energy change in coherent scatter
incident photon doesnt have enough energy to liberate electron from its boundstate
What is noise made of?
scatter
How does kVp and scatter link?
the more kVp, more scatter heads towards the image
when is a secondary radiation grid needed?
patient thickness is sufficient that body produces significant scattered radiation (due to increase in kVp and so more scatter heads towards image)
what happens when scatter reaches an image
reduces contrast
increases noise
what is the relation between reduction in beam intensity by absorption and atomic number of attenuating medium and energy of incident photon and physical density of medium
reduction in beam intensity by absorption is proportional to CUBE of atomic number of attenuating medium
reduction in beam intensity by absorption is INVERSELY proportional to the CUBE of energy of incident photon
reduction in beam intensity by absorption is PROPORTIONAL to physical density of medium
explain what an anti-scatter grid is
thin lead strips with radiolucent interspaces, ‘absorb’ scatter so they dont show up on image
what are the 3 types of anti-scatter grids
stationary
moving/oscillating/reciprocating
virtual
where can you find a stationary grid
built into a cassette
additional component that clips onto cassette
where can you find moving grid
table or wall bucky
where can you find virtual grid
algorithm for DR system
describe the grid structure and how does it work
fine tips of lead with radio-lucent spacers
lead absorbs low-energy photons arriving at an angle (scatter)
spaces allow primary beam through
what are the 3 shape of grids
parallel, focused, crossed
where is a parallel gird mainly used
bucky
why might a parallel grid need to be oscillating or reciprocating?
to blur out grid lines
what is a negative to using parallel and focused grids
requires larger source to image distance or indicated SID to avoid grid cut off at the edges if not moving
in what type of grid is focussed grids used?
stationary/fixed