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
what distance must focused grids be positioned at
1 or 1.8m
how are crossed grids more beneficial than parallel grids
parallel grids only clean up scatter in one direction, crossed grids are designed to do this in 2
ow is a crossed grid made
sandwiching 2 parallel grids perpendicular to each other
how do moving grids function in oscillating and reciprocal
oscillating - starts before exposure and moves evenly throughout procedure, driven by motor
reciprocal - movement is primed when tube is prepped, then moves on springs
what are the 3 different frequencies used in grids
low, medium, high
how many lines per cm and where is the low, medium and high frequency used?
low: 40-50 L/cm, used in bucky
medium: 50-60 L/cm, used in stationary grid
high: 60-70+ L/cm, used in stationary grid and DR system
what is a downside to using a taller grid despite it being able to absorb more scatter?
a taller grid will also absorb more of the primary beam
what is the focal range of a grid dependant on
geometry of lead strips
as grid ratio goes up, so does the signal comapred with the noise within the image
with an increased number of lines, the density of the strips increase and any primary photon that hits a stip will be removed, how will we compensate?
adding grid and adjusting exposure (increase)
define grid factor
amount by which we must increase the exposure to maintain the same dose to the image receptor
how is a virtual grid different from a physical on
These algorithms are capable of removing scatter radiation and improving image contrast for a wide variety of body thickness without having to adjust anything yourself/increase exposure
the use of a virtual grid allows the operator to reduce their exposure from the already optimal setting
what may happen if the get the distance wrong with out grids?
for either far or near focus-grids it will decenter the image
what is the upside down grid error
grid is used upside down
severe peripheral grid cutoff occurs
radiation passes through grid along central acid where the grid stops are most perpendicular
