Secondary radiation grids Flashcards

1
Q

What 2 types of incident radiation are part of secondary radiation

A

photo-electric effect
Compton scatter

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2
Q

when is the photoelectric effect at a maximum

A

when energy of incident photon is equal to or just greater than binding energy of electron in k shell

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3
Q

what isa the removed electron from photoelectric effect called

A

photoelectron

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4
Q

why does the photoelectric effect contribute to attenuation of x-ray beam

A

it passes through matter

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5
Q

describe Compton scatter

A

photon scatters off bound electron
results in change in direction
results in change in photon energy

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6
Q

what is the probability of the Compton effect dependant on

A

number of electrons per gram in absorbing material

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7
Q

describe Rayleigh/coherent scatter

A

photon scatters off bound electron
results in change of direction
no change in photon energy

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8
Q

why is there no energy change in coherent scatter

A

incident photon doesnt have enough energy to liberate electron from its boundstate

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9
Q

What is noise made of?

A

scatter

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10
Q

How does kVp and scatter link?

A

the more kVp, more scatter heads towards the image

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11
Q

when is a secondary radiation grid needed?

A

patient thickness is sufficient that body produces significant scattered radiation (due to increase in kVp and so more scatter heads towards image)

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12
Q

what happens when scatter reaches an image

A

reduces contrast
increases noise

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13
Q

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

A

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

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14
Q

explain what an anti-scatter grid is

A

thin lead strips with radiolucent interspaces, ‘absorb’ scatter so they dont show up on image

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15
Q

what are the 3 types of anti-scatter grids

A

stationary
moving/oscillating/reciprocating
virtual

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16
Q

where can you find a stationary grid

A

built into a cassette

additional component that clips onto cassette

17
Q

where can you find moving grid

A

table or wall bucky

18
Q

where can you find virtual grid

A

algorithm for DR system

19
Q

describe the grid structure and how does it work

A

fine tips of lead with radio-lucent spacers

lead absorbs low-energy photons arriving at an angle (scatter)

spaces allow primary beam through

20
Q

what are the 3 shape of grids

A

parallel, focused, crossed

21
Q

where is a parallel gird mainly used

22
Q

why might a parallel grid need to be oscillating or reciprocating?

A

to blur out grid lines

23
Q

what is a negative to using parallel and focused grids

A

requires larger source to image distance or indicated SID to avoid grid cut off at the edges if not moving

24
Q

in what type of grid is focussed grids used?

A

stationary/fixed

25
what distance must focused grids be positioned at
1 or 1.8m
26
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
27
ow is a crossed grid made
sandwiching 2 parallel grids perpendicular to each other
28
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
29
what are the 3 different frequencies used in grids
low, medium, high
30
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
31
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
32
what is the focal range of a grid dependant on
geometry of lead strips
33
as grid ratio goes up, so does the signal comapred with the noise within the image
34
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)
35
define grid factor
amount by which we must increase the exposure to maintain the same dose to the image receptor
36
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
37
what may happen if the get the distance wrong with out grids?
for either far or near focus-grids it will decenter the image
38
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
39