Secondary radiation grids

Question 1

What 2 types of incident radiation are part of secondary radiation

Answer 1

photo-electric effect
Compton scatter

Question 2

when is the photoelectric effect at a maximum

Answer 2

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

Question 3

what isa the removed electron from photoelectric effect called

Answer 3

photoelectron

Question 4

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

Answer 4

it passes through matter

Question 5

describe Compton scatter

Answer 5

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

Question 6

what is the probability of the Compton effect dependant on

Answer 6

number of electrons per gram in absorbing material

Question 7

describe Rayleigh/coherent scatter

Answer 7

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

Question 8

why is there no energy change in coherent scatter

Answer 8

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

Question 9

What is noise made of?

Answer 9

scatter

Question 10

How does kVp and scatter link?

Answer 10

the more kVp, more scatter heads towards the image

Question 11

when is a secondary radiation grid needed?

Answer 11

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

Question 12

what happens when scatter reaches an image

Answer 12

reduces contrast
increases noise

Question 13

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

Answer 13

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

Question 14

explain what an anti-scatter grid is

Answer 14

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

Question 15

what are the 3 types of anti-scatter grids

Answer 15

stationary
moving/oscillating/reciprocating
virtual

Question 16

where can you find a stationary grid

Answer 16

built into a cassette

additional component that clips onto cassette

Question 17

where can you find moving grid

Answer 17

table or wall bucky

Question 18

where can you find virtual grid

Answer 18

algorithm for DR system

Question 19

describe the grid structure and how does it work

Answer 19

fine tips of lead with radio-lucent spacers

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

spaces allow primary beam through

Question 20

what are the 3 shape of grids

Answer 20

parallel, focused, crossed

Question 21

where is a parallel gird mainly used

Answer 21

bucky

Question 22

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

Answer 22

to blur out grid lines

Question 23

what is a negative to using parallel and focused grids

Answer 23

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

Question 24

in what type of grid is focussed grids used?

Answer 24

stationary/fixed

Question 25

what distance must focused grids be positioned at

Answer 25

1 or 1.8m

Question 26

how are crossed grids more beneficial than parallel grids

Answer 26

parallel grids only clean up scatter in one direction, crossed grids are designed to do this in 2

Question 27

ow is a crossed grid made

Answer 27

sandwiching 2 parallel grids perpendicular to each other

Question 28

how do moving grids function in oscillating and reciprocal

Answer 28

oscillating - starts before exposure and moves evenly throughout procedure, driven by motor

reciprocal - movement is primed when tube is prepped, then moves on springs

Question 29

what are the 3 different frequencies used in grids

Answer 29

low, medium, high

Question 30

how many lines per cm and where is the low, medium and high frequency used?

Answer 30

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

Question 31

what is a downside to using a taller grid despite it being able to absorb more scatter?

Answer 31

a taller grid will also absorb more of the primary beam

Question 32

what is the focal range of a grid dependant on

Answer 32

geometry of lead strips

Question 33

as grid ratio goes up, so does the signal comapred with the noise within the image

Question 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?

Answer 34

adding grid and adjusting exposure (increase)

Question 35

define grid factor

Answer 35

amount by which we must increase the exposure to maintain the same dose to the image receptor

Question 36

how is a virtual grid different from a physical on

Answer 36

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

Question 37

what may happen if the get the distance wrong with out grids?

Answer 37

for either far or near focus-grids it will decenter the image

Question 38

what is the upside down grid error

Answer 38

grid is used upside down

severe peripheral grid cutoff occurs

radiation passes through grid along central acid where the grid stops are most perpendicular