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202 MEDIMAGE > 11. Scatter Control and Radiographic Grids > Flashcards

11. Scatter Control and Radiographic Grids Flashcards

1
Q

what 2 things are photoelectric radiation interaction responsible for

at what energies is it more likely to occur at

A

absorption of photons and tissue contrast

lower energies

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2
Q

contrast is improved (high) with ___ kV

A

decreased

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3
Q

what are compton radiation interaction responsible for

at what energies is it more likely to occur at

A

scattering of photons

higher energies

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4
Q

scatter is more likely to degrade image quality at ___ kV

A

increased kV

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5
Q

large IR has what effect on the scattered radiation detected

A

more scattered radiation detected

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6
Q

ideally what do beam restrictors do

A

decrease the field size to anatomy of interest

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7
Q

what 4 things happen when FS is decreased

A

decreases unnecessary tissue exposure

scatter decreases

scale of contrast shortens

visibility of detail increases

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8
Q

are 3 types of beam restrictors

A

aperture diaphragms/cones/cylinders

collimators

ancillary devices

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9
Q

what do aperture diaphragms cones used for

A

reduces unsharpness as compared to aperture diaphragm

gives specific shape of beam

closer collimation for specific shape

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10
Q

what do aperture diaphragms cones introduce and what shape does it have

A

introduce area of unsharpness = penumbra

almost divergent shape

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11
Q

what do aperture diaphragms cylinders used for

what is its effectiveness at reduction compared to the cones

A

further reduction in unsharpness as compared to cone

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12
Q

what 2 components do collimators have

A

light localizing field light

2 sets of shutters that permits infinite number of field size

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13
Q

what do the 2 sets of shutters do in a collimator

what does it permit

A

length and width of field independently controlled

permits rectilinear collimation

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14
Q

which shutters of the collimator reduces the penumbra

why

A

bottom shutters

geometric unsharpness around periphery of image- edge unsharpness

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15
Q

what does collimators do to the image edge

A

improves sharpness of recorded image edge

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16
Q

what does off focus radiation occur from

A

areas of the x-ray tube other than focal spot areas

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17
Q

upper shutters of the collimator reduces what in terms of radiation reaching the IR

A

reduces off focus radiation reaching IR

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18
Q

off focus radiation produces images where

A

beyond exposed field of radiation

image shadows

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19
Q

beam restriction does what

A

limit the x-ray beam field size

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20
Q

what 2 things does limiting the x-ray beam field size do

A

limit patient dose

reducing the amount of scatter

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21
Q

beam restriction needs to consider what

A

the full circular x-ray field

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22
Q

in terms of an aperture diaphragm there is significant unsharpness due to what

A

owing to proximity to x-ray source

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23
Q

what happens inside a collimator

A

light comes from lightbulb and mirror angle directs where x-ray beam is going to be

24
Q

why is there poor quality with wide collimation

A

lots of scatter

25
Q

the light reflected off the mirror is used to do what

A

project the coverage of x-ray beam

26
Q

the projection of the x-ray beam should be accurate to within +/- ___ % of the ___

A

+/-2% of the SID

27
Q

mirror adds what kind of filtration to the system

A

inherent filtration

28
Q

what are the 3 features of collimator housing

A

central ray must be marked

alignment light helps center beam with IR

some units project the AEC sensor locations in the light field

29
Q

what are Positive beam limitation devices/what do they do

A

automatically collimates beam to size of IR

30
Q

what are 2 types of ancillary devices

A

lead blockers/shields

lead masks attached to the collimator

31
Q

what do lead blockers do

A

absorb scatter

32
Q

what do grids do

A

absorb and limit the amount of scattered radiation that reaches the IR

improves the quality of radiograph

33
Q

less scatter on the image has what effect on contrast

A

increases contrast

34
Q

what do grids do in terms of the transmitted radiation

A

absorbs the transmitted radiation exiting the patient and therefore reduces the radiation reaching the IR

35
Q

what is the grid conversion formula

A

mAs1/mAs2 = grid ratio1/grid ratio2

36
Q

what are radiographic strips

A

device that has very thin lead strips with radiolucent interspaces, intended to absorb scatter radiation emitted from the patient

37
Q

what is the grid frequency

A

measure of the number of grid lines per unit distance

38
Q

what is the grid ratio equation in terms of h and D

A

ratio = h/D

h = height of lead strip
D = interspace distance
39
Q

where are grids placed

A

between the patient and the IR

40
Q

what is the grid frequency equation

A

freq = 1/(t + D)

t = thickness of lead strip

41
Q

increased grid ratio has what effect on scatter

A

more scatter removed = improved contrast

42
Q

what are the 2 grid types

A

linear and cross-hatched

43
Q

the focused grid does what to the x-ray beam and photons

A

matches the divergence of the x-ray beam and allwos more photons to reach the IR

44
Q

what happens if the grid isnt focused

A

if the grid isn’t focused, may eliminate some of the radiation you were going to use

45
Q

the convergent line or point of a focused grid falls within ___ ____

A

a focal range

46
Q

what is the grid conversion factor

A

measure of the required mAs adjustment to keep exposure constant with and without the grid

47
Q

what is the GCF equation

A

GCF = mAs with grid/mAs without grid

48
Q

increase in GCF has what effect on the required exposure

A

required exposure to the IR increases

49
Q

what is the limitation of stationary grids

A

stationary grids produce visible grid lines on the radiography

50
Q

what are reciprocating grids

A

slightly moving the grid during the x-ray exposure will blur the grid lines which will therefore be less visible

51
Q

reciprocating grid motion is controlled how

A

electronically

52
Q

what is the grid cutoff

A

decrease in the number of transmitted photons that reach the IR because of grid misalignment

53
Q

what are the 4 causes of grid unalignment

A

upside down focused grid

off level grid

off centre grid

off focus grid

54
Q

what is the moire effect

A

artifact demonstrated when a stationary grid is used during CR imaging and the grid frequency is similar to the laser scanning frequency

55
Q

higher ratio grids will increase what 3 things

A

scatter absorption

patient exposure

potential for grid cuttoff

202 MEDIMAGE (18 decks)

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