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Dx Imaging Physics > chapter 11 xray physics > Flashcards

chapter 11 xray physics Flashcards

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1
Q

compton predominates at higher what?

A

kVp settings

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2
Q

Compton is associated with that kind of e-?

A

scatter

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3
Q

what are 3 things that can be used to control scatter?

A

patient positioning, exposure technique, collimation

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4
Q

when there is less tissue to penetrate how does that affect the scatter?

A

it lessens it

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5
Q

how does higher kVp/lower mA technique affect scatter?

A

it will increase scatter

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6
Q

collimation does what?

A

reduces the field size to area of interest, reduces scatter, reduces pt. dose,

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7
Q

as the pt, becomes thicker what controlling mechanism of scatter if important?

A

collimation

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8
Q

why would you want to place pt. in recumbent position instead of a standing?

A

when they are thicker

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9
Q

More than___of the exiting (remnant) beam may be scatter

A

½

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10
Q

purpose of the grid is to do what?

A

reduce scatter “clean up”

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11
Q

what does scatter have an effect on?

A

it will reduce image contrast

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12
Q

what is the trade off when using a grid?

A

the increased exposure to the pt. and the improved dx. quality of image

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13
Q

what % can a grid reduce scatter by?

A

90-80

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14
Q

what is the grid ratio?

A

“height” of Pb strip divided by distance between the Pb strip

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15
Q

what does the height of the Pb strip refer too in recumbent xray technique?

A

the horizontal measurement from front to back

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16
Q

grid frequency =

A

lines/inch

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17
Q

grid selectivity is also known as

A

sigma

18
Q

sigma =

A

1^0 radiation t’mitted/scatter t’mitted

19
Q

are higher frequency grids generally less or more efficient than lower frequency grids of the same ratio?

A

they are less efficient

20
Q

the greater the lead in a grid, what happens to the selectivity?

A

it becomes greater

21
Q

what is the difference of effectiveness when using higher ratio grids vs lower ratio grids?

A

the effectiveness increases with higher ratio

22
Q

As SID decreases what margins of the grid are cut off?

A

lateral

23
Q

what are focused grids used for?

A

to reduce the grid cut off

24
Q

in a focused grid the grid strip are accommodating what?

A

accommodate the increasingly divergent angle of photons further away from central ray.

25
Q

focal range is defined as?

A

the limited range of SID that can be used without grid cutoff

26
Q

grids that are used for general radiography are what kind of grids?

A

focus

27
Q

what is the relationship between grid ratio and focal range?

A

an inverse relationship meaning that as grid ratio increases the focal range will decrease

28
Q

will the SID need to decrease or increase as you increase the grid ration?

A

it will need to decrease

29
Q

what are crossed grids ?

A

sandwhiching two parallel grids together so their grid strips are perpendicular

30
Q

what do cross grids reduce?

A

scatter photons

31
Q

how do cross grids negatively affect the image in an general radiograpy?

A

grid cutoff will occur in the superior to inferior angulation of the xray beam

32
Q

potter bucky diaphragm is known as the

A

moving grid

33
Q

is a moving grid a new technology or old?

A

old

34
Q

what is the purpose of the moving grid?

A

to blur the grid strips on the xray image

35
Q

the disadvantages of the moving grid?

A
  1. freezing of image may happen if exposure time is too short
  2. moving parts may require service
  3. motion of grid might show up on the image
  4. require 15% more exposure
36
Q

what may appear on the image screen if grid is not perpendicular to the central ray?

A

grid lines on the image

37
Q

grid cutoff on image will move towards what?

A

the displaced central ray

38
Q

when SID in not in the focal range where will the grid cutoffs be?

A

at the sides

39
Q

what are some common grid errors?

A
  1. grid not perpendicular
  2. central ray not in the center
  3. SID not in focal range
  4. grid upside down
40
Q

Air gap technique

A

space between the image receptor and object imaged cause scatter photon to diverge and not hit the image receptor.

41
Q

what are the drawbacks of air gap?

A

image magnification/distortion

Higher exposure is also necessary (about 10% increase per cm of air gap)

42
Q

What projection includes a natural “air gap”?

A

Looking top down on the pt. the image receptor and cervical spine will have a air gap –> creating scatter photons that do not end up hitting the image receptor

Dx Imaging Physics (8 decks)

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