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1126 Final Review > Grids > Flashcards

Grids Flashcards

1
Q

What is a grid constructed of?

A

Lead lines with an interspace material like plastic or Aluminum

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2
Q

Where are grids placed when in use?

A

Between the patient and the image receptor

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3
Q

What is the purpose of a grid?

A

To intercept some of the scatter in the remnant beam that reaches the IR

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4
Q

When the grid is placed below the patient will the penetration of the beam be affected?

A

No because the beam has already passed through the patient and penetrated

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5
Q

How thick are the strips within a grid?

A

0.005 inches (0.127 mm)

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6
Q

What do the grid lines absorb?

A

Unfocused random scatter that is produced by the patient

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7
Q

Who and when were grids invented?

A

They were invented in 1913 by Gustav Bucky

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8
Q

What are some issues produced by the original grids invented by Bucky?

A

They contained horizontal and vertical strips which created white line artifacts on the images (grid lines)

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9
Q

What is a reciprocating grid?

A

An updated grid created by Dr. Hollis Potter in 1920 that involved a moving grid which blurred out the grid lines on images

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10
Q

How much scatter is attenuated by modern grids?

A

70-80%

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11
Q

What affect do grids have on sharpness/details and magnification?

A

They have no affect

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12
Q

What is the grid ratio formula?

A

Grid Ratio = H/D

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13
Q

How is the grid ratio defined?

A

The relationship of the height (H) of the lead strips to the distance (D) between them

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14
Q

Which grid ratios provide higher contrast?

A

High grid ratio

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15
Q

What is the grid ratio for a grid that is 3 mm thick and has spaces 0.5 mm in width?

A

Grid Ration = 3/0.5 = 6 = 6:1

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16
Q

What happens to the beam that makes it through the strips?

A

They strike the IR and generate the image

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17
Q

What are the most common grid ratios?

A

6:1, 8:1 and 10:1

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18
Q

As the grid ratio goes up, what happens to the spaces between the lines on the grid?

A

The space between the lines get smaller, which allows more lines on a grid but less x-rays to get through

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19
Q

Why are grid ratios over 10:1 harder to use?

A

Because the space between the strips becomes smaller, which require more accurate tube to IR alignment and which can cause more repeats

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20
Q

Which grid ratios have a higher margin of error?

A

Higher grid ratios, mostly ones above 10:1

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21
Q

What is grid frequency?

A

The number of lead strips per inch when scanning across the grid (how frequently the grids appear)

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22
Q

What grid frequency do higher grid ratios have?

A

Higher frequency because the lines are closer together and you can pack more in

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23
Q

What should not be assumed when it comes to grid frequency?

A

That the thickness of the lead strips is the same

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24
Q

What is a critical factor in grid efficiency?

A

Grid ratio

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25
Q

What is the impact of having more lines on a grid?

A

Having more lines reduces the angle that scattered x-rays can pass through the IR, which improves contrast. However, if too many x-rays are blocked than its just as useless

26
Q

What impact do grids have on subject contrast?

A

Grids improve subject contrast in the resulting image because of the removal of scatter

27
Q

What impact do grids have on patient dose?

A

Negative impact in that dose needs to be increased to compensate for the grids, which increases patient dose

28
Q

What is the biggest indicator for use of a grid?

A

Part thickness - general rules is that parts greater than 10 cm require a grid

29
Q

As the part thickness increases, what grid ratio should be used?

A

A higher grid ratio

30
Q

What grid ratio should be used for a 10 cm thick part?

A

8:1 grid ratio

31
Q

What body parts have an exception to the part thickness/grid ratio relationship?

A

Chest, since the part is thick but the density of the tissue within is low

32
Q

What are the 3 causes of scatter and main reasons why grids need to be used?

A
  1. High levels of kVp
  2. Large field sizes
  3. Large body part thickness
33
Q

What interaction is the result of most scatter produced by the body?

A

Compton interaction

34
Q

What impact does kVp have on the production of scatter?

A

Minimal impact, since higher kVp is needed to adequate penetration and the amount of scatter produced as a result of increased kVp only is small

35
Q

What impact does field size have on the production of scatter?

A

An open field size exposes more tissue and objects to the x-ray beam, which causes more scatter to be produced

36
Q

What is the primary mechanism of controlling scatter when it comes to field size?

A

Collimation

37
Q

What impact does large part thickness have on the production of scatter?

A

Major impact, as the thicker the part the less absorption will occur and more scatter will be produced

38
Q

What are the 3 questions that should be asked prior to using a grid?

A
  1. Does the body part consist of more than 13 cm thickness of soft tissue? If so, use a grid
  2. Will a large field size be used? If so, a grid may need to be applied, even if the part is not 13 cm thick
  3. Will extremely high kVp be used (100 kVp+)? If so, a grid may need to be applied, even if the part is not 13 cm thick
39
Q

What is the Bucky factor?

A

A way to determine how much mAs is needed when using a grid vs not using a grid.

40
Q

What is grid selectivity?

A

Another way to evaluate the efficiency of a grid

41
Q

How is grid selectivity defined?

A

The ratio of primary radiation transmitted through the grid to scattered radiation transmitted through the grid

42
Q

What is needed to measure both Bucky factor and grid selectivity?

A

An ion chamber

43
Q

What is the most direct and scientific way to measure the effectiveness of a grid?

A

Contrast improvement factor (CIF)

44
Q

What does CIF measure?

A

Measures the contrast of an image with a grid and the contrast of an image without a grid

45
Q

How are CIFs measured?

A

Using a densitometer

46
Q

How does technique need to be adjusted when grids are in use?

A

Increase the exposure by 4x

47
Q

When moving from a Bucky to a tabletop how should the technique be adjusted?

A

Decrease the exposure by 1/4

48
Q

What happens if the exposure is not increased when using a grid?

A

The image would be light or grainy (photon starvation)

49
Q

What is the most common type of grid?

A

A focused grid in which the angle of the grid lines match the angle of divergence of the x-ray beam

50
Q

What focal ranges do focused grid types come in?

A

36-42 inches and 66-74 inches, which coincide with the two most common SIDs

51
Q

What will happen if focused grid types are used outside of the standard SID ranges?

A

The image cleanup will not be as good because the beam/grid angles will no longer align

52
Q

How are the lead strips positioned in a focused grid type?

A

The are tilted more and more as they move further from the CR

53
Q

What is the tilt of the lead strips called?

A

Canting

54
Q

What is another common type of grid used?

A

Parallel grid in which the lines are all parallel to each other

55
Q

What produces the most grid cutoff in images?

A

Incorrect SID and upside down placement of the grid

56
Q

What is a virtual grid?

A

An algorithm based grid that’s build into the software what has the same result of a physical grid

57
Q

What is the most important tool available to prevent the production of scatter radiation?

A

Collimation

58
Q

What are two advantages of using high kVp?

A

High kVp assures adequate penetration which produces adequate gray scale and High kVp allows for lower MAs which reduces patient dose

59
Q

As scatter increase, what is the affect on penumbra?

A

Has no affect

60
Q

If all tissues of interest are not penetrated adequately what gray scale will be reflected in the image?

A

Short gray scale

1126 Final Review (13 decks)

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