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Imaging > Week 6 - Scatter Control And Grids > Flashcards

Week 6 - Scatter Control And Grids Flashcards

1
Q

T/F
Scatter radiation is helpful in some ways, but not dianostically

A

True

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2
Q

What are 3 ways to control scatter?

A

Beam restriction, kvp selection, grids

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3
Q

How does collimation effect scatter?

A

Beam restriction leads to fewer primary photons created

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4
Q

How does kvp selection affect scatter?

A

Increased kvp increases the amount of Compton interactions created within the body

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5
Q

How do grids affect scatter?

A

Decrease scatter that is created after X-rays interact with the patient

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6
Q

What shape is the primary beam without collimation?

A

Circular

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7
Q

What is the most common beam restrictor used?

A

Collimator

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8
Q

What does off focus radiation mean?

A

Photons that are created outside the target (off the focal spot)

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9
Q

What are the top lead shutters of the collimator for?

A

Decreasing off focus radiation

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10
Q

What are the bottom lead shutters of the collimator used for?

A

Decreasing penumbra

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11
Q

Photons that were not produced at the focal spot

A

Off focus radiation

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12
Q

A geometric unsharpness around the periphery of the image, also known as edge unsharpness

A

Penumbra

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13
Q

Result of X-rays photons being created in all areas of the focal spot rather than at just a single point

A

Penumbra

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14
Q

Automatic collimation to size of IR

A

Positive beam limitation (PBL) device

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15
Q

T/F
mAs needs to be increased when collimation is increased

A

True

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16
Q

Increased collimation

A

Reduced field size

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17
Q

Decreased collimation

A

Increased field size

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18
Q

Flat sheet of metal (diaphragm) with opening in the center (aperture)

A

Aperture Diaphragm

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19
Q

What is a disadvantage to an aperture diaphragm?

A

Fixed field size

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20
Q

An aperture diaphragm that has an extended flange attached to it. The flange can vary in length and can be shaped as either a cone or cylinder

A

Beam restrictors: cone and cylinder

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21
Q

What does a higher kvp result in?

A

Decreased attenuation, increased transmission

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22
Q

What does a lower kvp result in?

A

Increased attenuation, decreased transmission

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23
Q

When increasing kvp, what should be accompanied to result in decreased scatter?

A

Decrease in mAs, less photons are needed to create an acceptable image

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24
Q

How do you identify if a low kvp was used?

A

Image will have a high contrast
(Greater variation of xray intensities exiting the patient)
(Less shades of grey)*

25
Q

How do you identify if an image was taken with a high kvp?

A

Lower image contrast
(Less variation in xray intensities exiting the patient)
(More shades of grey)*

26
Q

What are compression devices used for?

A

Applying compression during radiographic procedures (mammography)

27
Q

Xray photon passes through matter without interaction

A

Transmission

28
Q

What creates image fog?

A

Scatter exit radiation

29
Q

What is a drawback of using a grid?

A

Increased mAs = increased patient dose

30
Q

What anatomical part is a grid usually used for?

A

A part greater than 10cm thick

31
Q

What kvp is a grid usually used for?

A

Greater than 70 kvp

32
Q

Expresses the number of lead lines per unit length in inches, cm or both
Can range in value from 25-80 lines/cm

A

Grid frequency
(A typical grid frequency might be 40 lines/cm or 100 lines/in

33
Q

Defined as ratio of height of lead strips to the distance between them

A

Grid ratio

34
Q

h/D

A

Grid ratio
(Range 4:1 - 16:1)

35
Q

What are the two types of grid patterns based on the direction of lead strips?

A

Linear
Crossed grid or cross hatched

36
Q

Strips run only in one direction, more commonly used because it allows for tube angulation

A

Linear

37
Q

Strips run at a right angle to each other, no angulation can be used

A

Crossed grids

38
Q

What are the two types of grid focus based on the orientation of the lead lines

A

Parallel (non-focused)
Focused

39
Q

Lead lines and interspace are parallel to each other, primarily used in fluoroscopy and mobile imaging, best used at long SID

A

Parallel (non-focused)

40
Q

Central grid strips parallel, with increasing angulation as the move away from the centre, xray tube must be located along convergence line to match divergence

A

Focused

41
Q

Recommended range of SIDs that can be used with a focused grid

A

Focal range

42
Q

The range in which you can position the grid without producing grid cutoff

A

Positioning latitude
*less error = more positioning latitude
*higher grid ratio = more lead, less positioning latitude, more error

43
Q

Grid permanently mounted on the front surface of the IR

A

Grid cassette

44
Q

IR slides behind grid

A

Grid cap

45
Q

Matches size of cassette; taped to IR to prevent IR from slipping

A

Wafer or slip on grid

46
Q

Can cause grid lines or blur if moved during exposure

A

Stationary

47
Q

Grid moves slightly back and forth in lateral direction over the IR during entire exposure
*motor driven movement

A

Reciprocating

48
Q

Strips parallel to long axis of grid
*most commonly used

A

Long dimension gird

49
Q

Strips perpendicular to short axis of grid

A

Short dimension grid

50
Q

Electromagnet pulls grid then releases it during exposure

A

Oscillating grid

51
Q

mAs with grid/mAs without grid

A

Grid conversion factor

52
Q

A decrease in the number of transmitted photons reaching the IR due to misalignment of grid with divergence beam

A

Grid cut-off

53
Q

When a focused grid is placed upside down on the IR

A

Upside down focused

54
Q

Xray beam is angled across the lead strips
(Grid error)

A

Off level

55
Q

Central day is not aligned side to side with focused grid; divergence doesn’t match angle of lead strips

A

Off center

56
Q

Occurs when the SID is outside the recommended focal range

A

Off focus

57
Q

Occurs when the laser scanning lines are parallel to the grid lines

A

Moire effect

58
Q

What focal range does a typical grid have?

A

One that includes an SID of 40”(100cm) or 72”(180cm)

59
Q

How much air gap is equivalent to a 8:1 grid?

A

15cm

Imaging (5 decks)

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