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Radiology Vcast05 > Ch. 18 Image Production > Flashcards

Ch. 18 Image Production Flashcards

1
Q

What is a grid and how does it work?

A

It is a device that improves the contrast of the Rad image by absorbing scatter radiation before it reaches the IR.

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2
Q

What are the three things that happen in the body when exposed to radiation?

A

Passes through the body unaffected
Absorbed by the body
Interact and change direction

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3
Q

Absorption of the photons occur because of what type of interaction?

A

Photoelectric interaction

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4
Q

What causes scatter in the interaction with tissue?

A

Compton interaction

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5
Q

How do you define a photon as scatter?

A

When it interacts with the outer shell electron and as a result the scattered electron changes direction which can no longer be used to records exposure

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6
Q

What is the relationship between kVp and Compton scatter ?

A

Directly related

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7
Q

As kVp increases what happens to contrast (In film)?

A

results in low contrast (better contrast)

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8
Q

What increases scatter in a patient

A

Increase patient thickness
Larger field size
Decrease in atomic number of tissue(b/c low z# = less absorption

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9
Q

When should we use a grid?

A

When a body parts thickness exceeds 10cm

When kVp is above 60

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10
Q

What does grid consist of ?

A

Thin flat rectangular device that has a series of radiopaque (absorbed) lead strips side by side seaperating the strips by an interspace material that is radiolucent (passes through)

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11
Q

What is the material if the radiolucent interspaces?

A

Aluminum and plastic fiber

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12
Q

What is grid ratio defined as

A

The ratio of the height of the lead strips to the distance between the strips

Grid ratio formula= h(high of the lead strips)/D(interspace width)

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13
Q

If the height of the grid is constant and we were to decrease the distance between the lead strips (h) what would result?

A

Increase grid ratio

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14
Q

What is the relationship between distance of lead strips (D) and grid ratio if highs remains the same?

A

Inversely related

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15
Q

Higher grid ratios allow what ?

A

Less scatter passing to the IR through the interspace

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16
Q

Higher grid ratios are_____ but are prone to_____

A

Better, grid error

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17
Q

How are grid ratios rated?

A

Are sometimes rated by weight instrear or grid ratio

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18
Q

What is the term for expressing a grids weight?

A

Grams per square centimeter (g/cm^2) or in the US (g/in^2)

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19
Q

What is grid frequency?

A

The # of grid lines per inch or centimeter

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20
Q

What is the range of grid frequency in centimeters and inches ?

A

60-200 lines/inch; 25-80 lines/cm

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21
Q

What is the most common grid frequency?

A

85-103 lines/inch; 33-41 lines/cm

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22
Q

What aid the grid frequency of a very-high-frequency?

A

178 to 200 lines/inch; 70-80 lines/centimeter

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23
Q

What is a linear grid?

A

Grids with lead strips running in one direction

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24
Q

Two linear grids that are place one on top of the other so that they run at right angles

A

Cross-cross or cross hatched

25
Q

When are linear grids commonly used

A

In clinical practice

26
Q

Grid cutoff

A

When the primary beam is angled into the lead, which will lead to an unwanted about of primarily beam to be absorbed

27
Q

Parallel grids are made with?

A

Lead and interspaces running parallel to one another

28
Q

Focused grid

A

Made so that the center grid strips are parallel but as they move away from the center they become inclined

29
Q

Convergence line is?

A

Where the focused grid’s inclined lines meet in the middle

30
Q

Grid radius is what

A

Is the distance from the inclined focused grid lines and the convergence line

31
Q

Short-focal-range grids; which modality are they in?

A

14-18 inches or 36-46cm

Used in mammography (SID)

32
Q

Long-focal-range grids; which modality uses them?

A

60-72 inches; 152-183cm

Used for chest x- rays (SID)

33
Q

What is the usual size for a grid placed it the Bucky ?

A

17x19 in

34
Q

Reciprocating movement of a grid

A

A motor drives the grid back and forth at about 2-3cm

35
Q

How does the oscillating grid move

A

An electromagnet pulls the grid to one side and then releases it during exposure

36
Q

How do we calculate the grid conversion factor?

A

GCF= mAs with grid/mAs without grid

37
Q

What is he relationship between GCF and kVp?

A

no relation

38
Q

What is the relationship between mAs and GCF ?

A

Directly related

39
Q

What is the ICRU

A

The internal commission on radiologic units and measurements

40
Q

What criteria is measured to meant the ICRU guide lines for a grids performance?

A

Selectively and contrast improvement ability

41
Q

A grids selectivity is measured how ?

A

The percentage of scatter it eats up is greater than the percentage of primary beam it eats

42
Q

How can we calculate grid selectivity?

A

Selectivity= % primary radiation transmitted / % is scatter

43
Q

A _____ lead content will have greater selectivity

A

Higher

44
Q

How do you measure the contrast improvement factor(K)?

A

K= radiographic contrast with the grid/radiograph contrast without the grid

45
Q

What is the usual contrast improvement factor(k)?

A

1.5 to 3.5

46
Q

The _____ the K factor, the greater the contrast improvement

A

Higher

47
Q

Why are focused grids prone to grid error?

A

B/c the grids are made to go with the separation of the x-ray beam, which means the tube must be centered and the distance aligned

48
Q

What does a off-level error occur?

A

When the tube is angled across the long axis of the grid strips, which can also result in improper tube or grid positioning

49
Q

Using what grid will let you angle alone the long axis of the table ?

A

With a linear grid

50
Q

Can you angle with a criss-cross grid/cross-hatch grid ?

A

No

51
Q

What is an off-center grid error

A

When the grid is not aligned to the tube causing a decrease in exposure on the ENTIRE IMAGE

52
Q

What is off-focus in a grid?

A

When the distance is too great for the grid you are using; when it’s not in focal ranges CUT OFF AT THE EDGES

53
Q

Upside down grid is what

A

When a grid is used backwards and results in radiation absorbed away from the center ONLY THE CENTER IS SEEN; SIDES OF IMAGE IS CUT OFF

54
Q

When will the mortise effect occur in a grid?

A

When the grid lines must run in the same direction as the laser beam that is scanning then imaging plate (CR)

55
Q

what is photoelectric interaction

A

result in the complete absorption of the primary photon and the production of the second

56
Q

does Compton interaction add to diagnostic value of the image

A

it has no diagnostic value

57
Q

as scatter is increased b/c of kVp; what happens to contrast

A

contrast is impaired (weakened)

58
Q

how does thickness of the patient affect scatter

A

the thicker the patient the more scatter is produced but a higher atomic number will result in less scatter being produced

59
Q

what cause the amount of scatter to increase

A

increase in patient thickness, decreased atomic number of the tissue, and LARGE field size/ NO collimation

Radiology Vcast05 (18 decks)

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