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Rad tech subjects > physics ch 15,18 > Flashcards

physics ch 15,18 Flashcards

1
Q

what are two ways we can minimize scatter to the IR?

A

restrict the x Ray beam and use a grid

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2
Q

what is the purpose of “restricting the beam”?

A

to reduce the amount of primary photons that are emitted from the tube and collimator (less scatter photons )

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3
Q

what 2 principal factors affect the amount of scatter produced?

A

KV

material irradiated (volume )

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4
Q

kilovoltage affects beam ___________.

A

penetrability

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5
Q

KV is mostly selected based on _______and ________

A

size of the part and desired contrast

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6
Q

The amount of scatter created during an interaction is affected by the ________and ________

A

volume and atomic number of the material

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7
Q

The volume of irradiated material is controlled by ___________and ________

A

Field size and patient thickness

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8
Q

as the volume of irradiated tissue increases the amount of scatter _____________.

A

increases

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9
Q

volume increases as the ___________or_____________ increases

A

Field size or patient thickness

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10
Q

________field size equals __________scatter

A

decreased, decreased

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11
Q

how is KV and the field size controlled?

A

by the technologist

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12
Q

True or false
when the beam is restricted, less scatter radiation will reach the image receptor so the technical factors may need to be increased to compensate for the reduction in the overall IR exposure

A

true

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13
Q

what is the most common type of beam restrictor?

A

collimator

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14
Q

collimation decreases the amount of _______irradiated

A

tissue

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15
Q

what is an aperture diaphragm?

A

A flat sheet of lead with an opening cut in the middle

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16
Q

_________are rarely used but most effective at scatter control.

A

cones

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17
Q

what is the downside of using cones?

A

they have a fixed field size

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18
Q

what are two examples of ancillary devices?

A

lead blockers, lead masks

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19
Q

what is the most common beam limitation device used ?

A

variable aperture collimator

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20
Q

True or false

when using cones MAS must be increased to account for photons absorbed by the cone

A

true

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21
Q

The first set of shutters in a collimator, the upper shutters, are mounted as close as possible to the tube window to reduce the amount of______

A

off focus or stem radiation

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22
Q

define off focus or stem radiation

A

x-rays emitted from parts of the tube other then the focal spot

23
Q

how far from the collimator should the patient’s skin surface be to minimize skin exposure from off focus

A

15cm

24
Q

why is collimation not used in mammography?

A

Low technical factors are used

25
Q

how does scatter affect an image?

A

impairs image quality

26
Q

what is the role of the bottom shutters in collimators?

A

to reduce penumbra along the beam periphery

27
Q

when is a lead mask most commonly used

A

during cerebral angiography

28
Q

what is a lead blocker

A

A sheet of lead impregnated rubber

29
Q

A sensing device that activates motor to drive shutters into proper position

A

positive beam limitation device

30
Q

what type of grid is designed so that the central grid strips are parallel but become more inclined as they move away from the central axis

A

focused

31
Q

The greatest source of scatter comes from?

A

The patient

32
Q

what are the two things to remember when using a focused grid?

A

you must be at a particular SID and the central ray must be aligned perpendicular to the grid

33
Q

high ratio grids have ______ positioning latitude than low ratio grids

A

less

34
Q

True or false

as Bucky factor increases, radiographic technique and patient dose increase proportionately

A

true

35
Q

as the grid ratio gets higher contrast ________

A

improves

36
Q

what causes grid cut off?

A

misalignment or improper SID

37
Q

what happens on the image if the grid is upside down?

A

severe grid cut off toward the image edge

38
Q

what is the formula for grid ratio?

A

H/D

The height of the grid strip divided by the width of the interspace material

39
Q

what are the three types of grids?

A

parallel, cross hatched and focused

40
Q

describe a focused grid

A

The central grid strips are parallel and as the strips move away from the central axis they become more and more inclined

41
Q

True or false

The focused design results in a grid with lead strips designed to match the divergence of the x-ray beam

A

true

42
Q

in a focused grid, if the lead strips were extended the strips would intersect along a line in space known as the__________

A

convergence line

43
Q

what is the grid radius?

A

The distance from the face of the grid to the points of convergence of the ledge strips

44
Q

For the grid to be properly focused, the x-ray tube must be located along the_______

A

convergence line

45
Q

parallel grid’s function best at _____SID

A

long

46
Q

grid conversion factors

A

5:1 = 2
6:1 = 3
8:1 = 4
10/12:1 = 5
16:1 = 6

47
Q

what are the two criteria for measuring a grids performance?

A

selectivity and contrast improvement ability

48
Q

what is the only positioning error possible with a parallel Grid?

A

off level grid error

49
Q

what will a radiograph look like with an off level grid error

A

decreased exposure across the entire image (a lot of white)

50
Q

The greater the degree of lateral decentering the ________ the grid cut off

A

greater

51
Q

describe the airgap technique

A

place the patient at a greater OID creating an air gap between the patient and the IR. The amount of scatter reaching the IR will be reduced, contrast improved even though a grid is not used. this is equal to a 6:1 grid but the image will be magnified

52
Q

what is the primary disadvantage of the airgap technique

A

loss of sharpness due to increased OID

53
Q

The contrast improvement factor is dependent on the amount of scatter produced which is controlled by ______ and __________

A

KV P and volume of irradiated tissue

Rad tech subjects (16 decks)

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