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Advanced Image Aquisition > Control Of Scatter Radiation > Flashcards

Control Of Scatter Radiation Flashcards

1
Q

Define contrast

A

Arises from areas of light, dark, and shades of gray in the image

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2
Q

Define Contrast resolution

A

Ability to image adjacent similar tissues as different

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3
Q

Scatter puts what in the image

A

A density in the film that is not related to patient anatomy and makes the image less visible=noise

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4
Q

Scatter is produced when and destroys what

A

Produced during Compton interaction
High kV ranges
Scatter destroys contrast

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5
Q

Factors affecting scatter PRODUCTION

A

Volume of material irradiated
Atomic number of material being irradiated
KV

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6
Q

Ways to control scatter

A
  1. kVp selection
  2. Beam restriction
  3. Compression
  4. Grid use
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7
Q

What is the best way to reduce scatter?

A

Beam restriction

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8
Q

Aperture diaphragm

A 
Flat sheet of metal with a hole in the center 
Attached to tube head
Simple, cheap, easy to use
High penumbra
High off focus radiation
No light field
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9
Q

Comes/cylinders

A

Extended metal structure attached to tube head or collimator
Cheap, easy to use
Less penumbra and off focus than diaphragm

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10
Q

Collimator

A

Lead shutters at right angles to one another regulate field size
More complex, more expensive
Infinite field sizes, provides light source

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11
Q

PBL

A

Positive beam limitation devices

Automatic collimator a were required from 1974-1993

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12
Q

First stage shutters

A

Decrease off focus radiation

Fixed, don’t move

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13
Q

2nd stage shutters

A

Control field size and decrease penumbra

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14
Q

Lead strips and lead masks

A

Attach to collimator head used in oncology in size of tumor

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15
Q

Compression

A

Used for bigger patients to decrease patient thickness

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16
Q

Grid use

A

Doesn’t affect scatter production

Absorbs scatter after it’s been produced

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17
Q

What is the purpose of a grid?

A

Improves contrast of image by absorbing scatter
Use when: body part is greater than 10 cm thick
kV is greater than 60 in film and CR
kV is greater or equal to 90 in digital

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18
Q

What is a grid made out of?

A

Lead strips with Al or plastic fiber interspace

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19
Q

What is the Al interspace for?

A 
More durable
Easier for manufacture 
Nonhygroscopic
Acts as a grids grid
Absorbs more low energy primary also
20
Q

Grid ratio

A

Height/distance

Higher grid ratio absorbs more scatter

21
Q

Grid frequency

A

Number of grid lines/in
Range from 60-110 lines per inch
High frequency grids show less distinct grid lines

22
Q

Linear parallel grid

A

Strips in one direction, run parallel to each other.
Some grid cut off bc not canted along divergence of beam
Better for long SID

23
Q

What is grid cutoff?

A

Absorption of primary beam

24
Q

Linear focused grid

A

Central strips run parallel as strips move farther away from center they become more and more inclined
Less cut off
SID must be within focal range

25
Q

Convergence line

A

Imaginary line in space where strips would meet on linear focused grid

26
Q

Grid radius

A

Distance from face of grid to convergence line

27
Q

Cross cross or cross hatched grid

A

Lead strips in 2 directions at right angles of each other
More efficient than linear
Centering is crucial
Cannot angle tube against grid

28
Q

Stationary grid uses

A

Portable procedures
Most uprise procedures
Grid cassette

29
Q

Moving grid uses

A

Table top procedures
Usually focused grids
Blurs grid lines

30
Q

Single stroke mechanism

A

Spring, requires manual resetting for each exposure

31
Q

Reciprocating mechanism

A

Motor, back and forth

Most common

32
Q

Oscillating mechanism

A

Electromagnet circular motion

Vibrating

33
Q

Stroboscopic effect

A

Grid lines occurring at very short exposure times. With moving grids

34
Q

What grid to use up to 80 kV?

A

5:1 & 6:1

35
Q

What grid to use up to 100 kV?

A

8:1 & 10:1

36
Q

What grid to use over 100 kV?

A

12:1 & 16:1

37
Q

Grid conversion factor

A

Bucky factor
Tells how much to increase mAs with grid
B= mAs with grid/mAs without grid
B= incident radiation/transmitted radiation

38
Q

Selectivity of grid

A

Ability of grid to absorb scatter and not primary radiation
% primary radiation transmitted/% scatter radiation transmitted
Want a higher selectivity

39
Q

Contrast improvement factor

A 
K factor
Usually 1.5-3.5
If K=1, then no improvement is seen
K=radio graphic contrast with grid/without grid
Want higher number
Average gradient
40
Q

Off level

A

Tube angled across long axis of grid

Grid cutoff on entire image: underexposed

41
Q

Off center

A

Tube not centered along central axis of focused grid
Beam divergence my aligned
Grid cut off on entire image

42
Q

Off focus

A

Tube not within focal range o focused grid (SID)

Grid cut off along periphery

43
Q

Upside down

A

Tube side of focused grid against film
Line down the middle of image
Severe peripheral grid cut off

44
Q

Off center and off focus

A

Grid cut off on one side

45
Q

Air gap method

A

Use greater OID

Increase mAs 10% for every inch of air gap

46
Q

What is the grid conversion equation?

A

mAs2= mAs1 x GCF2/GCF1

47
Q

Lead content

A

Measured in g/cm^2
High lead content means high ratio with low frequency
High lead content means higher absorption of scatter
Lead strips become thinner as you add more

Advanced Image Aquisition (7 decks)

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