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Resident Physics Lectures

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8:1 to 12:1 for general work. 3:1 to 5:1 for mammography ... Reality: lead strips permit some scatter to get through to film. Lead. Interspace ... – PowerPoint PPT presentation

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Title: Resident Physics Lectures


1
Resident Physics Lectures
  • Christensen, Chapter 8
  • Grids

George David Associate Professor Department of
Radiology Medical College of Georgia
2
Purpose
  • Directional filter for photons
  • Ideal grid
  • passes all primary photons
  • photons coming from focal spot
  • blocks all secondary photons
  • photons not coming from focal spot

Focal Spot
Good photon
Patient
Bad photon
X
Grid
Film
3
Grid Construction
  • Lead
  • .05 thick upright strips (foil)
  • Interspace
  • material between lead strips
  • maintains lead orientation
  • materials
  • fiber
  • aluminum
  • wood

4
Grid Ratio
  • Ratio of interspace height to width

Lead
Interspace
h
w
Grid ratio h / w
5
Grid Ratio
  • Expressed as X1
  • Typical values
  • 81 to 121 for general work
  • 31 to 51 for mammography
  • Grid function generally improves with higher
    ratios

h
w
Grid ratio h / w
6
Lines per Inch
  • lead strips per inch grid width
  • Typical 103

25.4 Lines per inch
------------ W w
w thickness of interspace (mm) W thickness
of lead strips (mm)
7
Grid Structure
8
Grid Patterns
  • Orientation of lead strips as seen from above
  • Types
  • Linear
  • Cross hatched
  • 2 stacked linear grids
  • ratio is sum of ratios of two linear grids
  • very sensitive to positioning tilting
  • Rare only found in specials

9
Grid Styles
  • Parallel
  • Focused

10
Parallel Grid
  • lead strips parallel
  • useful only for
  • small field sizes
  • large source to image distances

11
Focused Grid
  • Slightly angled lead strips
  • Strip lines converge to a point in space called
    convergence line
  • Focal distance
  • distance from convergence line to grid plane
  • Focal range
  • working distance range
  • width depends on grid ratio
  • smaller ratio has greater range

Focal range
Focal distance
12
Grid Cassette
  • Grid built into cassette front
  • Sometimes used for portables
  • formerly used in mammography
  • low grid ratios
  • focused

13
Ideal Grid
  • passes all primary radiation
  • Reality lead strips block some primary

14
Ideal Grid
  • block all scattered radiation
  • Reality lead strips permit some scatter to get
    through to film

15
Grid Performance Measurements
  • Primary Transmission (Tp)
  • Bucky Factor (B)
  • contrast improvement factor (K)

16
Primary Transmission
  • Fraction of a scatter-free beam passed by grid
  • Ideally 100 (never achieved)

17
Measuring Primary Transmission
  • small area beam
  • scatterer in beam far from grid
  • virtually no scatter reaches grid
  • measure radiation intensity with without grid
  • ratio X 100 is Primary Transmission (Tp)

Focal Spot
Lead Diaphragm
Grid
Detector
18
Primary Transmission
  • Typical values 55 - 75
  • Theoretic calculation (fraction of grid that is
    interspace)
  • Tp () 100 X W / (Ww) where
  • W Interspace thickness
  • w lead strip thickness
  • actual transmission lt theoretical
  • primary attenuated byinterspace material
  • focusing imperfections

W
w
Ww
19
Bucky Factor
  • Radiation incident on grid----------------------
    ------------- transmitted radiation
  • indicates actual increase in exposure because of
    grids presence
  • due to attenuation of both primary secondary
    radiation

20
Bucky Factor Measurement
  • large x-ray field
  • thick phantom
  • ratio of intensity measurement with without grid

Grid
Detector
21
Bucky Factor
  • Measures fraction of radiation absorbed by grid
  • high ratio grids have higher bucky factors

22
Bucky Factor
  • Higher bucky factor means
  • higher x-ray technique
  • higher patient dose
  • typically 3-6

23
Contrast Improvement Factor
  • Ratio of contrast with without grid
  • Scatter reduces appearance of contrast

No Scatter
Scatter
24
Contrast Improvement Factor
  • Depends on
  • kVp
  • field size
  • phantom thickness
  • increase in any of above means
  • more scatter
  • less contrast
  • lower contrast improvement factor

25
Contrast Improvement Factor
  • Better contrast improvement with
  • higher ratio
  • more lead content in grid

26
Lead Content of Grid
  • Definition
  • weight per unit areagrams (Pb) / cm2 of grid

27
More Lines / inch at Same Ratio Means Less Lead
Content Contrast Improvement
  • thinner lead same ratio
  • less lead (less thickness, same height)
  • Same interspace dimensions

h
d
Grid ratio h / d
28
More Lines / inch at Same Ratio Means Less Lead
Content Contrast Improvement
  • thinner interspace less height to maintain
    ratio
  • less lead (less height, same thickness)

h
d
Grid ratio h / d
29
Lead Content of Grid
  • more lines / inch for same ratio means less lead
    content thus less contrast improvement
  • puts practical limit on lines per inch
  • same contrast improvement for 133 line 101 and
    80 line 81 grids

Grid ratio h / d
30
Grid Disadvantages
  • Increased patient dose
  • Positioning critical
  • poor positioning results in grid cutoff
  • loss of primary radiation because images of lead
    strips projecte wider

31
Grid Cutoff
  • focused grids used upside down
  • lateral decentering (or angulation)
  • focus- grid distance decentering
  • combined lateral focus-grid distance decentering

32
Upside Down Focused Grid
  • Dark exposed band in center
  • Severe peripheral cutoff

33
Lateral Decentering
  • uniform loss of radiation over entire film
  • uniformly light radiograph
  • no recognizable characteristic (dangerous)

34
Lateral Decentering
  • also occurs when grid at correct position but
    tilted
  • both result in uniform loss of intensity
  • no other clinical clues
  • may be mistaken for technique problems
  • Can be compensated for by over-exposing patient

35
Lateral Decentering
  • cutoff increases with
  • Higher grid ratio
  • Greater decentering distance
  • smaller focal distances

r b L ----- X 100 fo
L loss of primary radiation () r grid
ratio b lateral decentering distance
(inches) fo focal distance of grid (inches)
36
Lateral Decentering
  • Significant problem in portable radiography
  • Compensate by over-exposing patient
  • exact centering not possible
  • minimizing lateral decentering
  • low ratio grids
  • long focal distances

37
Distance Decentering
  • Grid too close or too far from focal spot
  • Darker center
  • All parallel grids have some degree of distance
    decentering
  • Focused to infinity

38
  • Near focus-grid decentering
  • target below convergent line
  • cutoff more severe than far decentering
  • Far focus-grid decentering
  • target above convergent line

X
39
  • Near focus-grid decentering
  • Far focus-grid decentering
  • cutoff at periphery
  • dark center
  • cutoff proportional to
  • grid ratio
  • decentering distance

40
Minimizing Distance Decentering Cutoff
  • low grid ratio
  • small fields

41
Combined lateral and focus-grid distance
decentering
  • Easy to recognize
  • uneven exposure
  • film light on one side, dark on the other

42
Combined lateral and focus-grid distance
decentering
  • Cutoff proportional to
  • grid ratio
  • decentering distance
  • Cutoff inversely proportional to grid focal
    distance
  • Less cutoff for longer focus grids
  • cutoff greater for near than for far distance
    decentering

43
Moving Grids
  • Motion starts with second trigger
  • Grids move 1- 3 inches
  • must be fast enough not to see grid lines for
    short exposures
  • Motion blurs out lead strip shadows
  • for single phase generators grid motion must not
    synchronize with pulses
  • note error in book, page 111 (omits not)

44
Moving Grid Disadvantages
  • Vibration Potential
  • May limit minimum exposure time
  • Increases patient dose
  • lateral decentering from motion
  • up to 20 loss of primary
  • evenly distributes radiation on film
  • stationary grid makes interspace gaps darker for
    same amount of radiation

45
Grid Tradeoff
  • Advantage
  • cleanup / scatter rejection
  • Disadvantage
  • increased patient dose
  • increased exposure time
  • increase tube loading
  • positioning centering more critical

46
Grid Selection
  • use low ratios for low kVp, high ratios for high
    kVp
  • book recommends
  • 81 below 90 kVp
  • 121 above 90 kVp

47
Air Gap Techniques
  • Principle
  • radiation scatters uniformly
  • decrease in scatter (most scatter misses film)
  • air gap decreases angle of capture increases
    angle of escape
  • Negligible attenuation in air gap

Angles of escape
48
Air Gap
  • air gap very effective in removing scatter
    originating closest to film
  • much of scatter nearest tube doesnt reach film

Much attenuation of scatter in the body
Air gap decreases capture angle
49
Air Gap Applications
  • Magnification Radiography including mammography
  • geometry causes air gap
  • Air Gap Chest Radiography
  • air gap used as alternative to grid
  • SID increased from 6 feet to 10 feet to maintain
    geometric unsharpness
  • Grid not used with air gap

50
Air Gap Optimization
  • Air gap more effective for thicker body parts
  • first inch of air gap most effective in contrast
    improvement
  • image sharpness deteriorates with increasing gap
    (magnification)
  • compensate with
  • greater SID
  • smaller focal spot

51
Mammo Cellular Grid
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