Resident Physics Lectures

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Resident Physics Lectures

• Christensen, Chapter 8
• Grids

George David Associate Professor Department of
Radiology Medical College of Georgia
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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
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Grid Construction

• Lead
• .05 thick upright strips (foil)
• Interspace
• material between lead strips
• maintains lead orientation
• materials
• fiber
• aluminum
• wood

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Grid Ratio

• Ratio of interspace height to width

Lead
Interspace
h
w
Grid ratio h / w
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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
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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)
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Grid Structure
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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

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Grid Styles

• Parallel
• Focused

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Parallel Grid

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

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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
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Grid Cassette

• Grid built into cassette front
• Sometimes used for portables
• formerly used in mammography
• low grid ratios
• focused

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Ideal Grid

• passes all primary radiation
• Reality lead strips block some primary

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Ideal Grid

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

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Grid Performance Measurements

• Primary Transmission (Tp)
• Bucky Factor (B)
• contrast improvement factor (K)

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Primary Transmission

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

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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
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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
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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

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Bucky Factor Measurement

• large x-ray field
• thick phantom
• ratio of intensity measurement with without grid

Grid
Detector
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Bucky Factor

• Measures fraction of radiation absorbed by grid
• high ratio grids have higher bucky factors

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Bucky Factor

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

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Contrast Improvement Factor

• Ratio of contrast with without grid
• Scatter reduces appearance of contrast

No Scatter
Scatter
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Contrast Improvement Factor

• Depends on
• kVp
• field size
• phantom thickness
• increase in any of above means
• more scatter
• less contrast
• lower contrast improvement factor

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Contrast Improvement Factor

• Better contrast improvement with
• higher ratio
• more lead content in grid

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Lead Content of Grid

• Definition
• weight per unit areagrams (Pb) / cm2 of grid

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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
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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
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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
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Grid Disadvantages

• Increased patient dose
• Positioning critical
• poor positioning results in grid cutoff
• loss of primary radiation because images of lead
  strips projecte wider

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Grid Cutoff

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

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Upside Down Focused Grid

• Dark exposed band in center
• Severe peripheral cutoff

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Lateral Decentering

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

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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

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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)
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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

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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

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

• Far focus-grid decentering
• target above convergent line

X
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• Near focus-grid decentering

• Far focus-grid decentering

• cutoff at periphery
• dark center
• cutoff proportional to
• grid ratio
• decentering distance

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Minimizing Distance Decentering Cutoff

• low grid ratio
• small fields

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Combined lateral and focus-grid distance
decentering

• Easy to recognize
• uneven exposure
• film light on one side, dark on the other

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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

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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)

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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

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Grid Tradeoff

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

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Grid Selection

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

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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
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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
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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

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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

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Mammo Cellular Grid