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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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Grids George David Associate Professor Department of
Radiology
Medical College of Georgia 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 Grid Construction Lead
Interspace ~ .05 thick upright strips (foil)
material between lead strips maintains lead orientation materials
fiber aluminum wood Lead Interspace Grid Ratio Ratio of interspace
height to width h Grid ratio = h / w w
Lead Interspace h w Grid ratio = h / w Grid Ratio Expressed as X:1
Typical values
8:1 to 12:1 for general work 3:1 to 5:1 for mammography Grid
function generally improves with higher ratios h w Grid ratio = h /
w Lines per Inch # lead strips per inch grid width Typical: 103
25.4
W + w w W w = thickness of interspace (mm) W = thickness of lead
strips (mm) Grid Structure 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 Grid Styles Parallel Focused Parallel
Grid lead strips parallel useful only for small field sizes
large source to image distances 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 Grid Cassette
Grid built into cassette front
Sometimes used for portables formerly used in mammography low grid
ratios focused Ideal Grid passes all primary radiation
Reality: lead strips block some primary Lead Interspace Ideal Grid
block all scattered radiation
Reality: lead strips permit some scatter to get through to film
Lead Interspace Grid Performance Measurements
Primary Transmission (Tp) Bucky Factor (B) contrast improvement
factor (K) Primary Transmission Fraction of a scatter-free beam
passed by grid
Ideally 100% (never achieved) Lead Interspace 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 Primary Transmission Typical values: 55 -
75%
Theoretic calculation: (fraction of grid that is interspace) Tp
(%)= 100 X W / (W+w) where W = Interspace thickness w = lead strip
thickness actual transmission < theoretical primary attenuated
by interspace material focusing imperfections w W W+w 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 Bucky Factor
Measurement
large x-ray field thick phantom ratio of intensity measurement with
& without grid Grid Detector Bucky Factor Measures fraction of
radiation absorbed by grid
high ratio grids have higher bucky factors Bucky Factor Higher
bucky factor means higher x-ray technique
higher patient dose typically 3-6 Contrast Improvement Factor
Ratio of contrast with & without grid Scatter reduces
appearance of contrast No Scatter Scatter Contrast Improvement
Factor
Depends on kVp field size phantom thickness increase in any of
above means more scatter less contrast lower contrast improvement
factor Contrast Improvement Factor
Better contrast improvement with higher ratio more lead content in
grid Lead Content of Grid Definition
weight per unit areagrams (Pb) / cm2 of grid 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 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 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 10:1 and 80 line
8:1 grids h d Grid ratio = h / d Grid Disadvantages Increased
patient dose Positioning critical
poor positioning results in grid cutoff loss of primary radiation
because images of lead strips projecte wider Grid Cutoff focused
grids used upside down
lateral decentering (or angulation) focus- grid distance
decentering combined lateral & focus-grid distance decentering
Upside Down Focused Grid
Dark exposed band in center Severe peripheral cutoff Lateral
Decentering uniform loss of radiation over entire film
uniformly light radiograph no recognizable characteristic
(dangerous) 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 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)
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
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 X Near focus-grid decentering Far
focus-grid decentering
target below convergent line cutoff more severe than far
decentering Far focus-grid decentering target above convergent line
X Near focus-grid decentering Far focus-grid decentering
cutoff at periphery dark center cutoff proportional to grid ratio
decentering distance Minimizing Distance Decentering Cutoff
low grid ratio small fields Combined lateral and focus-grid
distance decentering
Easy to recognize uneven exposure film light on one side, dark on
the other 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 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) Moving Grid Disadvantages
$$$ Vibration Potential May limit minimum exposure time Increases
patient dose lateral decentering from motion up to 20% loss of
primary evenly distributes radiation on film stationary grid makes
interspace gaps darker for same amount of radiation Grid Tradeoff
Advantage Disadvantage cleanup / scatter rejection
increased patient dose increased exposure time increase tube
loading positioning & centering more critical $$$ Grid
Selection use low ratios for low kVp, high ratios for high
kVp
book recommends 8:1 below 90 kVp 12:1 above 90 kVp Air Gap
Techniques Principle Negligible attenuation in air gap
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 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
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 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 Mammo Cellular
Grid