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

4. Grid Ratio Ratio of interspace height to width h Grid ratio = h / w w
Lead
Interspace h w
Grid ratio = h / w

5. 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

6. 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)

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
Lead
Interspace

14. Ideal Grid block all scattered radiation
Reality: lead strips permit some scatter to get through to film
Lead
Interspace

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)
Lead
Interspace

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 / (W+w) where
W = Interspace thickness
w = lead strip thickness
actual transmission < theoretical
primary attenuated by interspace material
focusing imperfections w W
W+w

19. Bucky Factor
Radiation incident on grid transmitted radiation
indicates actual increase in exposure because of grid’s 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 area grams (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 10:1 and 80 line 8:1 grids h d
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. 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

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 Disadvantage cleanup / scatter rejection
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
8:1 below 90 kVp
12:1 above 90 kVp

47. 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

48. Air Gap
air gap very effective in removing scatter originating closest to film
much of scatter nearest tube doesn’t 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