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

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

5. Grid Ratio Expressed as X:1 h Grid ratio = h / w 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. Ideal Grid passes all primary radiation
Reality: lead strips block some primary
Lead
Interspace

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

14. Primary Transmission Fraction of a scatter-free beam passed by grid
Ideally 100% (never achieved)
Lead
Interspace

15. 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

16. 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

17. Bucky Factor Measures fraction of radiation absorbed by grid
high ratio grids have higher bucky factors

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

19. More Lines / inch at Same Ratio Means Less Lead Content & Less Contrast Improvement
thinner lead & same ratio
less lead (less thickness, same height)
Same interspace dimensions
same contrast improvement for 133 line 10:1 and 80 line 8:1 grids h d
Grid ratio = h / d

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

21. Grid Cutoff focused grids used upside down
lateral decentering (or angulation)
focus- grid distance decentering
combined lateral & focus-grid distance decentering

22. Upside Down Focused Grid
Dark exposed band in center
Severe peripheral cutoff

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

24. Lateral Decentering also occurs when grid tilted
both result in uniform loss of intensity
no other clinical clues
may be mistaken for technique problems
Can be compensated for by increasing patient exposure

25. 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

26. 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

27. Near focus-grid decentering Far focus-grid decentering
cutoff at periphery
dark center
cutoff proportional to
grid ratio
decentering distance

28. Minimizing Distance Decentering Cutoff
low grid ratio
small fields

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

30. 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

31. 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

32. Grid Tradeoff Advantage Disadvantage cleanup / scatter rejection
increased patient dose
increased exposure time
increase tube loading
positioning & centering more critical
$$$

33. 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

34. Air Gap Techniques Principle Negligible attenuation in air gap
Angles of
escape
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

35. 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

36. Air Gap Applications Magnification Radiography including mammography
geometry causes air gap
Grid not used with air gap

37. Mammo Cellular Grid