1. Resident Physics Lectures
Christensen, Chapter 8 (year 1)
Grids
George David, MS, FAAPM, FACR
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
Image Receptor

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. Grid Structure

7. Grid Styles
Parallel
Focused

8. 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

9. Ideal Grid passes all primary radiation
Reality: lead strips block some primary
Lead
Interspace

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

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

12. Grid Disadvantages Increased patient dose Positioning critical
3-6 times
Positioning critical
poor positioning results in grid cutoff

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

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

15. Lateral Decentering also occurs when grid tilted
uniform loss of radiation over entire film
uniformly light radiograph
no recognizable characteristic (dangerous)
also occurs when grid tilted

16. Lateral Decentering Significant problem in portable radiography
Exact centering not possible

17. Distance Decentering Grid too far from or too close to focal spot
Far focus-grid decentering
Near focus-grid decentering
Grid too far from or too close to focal spot
cutoff at periphery
dark center

18. Combined lateral and focus-grid distance decentering
Easy to recognize
Uneven exposure
Image light on one side, dark on the other
Dangerous
Can mimic clinical conditions

19. Moving Grids Grids move ~1- 3 inches during exposure
Motion blurs out lead strip shadows

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

21. Air Gap Alternative to grid use
By-product of magnification radiography
Very effective in removing scatter originating closest to image receptor
Much attenuation
of scatter in the body
Air gap decreases
capture angle

22. Mammo Cellular Grid Similar to nuclear medicine collimator
Used in mammography