1. The Grid
Kyle Thornton
DMI 50B

2. What Is A Grid? Invented in 1913 by Gustaf Bucky
Consisted of a framework containing lead foil strips standing on edge, parallel and equidistant to each other
In 1920, Hollis Potter invented a mechanism for suspending the grid in a framework that moved between the patient and film
The motion eliminated the grid lines in the image
The grid is the most effective way to remove secondary radiation from large radiographic fields

3. What Does A Grid Do? A grid is an important radiographic tool
A grid absorbs scatter radiation before it reaches the film
A grid improves contrast on the film
A grid has a special composition and many different types
Used properly, the grid is a technologist’s best friend

4. Grid Construction Grid ratio Grid frequency Interspace material
Lead strips

5. Grid Ratio Three important dimensions of a grid
Grid thickness - T
Interspace material thickness - D
Grid height - h
Grid ratio is the height divided by interspace material thickness
Grid ratio = h/D

6. Why Is Grid Ratio Important?
Grid ratio determines how scatter radiation is “cleaned up”
The higher the grid ratio, the more cleanup
Grids of higher ratios require more technique
This results in a higher patient dose
Ratios range from 5:1 - 16:1
Mammo grids have very low ratios

7. Grid Ratio Equation
The distance between each grid strip is 200 m and the height is 2.4 mm. What is the grid ratio?
Hint: Ratio = h/D
Step 1 – Identify h
Step 2 – Identify D
Step 3 – ????
Step 4 - ????

8. Grid Frequency
The number of strips or lines per inch or centimeter is grid frequency
Higher frequencies display less lines
Higher frequencies affect patient dose
Higher frequencies are generally associated with higher ratios
Most grid frequencies are lines/inch
Mammo grids have very high frequencies, but low ratios

9. Interspace Material The material between the grid strips
Maintains a precise separation between the strips
Generally constructed from aluminum or plastic fiber
Aluminum has definite advantages over fiber

10. Grid Strips
Should be very thin and have high scatter absorption properties
Lead is best
The entire grid is encased in aluminum for protection
Sometimes it is further encased in plastic for more protection

11. Grid Performance
Contrast improvement factor
Bucky factor
Selectivity

12. Contrast Improvement Factor
Grids remove scatter radiation before it reaches the film
Therefore it improves contrast
Contrast improvement factor compares contrast improvement with a grid to that without a grid

13. Contrast Improvement Factor Equation
K = Radiographic contrast with grid
Radiographic contrast without grid
Most grids have a contrast improvement of
Contrast improvement is higher with higher ratio grids
Lead content also determines contrast improvement

14. Bucky Factor Also called grid factor
This compares the increased technique necessary for grid use
Bucky factor will increase with with increasing grid ratio
It will also increase with increasing kVp
B = Incident remnant radiation
Transmitted remnant radiation
The amount of radiation hitting the grid will always be greater than the amount hitting the film

15. Grid Selectivity Related to grid construction itself
The total lead content of the grid has an influence on selectivity
The more lead, the more cleanup
 = Primary radiation transmitted through grid
Scatter radiation transmitted through grid

16. General Rules Of Grid Characteristics
High ratio grids have high contrast improvement factors
High frequency grids have thin strips of interspace material and low contrast improvement factors
Heavy grids have high selectivity and high contrast improvement factors

17. Grid Types Linear parallel Crossed Focused Moving grids Single stroke
Reciprocating
Oscillating

18. Linear Parallel Grid Simplest to construct
The grid strips are parallel
Most latitude

19. Crossed Grid Two linear grids at right angles to each other
Was used primarily for pneumoencephalography
Used for high contrast studies
Very high cleanup
Not used very much
Must be centered exactly
Must be directly perpendicular to grid

20. Focused Grid The strips run on one axis and are tilted
Strips are parallel to the primary x-ray path across entire film
Must use within a proscribed distance

21. Moving Grids Single Stroke
Antiquated
Grid had to be cocked with a spring mechanism
Worked in synch with exposure time
The mechanism moved once throughout exposure
Had to be reset for each exposure

22. Reciprocating Grid Moves back and forth during exposure Motor driven
Does not have to be reset for each exposure

23. Oscillating Grid Similar to a reciprocating grid
Moves in a circular motion as opposed to back and forth

24. Grid mounted within Bucky Tray

25. Advantages And Disadvantages Of Moving Grids
No grid lines
Problems occur infrequently
Disadvantages
Mechanical problems may occur
Very infrequently, motion is detected on radiograph

26. Grid Cutoff A big problem with linear and crossed grids
Less of a problem with focused grids
The primary beam has been absorbed
Has a negative effect on image detail, density, and contrast

27. Grid Errors Off-center Beam is not centered to center of grid
Upside down
Focused grid only
Causes severe grid cutoff in periphery of film

28. More Grid Errors
Off-focus error
Focusing distance not observed
Focused grid only
Using the incorrect focal distance results in grid cutoff in the periphery of the image

29. Yet another grid error Off-center – off-focus
Partial grid cut-off occurs over the entire image

30. Even more grid errors… Off-level Beam is not perpendicular to grid
Grid is not perpendicular to the beam
Either way, you’re repeating that film

31. How many grid errors can there be?
The classic grid error
Focused grid placed upside down

32. Demo of Focused Grid Used Upside Down

33. Grid errors from the beam and grid’s perspective

34. Summary of grid errors and associated results
Off-level
cutoff across image; underexposed, light image
Off-center
Grid cutoff across image; underexposed, light image
Off-focus
Grid cutoff toward edge of image
Focused Grid Placed Upside-down
Severe grid cutoff toward edge of image
Off-center, off-focus
Grid cutoff on one side of image

35. Grid Selection Depends upon body part to be radiographed
Chest radiography uses high kVp
8:1 ratio can be used for most general work
Up to about 90 kVp
Focused grids are generally superior
Lower ratio grids offer more positioning latitude

36. Grids And Patient Dose
Patient dose increases with increasing grid ratio
High ratio grids are generally used for high kVp studies
Patient dose decreases with higher kVp use
Less radiation is absorbed in tissues with higher kVp

37. Suggested Grid Conversion Factors

38. Alternatives To Grid Use
Air-gap technique
OID is increased
Equal to approximately 8:1 grid
Increases magnification
Distance must be increased to overcome magnification
Not effective with high kVp