Chapter 17 The Grid

1
Chapter 17 The Grid

• So far we have discussed how kVp, patient size
  and collimation impact scatter radiation.
• As the part size and kVp increase, scatter is
  increased.
• Using low kVp reduces less scatter but increases
  patient dose.

2
The Grid

• Collimation reduces scatter radiation but that
  alone is not sufficient for larger body parts.
• With thick and dense body parts, almost all of
  the remnant rays are scattered many times. This
  results in reduced image contrast.

3
Grids

• An extremely effective means for reducing scatter
  radiation that reaches the film is called a grid.

4
Grid

• In 1913, Gustave Bucky demonstrated that strips
  of lead interspaced with radiolucent material is
  an effective means to reduce scatter radiation
  reaching the film.
• Only rays that travel in a relatively straight
  line from the source are allowed to reach the
  film.
• The others are absorbed by the lead.

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Grid

• Primary beam x-rays striking the interspace
  material are allowed to pass to the film.
• Secondary radiation that strike the interspace
  material may or may not pass on to the film.
• High quality grids will attenuate 80 to 90 of
  the scatter radiation.

6
Grid Construction

• There are three important aspects of grid
  construction
• Grid Ratio
• Grid Frequency
• Grid material

7
Grid Ratio

• There are three important dimensions on a grid.
• Width of the grid strip (T)
• Width of the interspace material (D)
• Height of the grid (h)

8
Grid Ratio

• High ratio grids are more effective in cleaning
  up scatter radiation because the angle of scatter
  allowed by the high ratio is less than permitted
  to pass by low ratio grids.

9
Grid Ratio

• High ratio grids are more expensive and harder to
  produce.
• The width of the interspace material is reduced
  while increasing the height of the grid material
  in order to increase the ratio.
• Ratios range from 51 to 161
• High ratio grids increase patient dose.

10
Grid Ratio

• 81 and 101 grids are the most popular ratios in
  general radiography.
• 81 grids are commonly found on single phase
  machines.
• 101 are often found on high frequency machines.

11
Grid Frequency

• The number of grid lines per inch or centimeter
  is called the Grid Frequency.
• Grids with high frequency show less distinct grid
  lines on the film.
• The higher the frequency of the grid, the thinner
  its strips of interspace material and the higher
  the ratio.

12
Grid Frequency

• The use of high frequency grids requires high
  radiographic technique and results in higher
  patient radiation dose.
• Grid frequency range from 25 to 45 lines per
  centimeter or 60 to 145 lines per inch.
• The advantage of high frequency grids is there
  are no objectionable grid lines on the image.

13
Grid Frequency

• High frequency grids allow the removal of a
  mechanism to move the grid during the exposure.
  This mechanism make the grid a Potter-Bucky
  Diaphragm instead of a grid holder.

14
Grid Material

• The most common grid material is lead because of
  its cost and ease of forming the strips.
• The interspace material is used to maintain a
  precise separation of the lead strips.
• Plastic fiber or aluminum is used as the
  interspace material.

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Grid Material

• Plastic fiber is more common as it does not
  attenuate the beam as it passes through the
  interspace.
• Aluminum interspace requires an increase in the
  technical factors by as much as 20.
• Plastic fiber can absorb moisture resulting in
  warping of the grid.

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Grid Material

• Aluminum is also easier to form and manufacture
  with high tolerances.
• Aluminum is used as the cover for the grid to
  protect it from damage and moisture.

17
Grid Performance

• There are three factors of grid performance.
• Contrast improvement factor improves as the ratio
  increases.
• Bucky factor is the amount of increase radiation
  required to produce the image or the measure of
  the penetration of both primary and secondary
  radiation.

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Grid Bucky Factor

• The Bucky Factor increases with the ratio and kVp
  used. At high kVp more scatter is provided and it
  has a harder time penetrating the grid.
• Different ratios are rated by the kVp needed to
  penetrate the grid. The Bucky factor is also used
  for technique adjustments for grid use.

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Grid Bucky Factor

• kVp limits by ratio.
• A 51 or 61 grid is limited to 80 kVp
• A 81 grid is limited to 90 kVp
• 101 or higher are used above 90 kVp.

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Grid Bucky Factor

• Grid ratio mAs increase kVp increase
• No grid 1X
• 51 2X 8 to 10
• 81 4X 13 to 15
• 101 5X 20 to 25
• 121 6X 20 to 25
• 161 8X 30 to 40

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Grid Selectivity

• The ideal grid would allow all of the primary
  radiation and none of the scatter radiation to
  pass through.
• The ratio of primary to scatter radiation is
  called the grid selectivity.
• Selectivity is influenced by the ratio of the
  grid.

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Grid Selectivity

• Selectivity is a function of the amount of lead
  in the grid.
• A heavy grid with the same ratio as a lighter one
  will contain more lead so its selectivity will
  be higher.

23
Grid Characteristics

• High ratio grids have a high contrast improvement
  factors.
• High frequency grids have a low contrast
  improvement factor.
• Heavy grids have high selectivity and high
  contrast improvement factors.

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Grid Types

• Three types of grids
• Parallel Linear Grids
• Crossed Grids
• Focused Linear Grids

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Parallel Grid

• Cheap and easy to manufacture.
• Problem Grid cutoff at the outer edge of the
  14X17 film.
• Cut off is most pronounced at short SID.

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Parallel Grid

• Cut off distance SID/ Grid Ratio.
• Parallel grids only reduce scatter in the
  direction of the grid lines.

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Crossed Grid

• Two parallel grids can be sandwiched together
  with the lines running across the long axis and
  short axis of the film.
• More efficient than parallel grid.

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Crossed Grid

• Grid cut off is the primary disadvantage of a
  crossed grid.
• The Central ray must be perfectly aligned with
  the center of the grid.
• Tube can not be angled.

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Focused Grids

• Focused grids are designed to minimize grid cut
  off.
• The grid lines are angled to match the divergence
  of the beam.

30
Focused Grids

• Focused grids are marked with an intended focal
  range and the side that should be towards the
  tube.

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Focused Grids

• If the tube is improperly aligned or the SID is
  under the focal range, grid cut off will occur.
• If the grid is placed backwards, cut off will
  occur.

32
The Bucky Grid

• If the grid moves during the exposure, the grid
  lines can be blurred out. This was discovered by
  Hollis Potter in 1920.
• There are two types used today, reciprocating and
  oscillating.
• The reciprocating design is moved by a motor
  during the exposure.

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The Bucky Grid

• The oscillating design is moved by an
  electromagnet in a circular pattern.
• The mechanism adds space between the patient and
  the film.
• The motion can move the film resulting in image
  blur.
• When they fail, the lines appear.

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The Bucky Grid

• For recumbent radiography, they are used
  extensively in medical radiography.
• They are more expensive and generally not
  available for 14 x 36 use.
• Most chiropractic office systems have a higher
  frequency stationary grid so the grid lines are
  not as pronounced.

35
Grid Problems

• If the beam is not properly aligned to the grid,
  cut off will occur.
• High ratio grids are more prone to cut off.
• Parallel and cross grids are prone to cut off.
• With focused grids there are four principle
  causes of grid cut-off.

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Grid Cut-off

• Is the density of the image of both knees the
  same?
• This is an example of grid cut-off. Some of the
  primary beam is being removed by the grid.

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Grid Cut-off Beam Angled

• If the tube is angled against the grid lines,
  grid cut-off will result.

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Grid Cut-off Grid Angled

• If the grid is is not perpendicular to the beam,
  grid cut-off will result.
• Most common problem.

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Beam Not Centered

• If the central ray is not properly centered to
  the center of the grid, grid cut off will happen.
• Common problem with mobile x-ray tables or
  ceiling suspended tubes.

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Off-Focus Grid

• If the SID is not within the focus range of the
  grid, grid cut off will happen with focused
  grids.
• Major problem with high ratio grids.
• More latitude with lower ratio grids.

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Grid Cut-off Grid Backwards

• If the grid is backwards, only the center of the
  beam will pass though the grid.
• Proper alignment must be maintained.

42
Air Gap Technique

• If the film is placed 10 to 15 cm away from the
  patient, the scatter generated my the patient
  will be dispersed away from the image receptor.
• We use this method for the lateral c-spine.

43
Air Gap Technique

• The neck is naturally this far away from the
  film.
• Exposure factor are comparable to an 81 grid.
• Significantly less exposure than using Bucky.

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End of Lecture

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