RADIOGRAPHIC GRIDS

1

• RADIOGRAPHIC GRIDS
• Introduction
• When an x-ray beam passes through the body, one
  of three things will occur with the x-ray
  photons
• Pass through unaffected.
• Be absorbed by the body (via the photoelectric
  effect).
• Interact and change direction, ie, be scattered
  (via the Compton effect) (Fig. 1).
• Scattered photons add to the overall graying of
  the image and therefore reduce contrast.
• Radiographic grids are used to reduce the amount
  of scattered radiation reaching the x-ray film
  and thereby increase image contrast.
• Grids are placed between the patient and the
  film.
• Grid Construction
• A grid consists of strips of a dense material
  (grid strips) separated by a material that is
  relatively transparent to x-rays (interspace
  material).
• The whole thing is enclosed in aluminium, which
  provides rigidity and keeps out moisture.
• The grid is designed to transmit direct x-rays
  but absorb scattered x-rays (Fig. 2).
• A grid is characterised by three quantities (Fig.
  3)
• The height of the grid, h.
• The width of the interspace materal, D.
• The width of the grid strips, T.

2

• Interspace Material
• Maintains precise separation between strips.
• Made of aluminium or plastic.
• Aluminium
• Higher atomic number and therefore provides
  better filtration of scattered x-rays.
• Produces less visible grid lines on film.
• Increases absorption of primary x-rays resulting
  in higher mAs being used, thus increasing patient
  dose.
• Does not absorb water.
• Easier to manufacture.
• Grid Strips.
• Should be very thin.
• Should be good absorbers of x-rays.
• Should be easy to shape and relatively
  inexpensive.
• Lead is most widely used.
• GRID CHARACTERISTICS
• Percentage Absorption
• Defined as
• (width of strip/(width of interspace width of
  strip)) x 100
• or T/(D T) x 100

3

• Grid Ratio
• Defined as
• height of grid / width of interspace or h/D
• Range from 51 to 161.
• High grid ratio grids
• Are more effective in cleaning up scatter
  radiation (up to 97) because the angle of
  scatter allowed is less (Fig. 4, 5).
• More difficult to manufacture.
• Increase patient radiation dose.
• Grid Frequency
• The number of grid strips per centimeter.
• Range from 25 to 45 lines per centimeter.
• Defined as
• 10,000 / (width of strip width of interspace)
  or 10,000 / (T D)
• Example what is the grid frequency of a grid
  with 30 mm wide strips and 300 mm wide
  interspaces?
• From above 10,000 / (30 300) 10,000 / 330
  30.3.
• High grid frequency grids
• Have thinner strips of interspace material.
• Show less distinct grid lines on radiograph.
• Increase patient radiation dose.

4

• Contrast Improvement Factor (k)
• Measurement which relates the ability of a grid
  to improve contrast.
• Most grids have a value between 1.5 and 2.5.
• Defined as
• k radiographic contrast with grid /
  radiographic contrast without grid
• Depends on
• X-ray spectrum.
• Patient thickness.
• Area irradiated.
• k is higher for high-ratio grids.
• Grid Factor or Bucky Factor (B)
• Measures the penetration of both primary and
  scattered radiation through the grid.
• Defined as
• B incident radiation / transmitted radiation
• or
• B patient dose with grid / patient dose
  without grid
• B increases as the grid ratio increases.
• B increase as kVp increases (Table 1).

5

• Selectivity (S)
• Defined as
• S primary radiation transmitted through grid /
  scattered radiation transmitted through grid
• The thicker the strip the greater the selectivity
  (and contrast improvement factor) because more
  scatter radiation is removed (Fig. 7).
• Grid Cut Off
• The undesirable absorption of primary x-rays by
  the grid (Fig. 8).
• Results in reduced optical density or total
  absence of film exposure.
• Most common with linear grids.
• Linear grid cut off is most pronounced when the
  grid is used at short source-image distances or
  with a large image size.
• The distance from the central axis at which
  complete cut off will occur is given by
• distance to cut off source to image distance
  / grid ratio
• Grid Types
• There are two types of grids
• Linear.
• Linear parallel.
• Linear focussed.
• Crossed
• Crossed parallel.
• Crossed focussed.

6

• Linear Parallel Grids.
• The simplest form.
• Lead strips are parallel.
• Can produce severe cut off.
• Cut off is worse at edge of film due to greater
  divergence.
• Can cause variation in optical density from
  centre of film to edge.
• Best used with large source to image distance and
  small exposure area.
• Linear Focussed Grids
• Designed to minimise grid cut off.
• The central strip is perpendicular to the plane
  of the film.
• The outer strips are then arranged with a slight
  incline toward the midline (Fig. 9).
• The anode should be positioned at the midpoint of
  the convergent lines or focal point of the
  strips.
• Focused grids are marked with their intended
  focal distance and the side that must face the
  tube.
• Crossed Grids
• Consist of two linear grids that are positioned
  perpendicular to each other (Fig. 10).
• Cleans scatter in two directions therefore is
  more efficient than linear grids.
• Central ray must coincide with centre of grid.
• Again best used with large source to image
  distance and small exposure area.

7

• Moving the Grid
• A shortcoming of grids is that they produce
  visible grid lines on radiographs.
• The solution is to move the grid during the x-ray
  exposure with a moving grid mechanism or
  Bucky.
• Grid Selection
• Depends on
• kVp.
• Degree of scatter removal.
• Patient dose.
• High kVp should use high grid ratio.
• As grid ratio increases, more scatter radiation
  is removed (Fig. 5).
• Effect on Patient Dose
• Using a grid requires more radiation than not
  using a grid.
• Patient dose increases with increasing grid
  ratio.
• For a particular grid ratio, patient dose
  decreases with increased kVp.
• Table 2 shows approximate entrance surface dose
  (ESD) values for an adult pelvis at 200
  film-screen speed.

8

• INVERSE SQUARE LAW
• Is a purely geometrical effect.
• Applies to radiation coming from a point source,
  such as (approximately) the focal spot of an
  x-ray tube, propagating through a medium that
  does not absorb or scatter the radiation (such as
  air).
• States that radiation intensity is inversely
  proportional to the square of the distance d from
  the source (Fig. 12)
• I ? 1/d2
• If we have intensity I1 at distance d1 and
  intensity I2 at distance d2, then
• I1 / I2 d22 / d12
• Example The dose from an x-ray tube operated at
  70 kVp and 200 mAs is 4 mGy at a distance of 90
  cm. What will be the dose at 180 cm?
• Dose is related to intensity and so the inverse
  square law applies with I2 4 mGy, d2 90 cm
  and d1 180 cm.
• From above I1 I2(d22 / d12) I2(d2 / d1)2
• 4(90/180)2
• 4(1/2)2
• 4(1/4)
• 1 mGy
• This example shows that when the distance is
  doubled, the intensity of radiation is reduced to
  one quarter.
• Conversely when the distance is halved, the
  intensity is increased by a factor of four.