Radiographic Grids II
By Professor Stelmark
Ideally, grids would absorb all scattered radiation and allow all transmitted
photons to reach the image receptor. In reality, however, some scattered
photons pass through to the image receptor and some transmitted
photons are absorbed.
Grid construction can be described by grid frequency and grid ratio.
• Grid frequency expresses the number of lead lines per unit length in
inches, centimeters, or both. Grid frequencies can range in value from
25 to 45 lines/cm (60 to 110 lines/inch). A typical value for grid
frequency might be 40 lines/cm or 103 lines/inch.
• Grid ratio is defined as the ratio of the height of the lead strips to the
distance between them.
Calculating Grid Ratio
What is the grid ratio when the lead strips are 3.2 mm high and separated
by 0.2 mm?
As grid ratio increases, for the same grid frequency, scatter cleanup improves
and radiographic contrast increases; as grid ratio decreases, for the same grid
frequency, scatter cleanup is less effective and radiographic contrast decreases.
10:1 ratio
frequency 100 lines/cm
10:1 ratio
frequency 50 lines/cm
5:1 ratio
frequency 100 lines/cm
5:1 ratio
frequency 50 lines/cm
8:1 ratio
frequency 100 lines/cm
4:1 ratio
frequency 50 lines/cm
Increasing the grid ratio for the same grid frequency will increase the
amount of lead content and therefore increase scatter absorption.
If the grid frequency is increased for the same grid ratio, there is overall less
lead content because the width of the interspace and or the thickness of the
lead strips have been decreased. Decreasing the overall lead content will
result in decreased scatter absorption.
Grid Ratio and Radiographic Density
As grid ratio increases, radiation exposure to the IR decreases; as grid ratio
decreases, radiation exposure to the IR increases.
Focused versus Parallel Grids
Focused grids have lead lines that are angled to approximately match the
divergence of the primary beam. Thus focused grids allow more
transmitted photons to reach the IR than parallel grids.
If imaginary lines were drawn from each of the lead lines in a linear focused grid,
these lines would meet to form an imaginary point, called the convergent point. If
points were connected along the length of the grid they would form an imaginary
line, called the convergent line. Both the convergent line and convergent point are
important because they determine the focal distance of a focused grid. The focal
distance (sometimes referred to as grid radius) is the distance between the grid
and the convergent line or point. The focal distance is important because it is used
to determine the focal range of a focused grid.
The focal range is the recommended range of SIDs that can be used
with a focused grid.
Stationary and Reciprocating Grids
When grids are stationary, it is possible to closely examine and see the grid
lines on the radiographic image. Slightly moving the grid during the x-ray
exposure blurs the grid lines.
Moving or reciprocating grids are part of the Bucky, more accurately called
the Potter-Bucky diaphragm.
The grid is located directly below the radiographic tabletop and just above the tray
that holds the IR. Grid motion is controlled electrically by the x-ray exposure
switch. The grid moves slightly back and forth in a lateral direction over the IR
during the entire exposure. These grids typically have dimensions of 17 × 17
inches (43 × 43 cm) so that a 14 × 17–inch (35 × 43 cm) cassette can be
positioned under the grid either lengthwise or crosswise, depending on the
examination requirements.
Focused grids usually are used as moving grids. They are placed in a holding
mechanism that begins moving just before x-ray exposure and continues moving
after the exposure ends. Two basic types of moving grid mechanisms are in use
today:
• reciprocating
• oscillating.
Disadvantages of Moving Grids
Moving grids require a bulky mechanism that is subject to failure. The distance
between the patient and the image receptor is increased with moving grids
because of this mechanism; this extra distance may create an unwanted
increase in magnification and image blur. Moving grids can introduce motion
into the cassette-holding device, which can result in additional image blur.
Reciprocating Grid
A reciprocating grid is a moving grid that is motor-driven back and forth several
times during x-ray exposure. The total distance of drive is approximately 2 cm.
Oscillating Grid
An oscillating grid is positioned within a frame with a 2- to 3-cm tolerance on all
sides between the frame and the grid. Delicate, springlike devices located in
the four corners hold the grid centered within the frame. A powerful
electromagnet pulls the grid to one side and releases it at the beginning of the
exposure. Thereafter, the grid oscillates in a circular fashion around the grid
frame, coming to rest after 20 to 30 seconds.
The Grid Conversion factor
Grid Ratio
• Non –grid
• 5:1
• 6:1
• 8:1
• 12:1
• 16:1
mAs Compensation
•
•
•
•
•
2 ( 2 x non-grid mAs)
3 ( 3 x non-grid mAs)
4 ( 4 x non-grid mAs)
5 ( 5 x non-grid mAs)
6 ( 6 x non-grid mAs)
Adding a Grid
If a radiographer produces a knee radiograph with a nongrid exposure using 2
mAs and next wants to use an 8:1 ratio grid, what mAs should be used to
produce a comparable-quality radiograph?
mAs grid
The Grid Conversion factor =
mAs non-grid
mAs grid
4 =
2
8 =
mAs grid
Changing the Grid Ratio
If a radiographer produces a knee radiograph with a 8:1 grid and exposure
using 2 mAs and next wants to use an 16:1 ratio grid, what mAs should be
used to produce a comparable-quality radiograph?
mAs new
GCF new
=
mAs old
GCF old
mAs new
6
=
2 mAs
4
mAs new
=
12 mAs/4
mAs new
=
3 mAs
Types of Grid Cutoff Errors
Grid cutoff can occur as a result of four types of errors in grid use. To
reduce or eliminate grid cutoff, the radiographer must have a thorough
understanding of the importance of proper grid alignment in relation to the
image receptor and x-ray tube.
UPSIDE-DOWN FOCUSED
Upside-down focused grid cutoff occurs when a focused grid is placed upside-down on
the image receptor, resulting in the grid lines going opposite the angle of divergence of
the x-ray beam. This appears radiographically as significant loss of density along the
edges of the image Photons easily pass through the center of the grid because the lead
lines are perpendicular to the image receptor surface. Lead lines that are more peripheral
to the center are angled more and thus absorb the transmitted photons.
OFF-FOCUS
Off-focus grid cutoff occurs when using an SID outside of the recommended focal
range. Grid cutoff occurs if the SID is less than or greater than the focal range. Both
appear the same radiographically as a loss of density at the periphery of the film
OFF-CENTER
Also called lateral decentering, off-center grid cutoff occurs when the central ray of the xray beam is not aligned from side to side with the center of a focused grid. Because of the
arrangement of the lead lines of the focused grid, the divergence of the primary beam does
not match the angle of these lead strips when not centered. Off-center grid cutoff appears
radiographically as an overall loss of density.
OFF-LEVEL
Off-level grid cutoff results when the x-ray beam is angled across the lead strips. It is the
most common type of cutoff and can occur from either the tube or grid being angled. Offlevel grid cutoff can often be seen with mobile radiographic studies or horizontal beam
exams and appears as a loss of density across the entire image. This type of grid cutoff is the
only type that occurs with both focused and parallel grids.