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?

The Grid Conversion factor =

mAs grid

mAs non-grid

4 =

mAs grid

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 old

mAs new GCF new

GCF old

=

mAs new

2 mAs=

6

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 FOCUSEDUpside-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-FOCUSOff-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-CENTERAlso called lateral decentering, off-center grid cutoff occurs when the central ray of the x-ray 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-LEVELOff-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. Off-level 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.