Presentation on theme: "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."— Presentation transcript:
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.
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.
Grids An extremely effective means for reducing scatter radiation that reaches the film is called a grid.
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.
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.
Grid Construction There are three important aspects of grid construction; Grid Ratio Grid Frequency Grid material
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)
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.
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 5:1 to 16:1 High ratio grids increase patient dose.
Grid Ratio 8:1 and 10:1 grids are the most popular ratios in general radiography. 8:1 grids are commonly found on single phase machines. 10:1 are often found on high frequency machines.
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.
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.
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.
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.
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.
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.
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.
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.
Grid Bucky Factor kVp limits by ratio. A 5:1 or 6:1 grid is limited to 80 kVp A 8:1 grid is limited to 90 kVp 10:1 or higher are used above 90 kVp.
Grid Bucky Factor Grid ratio mAs increasekVp increase No grid 1X 5:1 2X +8 to 10 8:1 4X +13 to 15 10:1 5X + 20 to 25 12:1 6X + 20 to 25 16:1 8X + 30 to 40
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.
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 it’s selectivity will be higher.
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.
Grid Types Three types of grids Parallel Linear Grids Crossed Grids Focused Linear Grids
Parallel Grid Cheap and easy to manufacture. Problem: Grid cutoff at the outer edge of the 14”X17” film. Cut off is most pronounced at short SID.
Parallel Grid Cut off distance = SID/ Grid Ratio. Parallel grids only reduce scatter in the direction of the grid lines.
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.
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.
Focused Grids Focused grids are designed to minimize grid cut off. The grid lines are angled to match the divergence of the beam.
Focused Grids Focused grids are marked with an intended focal range and the side that should be towards the tube.
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.
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.
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.
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.
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.
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.
Grid Cut-off Beam Angled If the tube is angled against the grid lines, grid cut-off will result.
Grid Cut-off Grid Angled If the grid is is not perpendicular to the beam, grid cut-off will result. Most common problem.
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.
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.
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.
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.
Air Gap Technique The neck is naturally this far away from the film. Exposure factor are comparable to an 8:1 grid. Significantly less exposure than using Bucky.
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