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Chapter 16 Beam-Restricting Devices. Three factors contribute to an increase in scatter radiation: Increased kVp Increased Field Size Increased Patient or Body Part Size . X-ray Interactions. a – some interact with the patient and are scattered away from the patient. b – some are absorbed

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Chapter 16 Beam-Restricting Devices

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chapter 16 beam restricting devices
Chapter 16 Beam-Restricting Devices
  • Three factors contribute to an increase in scatter radiation:
  • Increased kVp
  • Increased Field Size
  • Increased Patient or Body Part Size.
x ray interactions
X-ray Interactions
  • a – some interact with the patient and are scattered away from the patient.
  • b – some are absorbed
  • c - some pass through without interaction
  • d – some are scattered in the patient
  • c & d are image forming x-rays.
beam restricting devices
Beam-Restricting Devices
  • There are two principal means to reduce scatter radiation:
  • Beam Restricting Devices limit the field size to reduce scatter and primary radiation.
  • Grids to absorb scatter before it reached the image receptor.
beam restricting devices6
Beam-Restricting Devices
  • There are three principal types of beam restricting devices:
  • Aperture Diaphragm
  • Cones & Cylinders
  • Collimators
production of scatter radiation
Production of Scatter Radiation
  • Two kinds of x-rays are responsible for the optical density, or degree of blackening on a radiograph.
  • Those that pass through the patient without interacting called remnant ray.
  • Those that are scattered through Compton interaction.
kilovolt peak
Kilovolt Peak
  • As x-ray energy increases, the relativenumber of x-rays that undergo Compton Scattering increases.
  • The absolute number of the Compton interactions decrease with increasing energies but the number of photoelectric interactions decreases more rapidly.
field size
Field size
  • The size of the field or area being irradiated has a significant impact on scatter radiation.
  • Field size is computed in square inches or square cm
field size10
Field size
  • Scatter radiation increases as the field size increases.
  • The relative intensity of the scatter varies more when the field size is small than when the field is large.
field size11
Field size
  • When the field size is reduced, the resulting reduction in scatter will reduce the density on the image.
  • The mAs must be increased to maintain density.
  • The reduced scatter will improve contrast resolution resulting in improved image quality.
field size12
Field size
  • To change from a 14” x 17” to a 10” x 12” increase mAs 25%.
  • To change from a 14” x 17” to a 8” x 10” increase mAs 40%.
patient or part thickness
Patient or Part Thickness
  • More scatter results from imaging thick body parts compared to thin body parts.
  • There will be more scatter for a lumbar spine film compared to a cervical spine film.
  • As tissue thickness increases, more of the rays go through multiple scattering.
tissue thickness
Tissue Thickness
  • The relative intensity of scatter radiation increases with increasing thickness of the anatomy.
  • The amount of primary radiation also increases to compound the scatter.
patient thickness
Patient thickness
  • Normally body thickness is out of our control but we can change the method of imaging to improve image quality.
  • With obese patients, tissue thickness is reduced when taking the film recumbent due to compression.
  • Be sure and measure the patient recumbent.
types of beam restricting devices
Types of Beam Restricting Devices
  • There are three types of beam restricting devices.
  • Diaphragms
  • Cones
  • Collimators
types of beam restricting devices17
Types of Beam Restricting Devices
  • Large field sizes result in more scatter radiation that reduces image contrast.
aperture diaphragm
Aperture Diaphragm
  • Aperture diaphragms are basically lead or lead lines metal devices placed in the beam to restrict the x-rays emitted from the tube.
aperture diaphragm19
Aperture Diaphragm
  • Apertures are the simplest form of collimation.
  • In this case, the aperture is used to reduce exposure to the breast tissue.
aperture diaphragm20
Aperture Diaphragm
  • The width or size of the aperture is fixed and can not be adjusted.
  • The operator must be careful when placing the aperture in the beam.
cones and cylinders
Cones and Cylinders
  • Cones and cylinders are modifications to the aperture.
  • Cones are typically used in dental radiography.
cones and cylinders22
Cones and Cylinders
  • Most cone produce a round image on a rectangular film.
  • Cones are very effective at reducing scatter.
  • Hard to center.
variable aperture collimator
Variable Aperture Collimator
  • Proper collimation of the x-ray beam has the primary effect of reducing patient dose by restricting the volume of tissue irradiated.
variable aperture collimator24
Variable Aperture Collimator
  • Proper collimation also reduces scatter radiation that improves contrast.
light localizing collimator
Light Localizing Collimator
  • The light localizing variable aperture collimator is the most common beam restricting device in diagnostic radiography.
  • Not all of the x-rays are emitted precisely from the focal spot.
  • These rays are called off-focus radiation and they increase image blur.
  • First stage shutters protrude into the tube housing to control the off-focus radiation.
  • Adjustable second stage shutter pairs are used to restrict the beam.
  • Light localization is accomplished by a small projector lamp and mirror to project the setting of the shutters on the patient.
  • The light field and x-ray beam should match to avoid collimator cut-off.
  • A scale on the collimator is used to match the beam to the film size at fixed SID’s.
  • Many newer collimators a bright slit of light is provided to properly center the beam and the film.
  • Units manufactured between 1974 and 1994 has motorized shutters.
  • A sensor in the Bucky and the motor were used to automatically collimate the image to film size. This was called a positive-beam limiting (PBL) device.
  • Required by the FDA.
  • Requirement was repealed in 1994.
  • If the beam is not centered to the film, collimator cut-off will occur on the top or bottom of the image.
  • If the tube is not centered to the Bucky or the film is not pushed into the Bucky, side to side collimator cut-off will occur.
collimation rules
Collimation Rules
  • California required three borders of collimation to be seen on the film.
  • Collimation must be slightly less than film size or to the area of clinical interest, whichever is smaller.
  • ANY exposure beyond the film is unnecessary patient exposure.
chapter 17 the grid
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
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.
  • An extremely effective means for reducing scatter radiation that reaches the film is called a 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.
  • 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
Grid Construction
  • There are three important aspects of grid construction;
  • Grid Ratio
  • Grid Frequency
  • Grid material
grid ratio
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 ratio42
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 ratio43
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 ratio44
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
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 frequency46
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 200 lines per inch.
  • The advantage of high frequency grids is there are no objectionable grid lines on the image.
grid frequency47
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
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 material49
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 material50
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
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
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 factor53
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 factor54
Grid Bucky Factor
  • Grid ratio mAs increase kVp 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
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 selectivity56
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
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
Grid Types
  • Three types of grids
  • Parallel Linear Grids
  • Crossed Grids
  • Focused Linear Grids
parallel grid
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 grid60
Parallel Grid
  • Cut off distance = SID/ Grid Ratio.
  • Parallel grids only reduce scatter in the direction of the grid lines.
crossed grid
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 grid62
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
  • Focused grids are designed to minimize grid cut off.
  • The grid lines are angled to match the divergence of the beam.
focused grids64
Focused Grids
  • Focused grids are marked with an intended focal range and the side that should be towards the tube.
focused grids65
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
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 grid67
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 grid68
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
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
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
Grid Cut-off Beam Angled
  • If the tube is angled against the grid lines, grid cut-off will result.
grid cut off grid angled
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
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
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
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
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 technique77
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.
neutral lateral cervical spine
Neutral Lateral Cervical Spine
  • Part of all cervical spine series.
  • Hang the non-Bucky film holder on top of Bucky. Slide upper film holder up to place film in holder. Center holder and film to beam.
lateral cervical spine
Lateral Cervical Spine
  • At PCCW we use the air gap technique for taking the lateral cervical spine.
  • By using this technique, we save the patient 135.5 mrad exposure.