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Beam-Restricting Devices
• Three factors contribute to an increase in scatter radiation:
• Increased kVp
• Increased Field Size
• Increased Patient or Body Part Size.
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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.
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Relative Contributions of Scatter to the Radiographic Image
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Percent Interaction of Scatter and Percent Transmission through 10 cm of Tissue
kVp Photoelectric Compton Total %
Transmission
60 70 % 30% >99% <1%
70 60 % 40% > 99% < 1%
80 46% 52% 98% 2%
90 38% 59% 97% 3%
100 31% 63% 94% 6%
110 18% 83% 93% 7%
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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.
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Beam-Restricting Devices
• There are three principal types of beam restricting devices:
• Aperture Diaphragm
• Cones & Cylinders
• Collimators
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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.
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Kilovolt Peak
• As x-ray energy increases, the relative numberof 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.
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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
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Field size
• Scatter radiation increases as the field size increases.
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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.
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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%.
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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.
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Tissue Thickness
• The relative intensity of scatter radiation increases with increasing thickness of the anatomy.
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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 with compression.
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Types of Beam Restricting Devices
• There are three types of beam restricting devices.
• Diaphragms
• Cones
• Collimators
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Types of Beam Restricting Devices
• Large field sizes result in more scatter radiation that reduces image contrast.
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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.
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Aperture Diaphragm
• Apertures are the simplest form of collimation.
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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.
• It is always designed for a particular film size and SID
• Chest room has a fixed aperture diaphragm
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Cones and Cylinders
• Cones and cylinders are modifications to the aperture.
• Cones are typically used in dental radiography.
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Cones and Cylinders
• Most cone produce a round image on a rectangular film.
• Cones are very effective at reducing scatter.
• Hard to center.
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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.
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Variable Aperture Collimator
• Proper collimation also reduces scatter radiation that improves contrast.
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Light Localizing Collimator
• The light localizing variable aperture collimator is the most common beam restricting device in diagnostic radiography.
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Collimator
• 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.
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Collimator
• 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.
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Collimator
• Light localization is accomplished by a small projector lamp and mirror to project the setting of the shutters on the patient.
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Collimator
• 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.
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Collimator
• 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.
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Collimator
• 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.
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Collimator
• If the beam is not centered to the film, collimator cut-off will occur on the top or bottom of the image.
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Collimator
• 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.
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Collimation Rules
• Collimation must be slightly less than film size or to the area of clinical interest, whichever is smaller.