Week 4c Chapter 16 Beam Restricting Devices and Chapter 17 The Grid

Week 4c Chapter 16 Beam Restricting Devices and Chapter 17 The Grid

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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 .
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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 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 number 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.
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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. The relative intensity of the scatter varies more when the field size is small than when the field is large.
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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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The relative intensity of scatter radiation increases with increasing thickness of the anatomy. The amount of
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Week 4c Chapter 16 Beam Restricting Devices and Chapter 17 The Grid

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