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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
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 devices1
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 size1
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 size2
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 size3
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 devices1
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 diaphragm1
Aperture Diaphragm
  • Apertures are the simplest form of collimation.
  • In this case, the aperture is used to reduce exposure to the breast tissue.
aperture diaphragm2
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 cylinders1
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 collimator1
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.
collimator
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.
collimator1
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.
collimator2
Collimator
  • Light localization is accomplished by a small projector lamp and mirror to project the setting of the shutters on the patient.
collimator3
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.
collimator4
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.
collimator5
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.
collimator6
Collimator
  • 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.
collimator7
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.
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.
end of lecture

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