Scatter Control

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Scatter ControlGrid Use

• Denise Ogilvie
• October 2007

2
Objectives

• Identify factors that affect the amount of
  scatter radiation produced
• Describe methods used to control the amount of
  scatter radiation
• Describe the effect of beam restriction on image
  quality and patient dose
• Compare advantages and disadvantages of different
  beam restricting devises

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Objectives

• Describe the purpose of a grid
• Explain the construction of a grid, including
  materials used, grid ratio and grid frequency
• Differentiate between parallel and focused grids,
  stationary and moving grids
• Calculate changes in technical factors to
  compensate for changes in grid selection
• Be able to identify common errors made when using
  a grid on an image
• Know when to use a grid and when not to use.

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Scatter Radiation

• Scatter is radiation which is changed in
  direction as a result of interaction with some
  medium.
• Some of the photons energy is absorbed, leaving
  the resultant photon with a change in its
  direction and with less energy
• These scattered photons are detrimental to
  contrast of the image and also increase the
  patient dose

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Scatter Radiation

• Other sources of scatter materials beyond the
  image receptor table top may cause scatter to
  go back to the image.
• Two primary factors affecting the amount of
  scatter produced kvp and the irradiated material

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Scatter Radiation

• Kvp
• Affects the penetrability of the beam.
• Higher kVp, more photons go through patient to
  the IR, less absorbed by patient, higher scatter
  and less contrast on image
• Lower the kVp, increase in dose absorbed by
  patient, less fog on film, more contrasty image

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Scatter Radiation

• Irradiated Material
• Amount of scatter affected by volume and atomic
  number of irradiated material
• Volume is controlled by field size and patient
  thickness
• Increase in volume if field size increases and
  patient thickness increases.

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Scatter Radiation

• To reduce scatter smallest field size,
  compression of body part
• The higher the atomic number of the material the
  greater the absorption of photons and the less
  scatter eg bone compared to soft tissue

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Scatter Radiation
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Scatter Radiation

• Beam Restriction
• Aperture diaphragms, cones/cylinders, collimators
  3 types of beam restricting devices to control
  scatter and reduce patient dose

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Scatter Radiation

• Aperture Diaphragm
• Simplest, low cost
• Flat piece of lead with hole (of different sizes)
• Slides into slot at bottom of collimator
• Some resultant penumbra

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Scatter Radiation

• Cones and Cylinders
• Similar to diaphragm with extension cone or
  cylinder
• Slides into slot bottom of collimator
• Reduces penumbra

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Scatter Radiation
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Scatter Radiation

• Collimater
• More complex, most commonly used form of beam
  restriction
• Set of adjustable lead shutters
• Light mirror to show area of beam and
  collimation

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Scatter Radiation

• The bevelled edges of lead diaphragm compared to
  vertical edge.

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Radiographic Grids

• A device to absorb scatter radiation before it
  strikes the IR
• Made of thin Pb strips interspaced with
  radiolucent material usually aluminium
• Frequency number of lines per inch or cm eg 60
  lines per inch
• Grids with higher frequency have thinner Pb
  strips better for stationary grids so you dont
  see the lines
• The more Pb the better the scatter reduction

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Radiographic Grids

• Types
• Parallel Pb interspace running parallel to
  one another
• Focused central strips parallel, then become
  more angled as you move away from the centre
  angle matching that of divergent rays allows
  more transmitted photons to reach the IR

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Radiographic Grids

• Crossed grid 2 parallel grids on top of each
  other.
• May be parallel or focused

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Radiographic Grids

• Focal range recommended SID for that particular
  grid.
• For parallel grid focal range is from certain SID
  to infinity function better at longer SID

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Radiographic Grids

• Grid Ratio
• Ratio of height of Pb lines to distance between
  them
• Grid ratio increases, contrast increases

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Radiographic Grids

• The higher the grid ratio the more exposure is
  required

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Radiographic Grids

• Potter Bucky moving grid for better scatter
  clean up and improved image quality
• Grid is moved during the exposure to blur out
  grid lines.
• Movement must commence before exposure can be made

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Radiographic Grids

• Air gap technique
• Between patient and film
• Eliminates need of grid
• Gap of at least 15cm increase SID to reduce
  magnification
• The scatter from the body does not hit the IR

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Radiographic Grids

• Grid Errors
• Upside down grid peripheral grid cut off with a
  focus grid
• Check front of grid upper side has line down
  centre indicating direction of grid lines

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Radiographic Grids

• Off centre tube not centred to middle of grid.
• Result in decrease in exposure across entire
  image and visible grid lines
• The greater the decentering the greater the grid
  cut off

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Radiographic Grids

• Off level grid tube angled across long axis of
  Pb strips
• Show grid lines with decrease in exposure on image

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References

• Burns, E, Radiographic imaging a guide for
  producing quality images, Saunders 1992 1st edn
• Carlton, R, Adler, A, Principles of radiographic
  imaging an art and a science, 4th edn
• Fauber, T, Radiographic imaging exposure, 2000
• Kodak, The fundementals of radiography,11th edn
• Stockley, S, A manual of radiographic
  equipment,1st edn, 1986