Scatter Control _ Grid Use by maclaren1

VIEWS: 424 PAGES: 27

									Scatter Control
  Grid Use

    Denise Ogilvie
    October 2007
 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
 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.
           Scatter Radiation
 Scatter is radiation which is changed in
  direction as a result of interaction with some
 Some of the photon’s 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
              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
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
           Scatter Radiation
 Irradiated Material

 Amount of scatter affected by volume and
  atomic number of irradiated material
 Volume is controlled by field size and patient
 Increase in volume if field size increases
  and patient thickness increases.
           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
Scatter Radiation
           Scatter Radiation
 Beam Restriction

 Aperture diaphragms, cones/cylinders,
  collimators – 3 types of beam restricting
  devices to control scatter and reduce patient
               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
         Scatter Radiation
 Cones and Cylinders
 Similar to diaphragm
  with extension cone or
 Slides into slot bottom
  of collimator
 Reduces penumbra
Scatter Radiation
             Scatter Radiation
 Collimater
 More complex, most
  commonly used form of
  beam restriction
 Set of adjustable lead
 Light & mirror to show
  area of beam and
            Scatter Radiation

 The bevelled edges of
  lead diaphragm
  compared to vertical
          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
  don’t see the lines
 The more Pb the better the scatter reduction
          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
           Radiographic Grids
 Crossed grid – 2
  parallel grids on top of
  each other.
 May be parallel or
           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
           Radiographic Grids
 Grid Ratio
 Ratio of height of Pb
  lines to distance
  between them
 Grid ratio increases,
  contrast increases
          Radiographic Grids
 The higher the grid
  ratio the more
  exposure is required
           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
            Radiographic Grids
 Air gap technique
 Between patient and film
 Eliminates need of grid
 Gap of at least 15cm –
  increase SID to reduce
 The scatter from the body
  does not hit the IR
           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
           Radiographic Grids
 Off centre – tube not
  centred to middle of
 Result in decrease in
  exposure across entire
  image and visible grid
 The greater the
  decentering the
  greater the grid cut off
          Radiographic Grids
 Off level grid – tube
  angled across long
  axis of Pb strips
 Show grid lines with
  decrease in exposure
  on image

 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,

To top