Species: Feline   |   Classification: Miscellaneous


  • A radiographic image may be recorded in a number of ways:
    • X-ray film.
    • Digitally.
    • Camera.
    • Cine or video film.
  • The radiographic image is formed when x-rays passing through a patient and are selectively absorbed by structures within the patient.
  • The remaining x-rays fall on the film.
  • The image can be considered a shadowgram  Radiation physics: image production - shadowgram  .
  • The film is usually held in a cassette which contains, in addition, a pair of intensifying screens.
  • Standard sizes:
    • 18 x 24 cm.
    • 24 x 30 cm.
    • 30 x 40 cm.
    • 35 x 35 cm.
    • 35 x 43 cm.

Cassettes can be easily damaged by being dropped or mishandled.

Cassette construction

  • See diagram   Radiation physics: cassette construction  .


  • Conventional cassette is a light-proof box, hinged at one side.
  • Usually steel frame.
  • Fastenings secure but easy to use in the dark room.
  • The front is radiolucent made of:
    • Aluminum.
    • Plastic.
    • Carbon fiber.
  • Corner window where a piece of lead prevents radiographic exposure of film to allow for marking   Radiography: x-ray film  .
  • The back is radio-opaque to prevent scattered x-rays   Radiography: scattered radiation and grid  , reflected from the table-top, through and affecting the film:
    • Steel with a lead foil lining.

Pressure pad

  • A soft springy pad.
  • Keeps screen in close contact with film once loaded into cassette.
  • Film screen contact can be checked by placing chickenwire on cassette and exposing - if image is blurred in places, then contact is poor.

If cassette is bent or dented close contact between the screens and the film is lost resulting in image unsharpness   Radiography: image quality  .

Intensifying screen

  • Pressure pad keeps screen sprung against film.
  • If double-sided x-ray film is being used two screens will be present and the film is sandwiched between them.

Intensifying Screen

  • Screens convert incident x-ray photons to light which is mainly responsible for exposing the film.
  • Vastly reduces the amount of x-rays necessary to produce a visible image compared to non-screen film   →   reduced x-ray doses and exposure times.

Screen construction

  • See diagram of screen construction   Radiation physics: intensifying screen construction  .


  • Usually polyester
    • Inert.
    • Strong.
    • Chemically stable.
    • Radiolucent.

Reflecting layer

  • White pigment, spread evenly over screen, which reflects light from phosphor towards film.
  • Only present in high-speed screens.

Phosphor layer

  • A phosphor converts x-rays to light ( fluorescence).
  • ie absorbs electromagnetic radiation of a short wavelength and instantly re-emits it at a longer wavelength.
  • Wavelength (color) of light produced depends on the particular phosphor.
  • Formerly
    • Calcium tungstate.
    • Barium lead sulfate.
  • More modernrare-earthscreens are four times more efficient at producing light from x-rays and contain
    • Gadolinium oxysulfide.
    • Lanthanum oxybromide.
    • Yttrium tantalate.
  • Rare earth screens
    • Permit reduction in exposure factors by up to 90% (which reduces dose to patient and can cut tube current or exposure times).
    • Improve image quality.

Rare earth phosphors are less likely to produce light from scattered radiation so contrast is improved.


  • A protective layer of cellulose acetate.

Screen speed

  • Screen "speed" depends on type of phosphor used and size of phosphor crystals.
  • Afasterscreen will give more exposure of the film for a given x-ray exposure.
  • For a particular phosphor, screen speed increases and resolution diminishes with the size of crystals.
  • Intensification factoris the ratio of exposure without screens to exposure with screens to produce the same density on the radiograph.

Spectral emission

  • Different phosphors emit light of different wavelengths (colors) and it is important to match film's spectral sensitivity to emission spectrum of the screens   Radiation physics: screen spectral sensitivity    Radiation physics: screen spectral sensitivity  .

Failure to do this makes system very inefficient   →   under-exposure of films.

  • Calcium tungstateandLanthanumemit predominantly blue light   →   use with blue-sensitive (monochromatic) film   Radiation physics: film sensitivity  .
  • Gadoliniumemits predominantly green light   →   use with green-sensitive (orthochromatic) film   Radiation physics: film sensitivity  .
  • Yttriumemits predominantly ultra-violet light   →   use with UV-sensitive film.

Care of Intensifying Screens

  • Screens are very susceptible to damage from
    • Scratching.
    • Denting to the cassette.
    • Water splashes.
    • Chemical splashes.
  • Screen damage results in film artifacts - usually white marks   Radiography: film faults  .
  • Artifacts can also result from foreign bodies eg hairs or paper fragments in the cassette at time of exposure .
  • Regularly cleaning (weekly) is necessary using proprietary screen cleaners.
  • In addition these solutions inhibit the build up of static electricity on screens   →   static artifacts on films .
  • The cleaning solution is used to dampen a soft cloth which is wiped over the surface of the screen.
  • A dry swab is used to dry the screen.

Leave open in clean environment to air dry before reloading the cassette with film.

  • Screens which are regularly cleaned and carefully handled should last about 10 years.