Species: Canine   |   Classification: Miscellaneous

Cassette

  • 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.

Box

  • 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:
    • Aluminium.
    • 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 use , reflected from the table-top, passing through and affecting the film:
    • Steel with a lead foil lining.

Pressure pad

  • A soft felt pad.
  • Keeps screen in close contact with film once loaded into cassette.

    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.
Base
  • 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 cuts tube current).
    • Improve image quality.

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

Supercoat

  • A protective layer of cellulose acetate.

Screen speed

  • Screen "speed" depends on type of phosphor used and size of phosphor crystals.
  • Afasterscreeen will give more exposure of the film for a given x-ray exposure.
  • For a particular phosphur, 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.

    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.
  • Artefacts 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 re-loading the cassette with film.
  • Screens which are regularly cleaned and carefully handled should last about 10 years.