Species: Feline   |   Classification: Miscellaneous

Radiographic film properties and storage

Film speed
  • The term "speed" can be applied to films as well as screens   Radiography: cassette and intensifying screen  .
  • Afastfilm will become blacker for a given exposure than aslowfilm.
  • The speed of films is determined by the size and number of halide grains.
  • Larger grains give faster speedbut reduced resolution.
  • Speed is a relative value and cannot be compared across different manufacturers.
  • Speed is often used to describe a film-screen combination rather than one or the other in isolation.

Storage of x-ray films

  • Radiographic film is a delicate material and requires careful handling.
  • Ideal storage facilities:
    • Cool conditions (10-18°C).
    • Dry conditions (no more than 50% humidity).
    • Away from sudden changes in temperature and humidity.
    • Protected from light.
    • Protected from processing chemicals and their fumes.
    • Protected from ionizing radiation.
    • Film stored on edge (not stacked on top of one another as this may   →   pressure marks'.

Even when stored in ideal conditions film gradually deteriorates with age so stock should be rotated.

Film identification

  • It is essential that radiographs areclearly,correctlyandindeliblylabelled.
  • Information on each film should include
    • Owner and patient's name or identification number.
    • Date radiograph taken.
    • Hospital or practice name.
    • Anatomical marking ie left or right.
  • Methods of marking film:
    • "X-Rite" tape:
      • Self-adhesive, lead-impregnated tape on which information is written and then stuck to cassette before exposure.
      • Can be difficult to see on finished radiograph, especially if exposure has been high.

    This problem can be overcome by the use of special holders for the tape.

    • Lead numbers and letters:

      • Commonly used for anatomical marking but can also be used for identification.
      • Words formed from required combinations of letters and stuck to cassette before exposure.
      • Metal holders, into which the letters slide, are also available and can be stamped with practice name.

    • Light marker:

      • Photograph the identification information, written on a special piece of paper, onto a corner of the film which has been shielded from the main exposure by a lead blocker.
      • This marking can be done by manual marking of the film in the dark room or if cassette is inserted into machine before film is removed the identification window in the cassette automatically slides back and exposure is made.
    • Writingon the film after exposure (not ideal but better than nothing!).

Be sure to use permanent marker.

Types of radiographic film

  • Conventional x-ray film produces a negative image viewed with transmitted light.

Double-sided emulsion

  • See diagram of construction   Radiation physics: x-ray film construction  .

Film base

  • Provides the support for emulsion layers.
  • Usually polyester which has several appropriate properties:
    • Transparent.
    • Flexible.
    • Robust.
    • Chemically stable.
    • Non-flammable.

Substratum

  • Adhesive layer made from a mixture of gelatine and film base material.
  • Sticks emulsion to film base.

Emulsion

  • Consists of silver halide crystals (mainly silver bromide) suspended in gelatine.
  • Additives to emulsion may include
  • Color sensitizers:
    • Extend the spectral sensitivity of the silver halide.
    • Silver halides are naturally sensitive to x-rays, ultraviolet light and visible light up to the blue part of the spectrum).
    • Color sensitizers can extend this sensitivity to the green part of the spectrum.
    • This facilitates the use of green-emitting rare earthscreens   Radiography: x-ray film  .
    •  Monochromatic film  Radiation physics: film sensitivity  is sensitive up to the blue part of the visible spectrum.
    •  Orthochromatic film  Radiation physics: film sensitivity  is sensitive up to the green part of the visible spectrum.
    •  Panchromatic film  Radiation physics: film sensitivity  is sensitive to all parts of the visible spectrum.

It is important that safelights are chosen to complement the sensitivity of the film to prevent fogging.

  • Hardeners: make the emulsion more resistant to damage.
  • Bateriocides and fungicides.
  • Wetting agent: overcomes natural repulsion of gelatine for water allowing processing chemicals to penetrate the emulsion.

Supercoat

  • A thin layer of pure hardened gelatine to protect the emulsion.

Non-screen film

  • This is designed for use without intensifying screens   Radiography: cassette and intensifying screen  .
  • The image is formed solely by the interaction of x-rays with the film.
  • The emulsion layer is often thicker because x-rays can penetrate to a greater depth than light from intensifying screens and greater amounts of silver halide will partially compensate for lack of light to cause blackening.
  • This produces a greater depth of image.
  • Greater film latitude than screen film.

Some non-screen films are not suitable for development in an automatic processor as the cycle does not allow sufficient time for chemicals to penetrate the greater thickness of emulsion.

  • Supplied in light-proof, radiolucent envelopes.
  • Some dental non-screen film comes with an attached sachet of processing chemicals which are released onto film after exposure.
  • Uses:

Single-sided emulsion film

  • Constructed in a similar way to conventional film but emulsion is only coated onto one side of the film base.
  • Additional anti-halation layer between film base and emulsion to prevent light being reflected back into the emulsion from the base.
  • Single-sided emulsion film is often used in mammography cassettes containing only one screen which produce images of greater resolution than double-screen systems.
  • Uses:
    • Detailed orthopedic examination.
    • Duplicating film.
    • Multiformat cameras for hard-copy image production from CT or ultrasound.
    • Duplicating film produces a copy without reversal.
    • Acts in solarization region.
    • Exposed by light passing through original radiograph but the more light that reaches it, the lighter it becomes.

Polaroid film

  • This produces a positive image which is viewed with reflected light.
  • Processing facilities are not needed.
  • Difficult to read unless reader is very familiar with it.
  • Resolution limited.

Sensitometry

  • Study of emulsion's response to exposure.
  • Used to compare different films and intensifying screens.

Characteristic curve

  • Curve produced by subjecting film to a series of known exposures and plotting exposure againstdensity(amount of blackening).
  • An Aluminum step wedge is normally used to make exposures with gradually increasing thickness of wedge giving slight variations in density in adjacent areas.
  • Density is expressed as the ratio of (Log10 incident light ) divided by transmitted light.

The eye cannot perceive differences in density below 0.25 or above 2.8 so all information on radiograph must be recorded between these values.

Information obtained form characteristic curve

  • See characteristic curve graph   Radiation physics: characteristic curve  .
  • Basic fog: the inherent density of a film which has not been exposed.
    • Also called base fog or base + fog.
    • Caused by slight opacity of base material and small amount of density from development.
    • Increases with poor or extended storage.
    • Increases with increased development time or temperature.
  • Threshold: the point at which increase in exposure results in increased density on the film.
  • Maximum useful density: the density above which further density increases cannot be perceived by the human eye.
    • Ideally all densities on the film recorded in the straight line part of the graph so that good contrast is achieved.
  • D-max: maximum achievable density.
    • Beyond this point increased exposure does not cause an increased density.
  • Gamma: indicates the contrast of the film.
    • Measured from the straight line of the curve.
    • The gradient of the line indicates the contrast of the film   Radiation physics: characteristic curve - film latitude  .
    •  High gamma- a small increase in exposure produces a large difference in radiographic density.
    •  Low gamma- a larger exposure is required to achieve the same increase in density producing a low contrast film with highlatitude.
    • Film latitude is the range of density produced on a given film.

Not to be confused withexposure latitudewhich is the range of exposures producing a useful density on film.

  • Compare film speeds: a "fast" film requires less exposure to produce the same density change than a "slow" film   Radiation physics: characteristic curve - film speed  .

Film latitude and speed are inherent in the emulsion and CANNOT be altered by the radiographer.