Chapter 13 Radiographic Film

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Chapter 13 Radiographic Film

• Remnant Radiation the x-rays that interact with
  the x-ray film.
• Few of the original x-rays actually make the
  image. The remnant radiation is the image forming
  radiation that passes completely through the
  patient.

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Types of X-ray

• A- X-rays scatter by Compton interactions
• B- x-rays absorbed by photoelectric absorption
• C- X-rays that exit the patient without
  interaction.

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Remnant Radiation

• The beam started as a relatively uniform
  intensity as it exited the tube.
• Upon striking the patient, the beam is attenuated
  by the patient some were absorbed, others are
  scattered.
• Those that actually hit the film are referred to
  as the useful or remnant radiation.

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Remnant Radiation

• The remnant radiation consists of x-rays
  scattered away from the receptor and the useful
  beam.
• The film is sandwiched between radiographic
  intensifying screens in a protective cassette.

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Intensifying Screens Film

• The intensifying screens change the x-rays into
  visible light. The visible light exposes the
  radiographic film.
• Radiographic film is similar in construction and
  characteristics to photographic film.
• Its spectral response is different from
  photographic film but is mechanism of operation
  is the same.

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Film Construction

• Radiographic Film has two basic parts.
• Base
• Emulsion
• Most film has two layers of emulsion so it is
  referred to as Double Emulsion Film

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Film Construction

• An adhesive layer attaches the emulsion to the
  base.
• The emulsion is enclosed in a protective layer or
  overcoat.

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Radiographic Film Base

• Initially x-ray were taken on glass plates.
• In 1918 cellulose nitrate bases film replaced
  glass due to WWI and a shortage of glass.
  Cellulose Nitrate was flammable so x-ray film was
  a fire hazard. Several severe hospital fire were
  caused by the x-ray film.

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Radiographic Film Base

• 1920 Cellulose triacetate or safety base was
  introduced. Not as flammable.
• Polyester base replaced Cellulose Triacetate in
  the 1960s. Still used today. It is semi-rigid
  and about 150 to 300 µm thick.

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Emulsion

• The emulsion is the heart of the film. The x-rays
  or light from the intensifying screens interact
  with the emulsion and transfer information to the
  film
• The emulsion consists or a very homogeneous
  mixture of gelatin and silver halide crystals
  about 3 to 5 µm thick.

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Gelatin

• The gelatin is clear so it transmits the light to
  the silver halide crystals.
• It is porous so the processing chemicals can
  penetrate to the silver halide crystals.
• The primary function of the gelatin is to provide
  a support medium for the silver halide crystals
  by holding them in place.

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Silver Halide Crystals

• 98 Silver Bromide
• 2 Silver Iodide
• Tabular shape used most commonly for general
  radiography.
• About 1µm thick for screen film exposure.

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Silver Halide Crystals

• The differences in speed, contrast and resolution
  depend upon the process by which the silver
  halide crystals are manufactured and by the
  mixture of these crystals into the gelatin.
• Size and concentration of crystals have a primary
  influence on speed.

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Manufacture

• The manufacturers closely guard the mixture they
  use to manufacture their film.
• Manufacture is in total darkness with protection
  for radiation. From the time that the emulsion
  ingredients are brought together until the film
  is packaged, no light is present.

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The Latent Image

• The latent image is the invisible change in the
  silver halide crystals.
• The interaction between the photons and the
  silver halide crystals produces the latent image
  or manifest image.
• This interaction is sometimes referred to as the
  photographic effect.

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The Latent Image

• This process is not well understood and is the
  subject of considerable research.
• The following is the Gurney-Mott theory.

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Producing the Latent Image

• Radiation interaction releases electrons.
• Electrons migrate to the sensitivity center.
• At the sensitivity center, atomic silver is
  formed by attracting an interstitial silver ion.

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Producing the Latent Image

• The process is repeated many times resulting in
  the build up of silver atoms.
• The remaining silver halide is converted to
  silver during processing.

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Producing the Latent Image

• The resulting silver grain is formed.
• Silver halide that is not irradiated remain
  inactive. The irradiated and non-irradiated
  silver halide produces the latent image.

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Types of X-ray Film

• Two main types
• Screen film used with intensifying screens.
• Single emulsion- emulsion on one side of base.
• Double emulsion used with two screens.
• Direct exposure film or non-screen film.
• Special purpose Duplication, Cine, Dental

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Standard Screen-Film Sizes

• English Units
• 8 x 10
• 10 x 12
• 11 x 14
• 7 x 17
• 14 x 17
• 14 x 36

• SI Units
• 20 x 25
• 24 x 30
• 30 x 35
• 18 x 43
• 35 x 43

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Screen Film Factors

• Main factors to be considered when selecting film
• Contrast Speed
• Crossover
• Spectral matching
• Reciprocity Law
• Safelights

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Contrast

• Most manufacturers offer multiple contrast
  levels in their film lines.
• High contrast film has low latitude
• Medium contrast has medium latitude
• Low contrast has high latitude
• High contrast has small uniform grains
• Low contrast has larger grains and wide range in
  size.

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Speed

• The size and shape of the silver halide crystals
  are the main factors that determine speed.
• Faster speed films are almost always double
  emulsion.
• Light spectrum from screens must match to achieve
  optimum speed.

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Crossover

• Crossover is the exposure of an emulsion by light
  from the opposite-side radiographic intensifying
  screen.
• Modern tabular grain film with a dye or crossover
  control layer has reduced crossover.

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Spectral Matching

• The most important consideration in selecting
  screen film is spectral absorption matching.
• The material in the screens will determine the
  color of light emitted by the screens.
• Special dyes in the film are used to match the
  screens to film.

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Spectral Matching

• Calcium Tungstate screen emit blue and blue
  violet light. All film will respond to blue and
  violet light.
• Rare earth screens emit blue-green light. Green
  sensitive film must be used. It is referred to as
  orthochromatic film. It will respond to blue
  green light spectrums.

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Spectral Matching

• If the light spectrum does not match, there will
  be a significant loss of speed.
• Kodak Lanex Regular Screens are rated at 400
  speed with orthochromatic film and 200 speed with
  blue sensitive film.

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Reciprocity Law

• Exposure Intensity x time Constant Optical
  Density
• So mA x time (s) mAs
• As long as the product of mA and time are the
  same, the optical density should be the same.
  Right?
• Wrong !!!

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Reciprocity Law

• There are times when the reciprocity law does not
  work with screen film.
• Very short exposure times (1 ms) and long
  exposure times (1 second or more).
• The result is a loss of speed.

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Approximate Reciprocity Law Failure

• Exposure Time
• 1 ms
• 10 ms
• 100 ms
• 1 s
• 10 s

• Relative Speed ()
• 95
• 100
• 100
• 90
• 60

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Safelight

• Working with film in the darkroom requires
  special lighting to avoid exposure of the film.
• Filters are used to avoid exposure of the film.

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Safelights

• A red filter is used for blue-green sensitive
  film.
• The color is not the only concern, the wattage of
  the bulb and distance from the counter top is
  also very important.
• Maximum wattage is 15w.
• Distance 60 from counter top.

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Special Film Types

• Direct exposure once used for small body parts
  measuring less than 10 cm. Requires 10 to 100
  time more exposure. No longer used.
• Single emulsion film once used for extremities
  but now most extremity cassettes are double
  screen type. Again required more exposure.

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Special Film Types

• Mammography Film Only single emulsion film
  currently used in modern radiography.
• Laser Film Used in medical radiography with a
  laser printer for digital radiography, CT and
  MRI. Modern units are dry chemical printers.
  Similar to laser printers except image is printed
  on film.

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Special Film Types

• Duplication Film special single emulsion film
  used to copy x-ray films. Sensitive to UV or blue
  light. Never used in cassettes.
• Subtraction Film used in angiography to do
  subtraction where the bone is removed for better
  visualize the arteries. Not used in chiropractic.

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Special Film Types

• Spotfilm Special roll film of 70 to 105 mm width
  used in fluoroscopy in medical radiography. Can
  be processed in x-ray film processor.
• Cine film 16 mm or 135 mm black white film
  used in coronary angiography. Requires motion
  picture film processor.

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Handling and Storage of Radiographic Film

• X-ray film is a sensitive radiation detector and
  it must be handled in an area free of radiation.
• Film storage must be shielded.
• The darkroom adjacent to the x-ray room must be
  shielded.
• If film use is low more shielding may be required.

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Handling and Storage of Radiographic Film

• Improper handling of the film will result in poor
  image quality due to artifacts.
• Avoid bending, creasing or otherwise rough
  handling the film. Avoid sharp objects contacting
  the film.
• Hands must be clean and dry.
• Avoid hand creams, lotions or water free hand
  cleaners.
• Static electricity or a dirty processor can cause
  artifacts.
• Artifacts must be avoided.

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Handling and Storage of Radiographic Film

• Heat and Humidity must be controlled. Film is
  sensitive to heat and humidity from the time it
  is manufactured until the time it is viewed.
• Heat and humidity causes fog or a loss of
  contrast. Film should be stored at 20º C (68º F).
• Humidity should be between 40 and 60.

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Handling and Storage of Radiographic Film

• Light will expose the film. Film must be handled
  and stored in he dark.
• If low level diffuse light exposes the film, fog
  is increased.
• Luminous watches, cell phone and darkroom light
  leaks should be avoided.
• Bright light causes gross exposure.

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Handling and Storage of Radiographic Film

• Shelf life. All film is supplied in boxes with an
  expiration date.
• Most film is supplied in boxes of 100 sheets.
• The 14 x 36 size is supplies in 25 sheet boxes
  with each sheet interleaved with paper.
• The oldest film in stock should always be used
  first. Rotation is important.
• Expired will loose speed and contrast and have
  increased fog.

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End of Lecture