Film Processing

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Film Processing

• Film exposure
• Film processor
• Artifacts
• Quality assurance

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Film emulsion

• Silver halide crystals used for film emulsions
  have defects in their crystal structure
  introduced by silver sulfide (AgS)
• This defect gives rise to its optical properties
  in the region of a sensitivity speck
• The sensitivity speck is essentially a protrusion
  of positive charge that reaches the surface of
  the crystal

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Conceptual illustration of tabular grain silver
halide crystals
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Latent image

• Loosely bound electrons in the silver halide can
  absorb light energy and drift as free electrons
  in the emulsion
• The free electrons can come in contact with the
  positive sensitivity speck, where ionic silver is
  reduced to metallic silver by the free electron

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Development

• Silver halide crystals that have not been exposed
  in the emulsion are inert to the effects of the
  chemical developer
• Metallic silver atoms in those which have been
  exposed act as a catalyst for reduction of
  remaining silver ions in that crystal into a
  grain of metallic silver
• Each blackened grain contributes slightly to the
  overall OD
• Darker areas of the film have a higher
  concentration of grains (per square millimeter)
• Grains are 1 to 5 ?m in size

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Development stage
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Film processor

• Automatic film processors must deliver consistent
  performance
• Processor controls both the temperature and
  chemical concentrations
• The time that the emulsion spends in each of the
  chemical baths is regulated by the length of the
  film path through each tank, which is governed by
  the depth of the tank and by the film speed

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Functional components of a film processor
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Daylight section of a film processor
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Darkroom section of a film processor
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Roller assemblies (crossover racks removed)
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Development

• Developer solution includes
• Water
• Developing agents (hydroquinone and phenidone)
• Activator (sodium hydroxide)
• Increases pH of the solution causes gelatin to
  swell
• Restrainer (potassium halides)
• Limit development to only exposed silver halide
  crystals
• Preservative (sodium sulfite)
• Hardener (glutaraldehyde)

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Fixing

• Fixing solution includes
• Activator (acetic acid)
• Stop developer action by driving pH down
• Clearing agent (ammonium thiosulfate)
• Removes undeveloped silver halide from emulsion
• Hardener (potassium chloride)
• Shrinks and hardens emulsion
• Preservative (sodium sulfite)

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HD curve effects

• If developer concentration is too high,
  overdevelopment may occur.
• Higher ODs cannot be increased too much because
  the number of silver grains in the emulsion
  limits these OD values
• Underdevelopment occurs if the developer
  concentration is too low or if the temperature of
  the developer is too low

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Effects of processing on HD curve
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Other artifacts

• Water spots can occur on the film if the
  replenishment rates are incorrect, if the
  squeegee mechanism is defective, or if the dryer
  is malfunctioning
• A slap line is a plus-density line perpendicular
  to the direction of film travel that occurs near
  the trailing edge of the film. Caused by the
  abrupt release of the back edge of the film as it
  passes through the developer-to-fixer crossover
  assembly

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Other artifacts (cont.)

• Pick-off artifacts are small, clear areas of the
  film where emulsion has flecked off the film
  base. Can be caused by rough rollers, nonuniform
  film transport, or a mismatch between film
  emulsion and chemicals
• Wet pressure marks occur when the pinch rollers
  apply too much or inconsistent pressure to the
  film in the developer or in the
  developer-to-fixer crossover racks

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Other artifacts (cont.)

• Several types of shoe marks can occur.
• Result when film rubs against guide shoes during
  transport
• Manifests as a series of evenly spaced lines
  parallel to the direction of film transport
• Plus-density marks are caused by the guide shoes
  in the developer tank
• Minus-density shoe marks often occur in the
  fixer-to-washer crossover

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Other artifacts (cont.)

• Runback artifacts look like fluid drips and occur
  at the trailing edge of a film
• No squeegee action from rollers between developer
  and fixer tanks
• As film descends into fixer tank, excess
  developer runs back at the trailing edge
• Oxidized developer on the developer-to-fixer
  crossover assembly can cause this artifact
• Chatter is a periodic set of lines perpendicular
  to the film transport direction caused by binding
  of the roller assembly in the developer tank or
  in the developer-to-fixer crossover assembly

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Processor quality assurance

• Because of their chemical nature and the degree
  of amplification that film processing performs,
  x-ray processors are typically the least precise
  instruments in the generation of diagnostic
  images
• Quality control of film processors is a standard
  of practice
• Purpose of processor QA is to recognize problems
  with processor performance before those problems
  cause a diagnostic exposure to be compromised

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Sensitometer

• A sensitometer is used to expose film to a
  standardized range of light exposures
• Sensitometer shines light through an optical step
  tablet, which is a sheet of film that has
  different density steps on it
• Used under darkroom conditions

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Schematic diagram of a film sensitometer
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Typical sensitometer

• Select blue or green light to match film emulsion
• Audible alert that exposure is finished

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Densitometer

• A densitometer is used to measure the OD on a
  film
• Used in standard room lighting conditions
• Film is placed in the aperture of the device,
  readout button is pressed, and the OD is displayed

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Functional components of a film densitometer
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Typical densitometer

• Interchangeable aperture plates
• Useful for general spot measurements from film

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Automatic scanning densitometer

• Automatic film feed
• Reads OD for all steps
• Printer or computer connection for data recording

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QA record from automatic scanning densitometer
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Monitoring film processor performance