Lecture 6 Image recording + processing

• What are the 4 layers of x-ray film pack?

  }  Waterproof, light proof outer packing

  }  Black card for physical strength (not to stop light)

  }  Lead back scatter shield (not for radiation protection but to keep the image clear of secondary shadows)

  }  Film
• Label the components of an x-ray film

  A) water and light proof outer cover

  B) lead foil back-scatter shield

  C) film with photographic emulsion

  D) black card, physical support and light shield

  
• What does a dental x-ray film consist of?

  polyester base, to maintain shape and form but remaining flexible, containing slight colour dye to stop excessive flaring of the image. On each side there is a scrubbing layer and an active emulsion (silver halide, as halides are sensitive to electromagnetic radiation, silver bromide in dental films)
• What makes the silver bromide an active emulsion?

  molecules are suspended in gelatine
• The x-ray film is dupitized; this means it has emulsion on both sides of the base. This arrangement allows a diagnostic level of density to be achieved at half of the dose that would otherwise be needed.
• what is the developer?

  reducing agent
• The latent image in photography is visible in the developer
• Developer, a reducing agent, donates electrons to materials
• Crystals in the film emulsion are surrounded by a negative charge which repels the electrons of the developer
• After exposure, crystals with dissociated molecules have a defect in the negative charge shield, called the sensitivity spec, allowing electrons from the developer to penetrate
• Silver released by exposure from the silver bromide has a positive charge and is attracted to the negative charge created by the electrons donated by the developer
• Further exposure causes the dissociation of more Silver-bromide molecules, making the image not permanent
• What are the 5 steps of processing an radiograph?

  Development, rinse/squeeze, fix, rinse/squeeze, wash, dry
• Why are automatic processing of x-ray films faster?

  }  The chemicals are more concentrated and faster acting

  }  The process takes place at higher temperatures

  }  A disadvantage is the two factors above will reduce the contrast of the image
• Intensifying screens are fluoroscopic materials the convert high energy x-ray photons into an increased number of light photons

  }  The purpose of the screen is to reduce the radiation dose to the patient.

  }  They do this because the emulsion is more sensitive to light than to x-rays, the increase in the number of photons gives the required density at lower doses
• What are some examples of digital image recording?

  • Direct (DDR) either charge coupled devices or wired systems
  • Indirect (CDR) photo phosphor plates
• Uses of Phosphor Plates: With indirect digital imaging the timing of the release of the lower energy photons is controlled by external influences. The image is stored as electrons held in a state of high energy excitation, they will remain here until released by another external influence.
• Film pros and cons:

  • records a wider range of grey scales
  • contains more image information
  • vulnerable to extreme environments
  • processing faults and artifacts common
  • COSHH regulations
• Digital pros and cons:

  • image manipulation (density and contrast)
  • ease of image transfer
  • good for teaching
  • no COSHH
  • sensors or plates are expensive and easily damaged
• Film processor QA: activity fall off due to fall in fluid levels, temperature, exhaustion of chemicals
• For digital phosphor plates stepped wedge tests are to identify plate artefacts rather than a change in the response to x-ray photons.
• What is the amount of background radiation due to dental radiography?

  5%
• how long for a full manual process?

  1hr
• perfect image?

  • long focal to film distance
  • long focal to object distance
  • short object to film distance
• moving object further from film causes magnification and unsharpness