Intensfiyng screen

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Intensifying Screens

• RT 104

Joanna Miranda Celjhon Ariño
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Cassettes

• Cassettes serve 3 important functions
• Protect film from exposure to light
• Protect film from bending and scratching during
  use.
• Contain intensifying screens, keeps film in close
  contact to screen during exposure.

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CASSETTE or FILM HOLDER

• The CASSETTE is used to hold the film during
  examinations. It consist of front and back
  intensifying screens, and has a lead (Pb)
  backing. The cassette is light tight

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Cassette Features - Front

• Exposure side of cassette is the front.
• Made of radiolucent material easily penetrated
  by x-rays, lightweight metal alloy or plastic
  material made of resin.
• Intensifying screen mounted to inside of front.

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Cassette Features Back

• Back made of metal or plastic
• Inside back is a layer of lead foil prevents
  backscatter that could fog the film
• Inside foil layer is a layer of padding
  maintains good film/screen contact
• Back intensifying screen mounted on padding
• Has the ID blocker (patient identification)

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Image creation

• 1 of xray photons that leave patient
• Interact with phosphors of intensifying screens
• 100s of light photons created to make image on
  film
• Light photons expose silver halide crystals in
  the film emulsion
• Turn black metallic silver after procession

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Intensifying Screens

• Flat surface coated with fluorescent crystals
  called phosphors
• that glow, giving off light when exposed to
  x-rays.

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Intensifying Screens Phosphors

• RARE EARTH (emits green light)
• Developed in 1980s
• Most efficient most common in use today
• CALCIUM TUNGSTATE (blue light)
• Not as efficient

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Rare Earth Screens

• Gadolinium
• Lanthanum
• Yttrium
• Found in low abundance in nature

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Cardboard Cassettes

• Direct x-ray exposure to film required
• 25 to 400 times more radiation to create an
  image on the film
• BETTER DETAIL THAN FILM SCREEN (NO BLURRING OF
  IMAGE FROM LIGHT)
• ALL EXPOSURE MADE FROM X-RAY PHOTONS
• BIG DOSE TO THE PATEINT

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INTENSIFYING SCREENS

• DISADVANTAGES
• less detail than direct exposure
• (detail better with rare earth than
• calcuim tungstate screens)
• ADVANTAGES
• Reduce patient exposure
• Increase x-ray tube life

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Screen Construction

• Polyester plastic base support layer
• Phosphor layer active layer
• Reflective layer increases screen efficiency by
  redirecting light headed in other directions
• Protective coating

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Intensifying screens
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Phosphor Layer

• Active layer x-ray photons converted to light
  photons
• Photoelectric Effect

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Screen Speed

• A relative number that describes how efficiently
  x-rays are converted into usable light
• Ranges from 100 (slow) to 1200 (fast)

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Screen Speed

• Greater efficiency less exposure faster
• -Standard screen speed class of 100
• -200 screen speed is twice as fast
• Speeds for routine work 200 800
• Speeds for high detail 50 100
• Increasing speed also increases image noise

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SCREEN SPEEDS

• FASTER SPEED REDUCES PATIENT EXPOSURE
• FASTER SPEED - REDUCES IMAGE DETAIL INCREASES
  NOISE (LIGHT BLURING AROUND IMAGE)

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Technique Changes

• Relative Speed Film speed Screen speed
• mAs 1 RS 2
• mAs 2 RS 1
• They are inversely related

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Image Noise

• Speckled background on the image
• Caused when fast screens and high kVp techniques
  are used. Noise reduces image contrast
• The percentage of x-rays absorbed by the screen
  is the detective quantum efficiency (DQE)
• The amount of light emitted for each x-ray
  absorbed is the conversion efficiency (CE)

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SCREEN SPEEDS

• Quantum Mottle causing a grainy, mottled or
  splotch image
• Often results of using very fast-speed
  screen-film systems

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• The light photons generated in the intensifying
  screen are emitted by phosphor crystals.
• These crystals are significantly larger than the
  silver halide crystals in the film
• use of a screen reduces image sharpness somewhat
• Some examinations requiring extremely fine detail
  use screens with small crystals.

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Image Quality
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Rare Earth Screens

• Have higher DQE (detective quantum efficiency).
  Higher x-ray absorption abilities.
• Have higher CE (conversion efficiency). More
  light emitted per x-ray absorbed by the screen.

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Spatial resolution

• The use of intensifying screens lowers spatial
  resolution compared with direct-exposure
  radiographs.
• Spatial resolution
• expressed by the
• number of line pairs
• per millimeter
• (lp/mm)

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The higher the lp/mm the smaller the object that
can be imaged

• Very fast screens 7 lp/mm
• Fine-detail screens 15 lp/mm
• Direct-exposure screens 50 lp/mm

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Screen speed vs Spatial resolution
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Wire mesh test check for screen-film contact.
Good contact
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Wire mesh test check for screen-film contact.

• Warped cassette poor contact

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Care of Screens

• Image artifacts can appear if screens are
  modified
• Small scratches can leave artifacts
• Dirty screens can leave artifacts
• Screens should be cleaned once each month with
  manufacturers cleaner with antistatic compounds

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Questions?