Chapter 15 Radiographic Intensifying Screens And Cassettes

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Chapter 15 Radiographic Intensifying Screens And
Cassettes

• There are three key parts of the Image Receptor
  for Conventional Radiography
• Film to record the image
• Intensifying Screens to expose the film
• Cassette to protect the screens and film

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Radiographic Intensifying Screens

• Less than 1 of the incident x-rays interact with
  the film to contribute to the latent image.
• The intensifying screens converts the remnant
  radiation to light than produces the latent
  image. They act as an amplifier of the remnant
  radiation.

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Radiographic Intensifying Screens

• About 30 of the x-rays striking the screens
  interact with the screens producing a large
  number of visible light photons.
• The use of intensifying screens results in
  considerable lower radiation dose to the patient
  but has the disadvantage of causing a slight
  blurring of the image.

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Radiographic Intensifying Screens

• Most conventional radiographic cassettes have a
  pair of screens that sandwich the film. This
  design used double emulsion film.

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Screen Construction

• Four Distinct Layers
• Protective Coating
• Phosphor
• Reflective layer
• Base

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Protective Coating

• Coating is transparent to light.
• Resistant to abrasion and damage from handling.
• Resistant to static electricity
• Provide a surface for cleaning while protecting
  the phosphors.

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Phosphors

• The active layer of the screen is the phosphors.
• The phosphors emit light when stimulated by
  x-rays.
• Prior to 1970 the most common phosphor was a
  crystalline form of Calcium Tungstate.

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Phosphors

• Modern screens use rare earth elements such as
• Gadolinium
• Lanthanum
• Yttrium

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Properties of Phosphors used in Intensifying
Screens

• High atomic number so x-ray absorption will be
  high. Quantum Detective Efficiency
• Emit a large amount of light per x-ray
  absorption. Conversion Efficiency
• Light must be of proper wavelength to match the
  sensitivity of the film Spectral Matching

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Properties of Phosphors used in Intensifying
Screens

• Phosphor Afterglow should be minimal.
• Phosphor should not be affected by heat humidity
  or other environmental conditions

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Influences of the Action of Intensifying Screens

• Thickness of the phosphor Layer
• Concentration of the crystals
• Size of the crystals.

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Reflective Layer

• The light from the phosphors is emitted
  isotropically.
• Without a reflective layer, only half of the
  light would interact with the film.
• The reflective layer redirects the light to the
  film.

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Reflective Layer

• Some screens have special dyes that absorb the
  light photons coming at a large angles.
• These photons would increase the image blur.
• Only the photons perpendicular to the film are
  emitted. The dye increases spatial resolution but
  reduce speed.

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Base

• The base is the layer farthest from the film.
• It is usually made of polyester. The bas should
  be
• Rugged and moister resistant
• Can not be damaged by radiation or discoloration
• Chemically inert, flexible and free of
  impurities.

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Luminescence

• The x-ray photon is absorbed by the target atom.
• The outer shell electron is raised to an excited
  state.
• It returns to a ground state with emission of a
  light photon.

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Luminescence

• Any material that gives of light in response to a
  stimulus is a luminescent material.
• Two types of luminescent material.
• Fluorescent gives off light only during
  stimulus. Good for screens
• Phosphorescence continues to give off light
  after stimulus. Bad for screens called Lag or
  Afterglow.

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Properties of Screens

• Phosphor composition Rare earth screens are very
  efficient in conversion of x-ray to light.

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Properties of Screens

• Phosphor thickness The thicker the phosphor
  layer, the higher the number of x-rays converted
  to light.
• High speed screens have thick layer. Detail
  screens have a thin layer.
• Reflective layer will increase speed and blur

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Properties of Screens

• Dye Light controlling dyes are added to control
  the light spread to improve spatial resolution.
• Crystal size Larger crystal produce more light
  per interaction. Detail screens have small
  crystals.

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Properties of Screens

• Concentration of crystals The higher the
  concentration of crystals, the higher the speed.

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Image Noise

• Rare earth screens have increased speed for two
  reasons
• Detective Quantum Efficiency or the ability to
  absorb the photons ( High Z)
• Conversion Efficiency Amount of light emitted
  per x-ray.

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Image Noise

• Conversion Efficiency High conversion efficiency
  results in increases image noise.
• Noise appears as a speckled background.
• It occurs with fast screens and use of high kVp.
• The factors that make rare earth screens have
  greater speed also contribute to increased noise.
• Increased conversion efficiency results in lower
  exposure. Less x-rays results in an increased
  quantum mottle.

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Spatial Resolution

• Image detail is the result of spatial resolution
  and contrast resolution.
• Generally the conditions that increase speed
  reduce spatial resolution.

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Spatial Resolution

• When screens phosphor reacts with x-rays a larger
  area of the film is exposed than what would be
  exposed by radiation alone.
• This results in reduced spatial resolution and
  more blur.

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Spatial Resolution

• Direct exposure can resolve 50 lp/mm with a very
  small focal spot.
• High speed screens can resolve 7 lp/mm.
• Detail screens can resolve 15 lp/mm
• The unaided eye can resolve 10 lp/mm.

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Spatial Resolution

• High speed screens have thick layers of crystal
  and /or large crystals.
• High detail screens have a thin layer of small
  crystals.

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Screen film Combinations

• Screens in pairs and double emulsion film is the
  standard of the industry. Less than 1 of the
  image is produced by the x-ray photons.
• Each screen contributes relatively evenly in the
  production of the image.

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Cassettes

• The cassette is a rigid holder for the film and
  screens.
• It will contain some form of compression to push
  the film in close contact with the screens.
• The front of the cassette is made of a
  radiolucent material with low absorption
  characteristics.

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Cassettes

• The back of the cassette may contain some form of
  metal that can absorb x-rays that are not
  absorbed by the screens.
• Sometime with cassettes that do not adequately
  absorb the rays, back scatter will result from
  scatter radiation from the cassette holder or
  near by wall.

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Spectrum Matching

• For the screen to work at maximum efficiency, the
  light absorption characteristics of the film must
  be matched to the light emitted from the screens.
• This is called spectrum matching.

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Spectrum Matching

• Calcium Tungstate emits a broad blue spectrum.
• Rare earth emits a green spectrum.
• The film, screens and safelight must match.

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Asymmetric Screens

• Screens in the cassette can be of two types or
  speeds. Some people use two different speeds in
  cassette for full spine radiography.
• When types of screens are different, they are
  referred to as Asymmetric screens. One side may
  be high contrast and the other side wide
  latitude. The combined image is superior.

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Care of Screens and Cassettes

• High quality radiography requires that the
  screens be clean and free of artifacts.
• Avoid touching the screens with your hands.
• Clean the screens with screen cleaner.
• Do not slide the film in or out when loading the
  cassette.

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Care of Screens and Cassettes

• Keeping the dark room clean will help reduce dirt
  or dust getting into the cassette.
• Dont stack the cassette on top of each other as
  the weight can damage the cassette.
• Load the film completely in the cassette.

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Care of Screens and Cassettes

• Clean the screens at least quarterly. California
  requires monthly.
• Use only specially formulated screen cleaner with
  anti static properties.
• Never use alcohol to clean screens.
• Make sure they are dry before reloading with film.

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Examples of Screen Problems

• The hinge of the cassette has failed, resulting
  in a light leak.

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Cassette Screen Problems

• Card inside cassette

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Cassette Artifacts

• This cassette popped partly open.
• With cassette artifacts, think about how the
  cassette opens.
• If the cassette pops open do not use the film.

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Cassette Artifacts

• Dirty screens will appear as white spots on the
  film.
• This film also has some static electricity
  artifacts.

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Cassette Artifact Dirty Screens

• Dirty or damaged screens will cause white spots
  on the image.

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Dirty or Damaged Screens

• Dirty or damaged screen will cause white spots on
  the image.

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Dirty Damaged Screens

• The white spots on this film are the result of
  damaged or worn out screens.
• Never use alcohol or detergents to clean screens.

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Poor Screen Contact

• Poor screen contact will cause an an area of the
  image to appear cloudy and blurry. Common reasons
  for poor contact include
• Worn contact felt
• Loose, bent or broken hinges
• Loose bent or broken latches
• Warped screen

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Poor Screen Contact

• Common reasons for poor contact include
• Warped cassette front or frame.
• Sprung or cracked cassette frame.
• Foreign matter in the cassette.
• Screen contact is tested using a wire mesh test
  tool.
• The wire mesh is placed on top of the cassette.

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Poor Screen Contact

• A radiograph is taken and the film processed.
• The image is viewed from 2 to 3 meters from the
  view box.
• Poor contact will appear as a cloudy and blurry
  area on the film.

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Poor Screen Contact

• Test the cassette when they are purchased and
  then twice yearly.

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23.4 Screen Contact Testing

• Procedure
• Clean screens and let them dry. Use screen
  cleaner design for the screen used.
• With a felt tip pen, write an identification
  number on the screen next to the I.D. and on the
  back of the cassette.
• Load cassettes.

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Screen Contact Testing

• Procedure
• Set SID to 40 Table Top
• Place cassette on table.
• Place wire mesh tool on cassette.
• Set collimation to film size.
• Make exposure and process film.

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Screen Contact Testing

• Procedure
• Hang film on view box.
• Step back 72 from view box and view film.
• Areas of increased density or loss of resolution
  indicates poor contact or stained screens.

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Poor Screen Contact

• There is a loss of detail in the thoracic and
  lumbar spine due to poor screen contact.
• This was a new cassette.

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Poor Screen Contact

• Note the blurry image in the spine but sharp
  image of the ribs.
• The screens were not in proper contact in the
  middle of the cassette due to a bow in the
  cassette back.

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End of Lecture