Radiographic Intensifying Screens

1
Radiographic Intensifying Screens

• 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

2
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.

3
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.

4
Radiographic Intensifying Screens

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

5
Screen Construction

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

6
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.

7
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

9
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

10
Properties of Phosphors used in Intensifying
Screens

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

11
Influences of the Action of Intensifying Screens

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

12
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.

13
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.

14
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.

15
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.

16
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.

17
Properties of Screens

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

18
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

19
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.

20
Properties of Screens

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

21
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.

22
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 incresed
  quantom mottle.

23
Spatial Resolution

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

24
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.

25
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.

26
Spatial Resolution

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

27
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.

28
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.

29
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.

30
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.

31
Spectrum Matching

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

32
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.

33
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.

36
Examples of Screen Problems

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

37
Cassette Screen Problems

• Card inside cassette

38
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.

40
Cassette Artifact Dirty Screens

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

41
Dirty or Damaged Screens

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

42
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.

43
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

44
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.

45
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.

46
Poor Screen Contact

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

47
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.

48
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.

49
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.

50
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.

51
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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