Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 9
• X-Ray Intensifying Screens

George David Associate Professor Medical College
of Georgia Department of Radiology
2
Screens General Principles

Photon

• Convert x-rays to light
• many light photons created per x-ray photon
  absorbed in screen
• Light photons have much less energy
• light from screen exposes film
• film much more sensitive to light than to x-rays
• screens substantially reduce patient dose
• Factor of 100s
• screen use virtually universal

Screen
Light

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Radiographic Cassette

• light tight container for film
• holds film in tight contact with screens over
  entire surface
• gaps drastically increase image unsharpness
• All non-mammo cassettes use two screens
• One above film
• One below film

Cassette
Screens
Film
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Radiographic Cassette

• Two screens produce more light
• Less radiation required to achieve a given
  optical density
• Requires two emulsions on film
• One above one below

Cassette
Screens
Film
5
Double-Emulsion Film Advantages

• easier to manufacture
• emulsion shrinks when it dries
• Having two emulsions minimizes curling
• photographic advantage
• faster system
• two screens used
• each emulsion optimally captures light produced
  by its screen

double emulsion film
screens
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Bad Film-Screen Contact

• contact tested by imaging wire screen mesh placed
  on top of cassette
• poor contact areas appear fuzzy, dark

7
Radiographic Cassettes

• screens require regularly cleaning
• Dust, dirt, paper, hair,etc prevent screen light
  from reaching film
• Causes white dots on image

8
Radiographic Cassettes

• mammography cassettes can trap air between film
  screen when closed
• results in poor contact
• must allow time for air to bleed off
• 10 minutes

9
Fluorescence in Radiology

• Light emitted by crystals
• inorganic salts called phosphors
• older phosphor materials
• calcium tungstate
• original phosphor material used in radiology
• emits blue light
• zinc cadmium sulfide

10
Newer Phosphors

• film screens
• barium strontium sulfate
• yttrium
• rare earths
• gadolinium
• lanthanum
• tantalates

• image tubes
• cesium iodide

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Screen Features

• Advantages over direct film exp.
• Drastically decreased patient dose (X 100s)
• Shorter exposure times
• Configuration
• cassette sandwichesfilm between 2 screens

12
Screen Construction

• plastic protective coat
• phosphor layer
• reflecting layer
• base support layer

One screen
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Screen Construction

• Protective Layer
• applied over phosphor
• made of plastic
• approximately .7 - .8 mils thick
• Functions
• prevents static electricity
• provides physical protection
• provides surface suitable for cleaning
• Phosphor Layer
• contains phosphor crystals
• approximately 1 - 4 mils thick

14
Screen Construction

• Reflecting Coat
• reflects light emitted toward back of screen
• phosphors emit light in all directions
• not all screens have reflecting coat
• Reduces resolution
• made of white substance (titanium dioxide)
• 1 mil thick
• Base Layer
• Mechanical support
• cardboard or polyester plastic
• approximately 7 - 10 mils thick

15
Resolving Power

• Maximum number of line pairs (line space) per
  millimeter resolved by screen-film system
• line space have equal width
• Typical values
• Film
• 100 line pairs per mm
• Film / screen systems
• 10 line pairs per mm maximum

16
Imaging Process
Photon must be absorbed by a screen
Screen must emit light
Light must reach film
Light must expose film
Each step above has an associated efficiency
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Fraction of Beam Absorbed By Screen Pair
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Absorption Comparison

• Atomic Number
• tungsten of calcium tungstate higher than rare
  earth, more photoelectric interaction
• K-Edge
• tungsten 69.5 keV
• Yttrium 17 keV
• Barium 37 keV
• Lanthanum 39 keV
• Gadolinium 50 keV
• Lower K-edge greatly increasesabsorption in
  diagnostic energy range

Absorption
Photon Energy
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Thicker Phosphor

• Thicker phosphor increases absorption
• Increases speed
• Reduces patient exposure
• Diffusion of light causes unsharpness
• light travels further from point of origin in
  screen to film

Screen
Film
20
2 Screens Double-Emulsion Film

• Why use 2 thin emulsions rather than 1 thicker
  one?
• light produced closer to emulsion
• less light spread

X-Ray
X-Ray
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Crossover

• light from one screen exposes opposite emulsion

X-Ray
Top Screen
Top Emulsion
Film
Bottom Screen
Bottom Emulsion
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Crossover
X-Ray

• poorer resolution
• light travels further, spreads more
• caused by incomplete absorption of light by
  adjacent emulsion

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Intrinsic Screen Efficiency

• Efficiency of energy conversion from x-rays to
  light
• 5 for calcium tungstate
• 850 light photons per x-ray photon absorbed
• up to 20 for newer phosphors such as rare earth
• Can be as high as 45 for direct digital DR
  systems

24
Rare Earth Screens

• commercially available since 1973
• much higher conversion efficiency than Calcium
  Tungstate (20 vs. 5)
• rare earth produces about 4 times as many light
  photons per x-ray ray photon absorbed
• examples
• terbium-activated gadolinium oxysulfide
• thulium-activated lanthanum oxybromide

25
Screen Efficiency

• ability of light emitted by phosphor to escape
  screen expose film
• typically half of light emitted by screen does
  not reach film

26
Emission Spectrum

• Screens light spectrum must match films color
  sensitivity
• optimize speed by matching film response to
  screen light

27
Emission Spectrum

• Calcium Tungstate
• Somewhat continuous blue spectrum
• (430 nm wavelength)
• Gd2O2STb
• narrower green spectrum
• (544 nm wavelength)
• most but not all rare earth screens emit
  predominantly green light

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Intensification Factor

• exposure required without screen-----------------
  ----------------------------exposure required
  with screen
• for calcium tungstate
• intensification factor increases with kVp
• thicker body parts cause increase
• filtering raises effective kVp
• small number of x-ray photons interact directly
  with film
• negligible film darkening contribution

29
Screen Speed depends on

• phosphor layer thickness
• thicker screen
• faster
• poorer detail because of light spread or
  diffusion
• light produced further from film
• size of phosphor crystals
• presence or absence of light-absorbing dye
• dye reduced lateral light diffusion
• better resolution
• poorer efficiency (lower speed)
• phosphor efficiency

30
Ways to Increase Screen Speed

• increase thickness of phosphor layer
• Change to different phosphor material with higher
  absorption efficiency
• More absorption for given thickness
• Change to different phosphor material with higher
  conversion efficiency
• More light per absorption

31
Rare Earth Speed

• speed of rare earth screens vary as function of
  kV
• rare earth speed greatest at about 80 kV
• slight fall-off at higher kVs
• significant fall-off at lower kVs (lt 70)
• Phototimers must compensate

32
Quantum Mottle

• Image noise determined by of x-ray photons
  absorbed by screen
• quantum mottle dictates ultimate limit in speed