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Resident Physics Lectures

Screens: General Principles. Convert x-rays to lightmany light photons created per x-ray photon absorbed in screenLight photons have much less energylight from screen exposes filmfilm much more sensitive to light than to x-raysscreens substantially reduce patient doseFactor of 100'sscreen use

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Resident Physics Lectures

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    1. Resident Physics Lectures Christensen, Chapter 9 X-Ray Intensifying Screens

    2. Screens: General Principles 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 100’s screen use virtually universal

    3. 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

    4. Radiographic Cassette 2 screens produce more light Require less radiation to be used Requires two emulsions on film One above one below

    5. Double-Emulsion Film Advantages easier to manufacture emulsion shrinks when it dries 2 emulsions minimizes curling faster system Less radiation each emulsion optimally captures light produced by “its” screen

    6. Film-Screen Contact Test

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

    8. 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

    9. Newer Phosphors image tubes cesium iodide

    10. Screen Features Advantages over direct film exp. Drastically decreased patient dose (X 100’s) Shorter exposure times Configuration cassette sandwiches film between 2 screens

    11. Screen Construction plastic protective coat phosphor layer reflecting layer base support layer

    12. Screen Construction Protective Layer Plastic .7 - .8 mils thick Functions prevents static electricity provides physical protection provides surface suitable for cleaning Phosphor Layer phosphor crystals 1 - 4 mils thick

    13. 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 Base Layer Mechanical support cardboard or polyester plastic 7 - 10 mils thick

    14. Resolving Power Maximum # 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

    15. Imaging Process

    16. Fraction of Beam Absorbed By Screen Pair

    17. 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 increases absorption in diagnostic energy range

    18. Thicker Phosphor More absorption Increases speed Reduces patient exposure Diffusion of light causes unsharpness light travels further from point of origin in screen to film

    19. Crossover light from one screen exposes opposite emulsion

    20. Crossover poorer resolution light travels further, spreads more caused by incomplete absorption of light by adjacent emulsion

    21. 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

    22. Rare Earth Screens Much higher conversion efficiency than previous systems (20% vs. 5%) Produces about 4 X as much light

    23. Screen Efficiency ability of light emitted by phosphor to escape screen & expose film typically half of light emitted by screen does not reach film

    24. Emission Spectrum Screen’s light spectrum must match film’s color sensitivity optimize speed by matching film response to screen light

    25. 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

    26. 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

    27. 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

    28. 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 kV’s significant fall-off at lower kV’s (< 70) Phototimers must compensate

    29. Quantum Mottle Image noise determined by # of x-ray photons absorbed by screen quantum mottle dictates ultimate limit in speed

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