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

    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

    6. Bad Film-Screen Contact

    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 image tubes cesium iodide

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

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

    13. 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

    17. Fraction of Beam Absorbed By Screen Pair

    18. 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

    19. 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

    20. 2 Screens & Double-Emulsion Film Why use 2 thin emulsions rather than 1 thicker one? light produced closer to emulsion less light spread

    21. Crossover light from one screen exposes opposite emulsion

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

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

    27. Emission Spectrum Calcium Tungstate Somewhat continuous blue spectrum (430 nm wavelength) Gd2O2S:Tb narrower green spectrum (544 nm wavelength) most but not all rare earth screens emit predominantly green light

    28. 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 kV’s significant fall-off at lower kV’s (< 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

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