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X-Ray Tube Construction: Principles and Components

Understand the construction of x-ray tubes, including components like the housing and electrodes. Learn about the principles behind x-ray production, including the role of electrons and the requirements for producing x-rays. Discover how characteristic radiation and Bremsstrahlung contribute to x-ray emission. Explore exposure parameters and beam intensity in relation to filament voltage and high voltage sources. Gain insights into focal spots, resolution limits, and heat ratings.

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X-Ray Tube Construction: Principles and Components

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  1. Resident Physics Lectures (1st Year) • Christensen, Chapter 2A X-Ray Tube Construction George David Associate Professor Department of Radiology Medical College of Georgia

  2. * X-Ray Tube • Converts Energy • FROM • electrical energy • To • Heat • > 99% of incident energy • X-Rays • < 1% of incident energy • Good! Our desired product

  3. * Heat and X-Ray Tubes • Bad! Ultimately destroys tubes • Limits single exposure beam intensity • Limits ability to make repeated exposures over time

  4. * X-Ray Tube Components • Housing • Visible part of tube • Glass Enclosure(insert) • Vacuum • Electrodes • Cathode • Filament • Anode • Target

  5. * Tube Housing • Shields against leakage radiation • Electrical insulation

  6. Tube Housing Filled with Oil • Helps cool tube • Electrical insulation • Bellows on end of tube allows oil to expand when hot. Insert

  7. filament target Inside the Glass Insert • Filament • Similar to light bulb • Glows when heated • Target • Large (usually) tungsten block

  8. * X-Ray Tube Principle • Filament heated • Electrons released (“boiled” off) • Thermionic emission

  9. + * X-Ray Tube Principle • Positive (high) voltage applied to anode relative to filament • electrons accelerate toward anode target • Gain kinetic energy • electrons strike target • electrons’ kinetic energy converted to • heat • x-rays

  10. + Electron Role • Electron carries energy as kinetic energy • Higher energy electron moves faster • Electrons controlled by electric fields • Accelerated • Steered

  11. Requirements to Produce X-Rays • Filament Voltage • High Voltage anode filament filament voltage source + high voltage source

  12. Cathode (filament) • Coil of tungsten wire • similar to light bulb filament • Cathode is source of electrons • filament heated by electric current

  13. X-Ray Production(cont.) • X-Rays produced by 2 distinct processes • Characteristic radiation • Bremsstrahlung

  14. Characteristic Radiation Interaction of high speed incident electron with orbital electron of target • #1: Electron from filament removes inner-shell orbital electron from atom • #2: electrons from higher energy shells cascade down to fill vacancies • #3: characteristic x-ray emitted L K - + ~ + ~ + ~ #1 - Electron from Filament - - #2 - #3

  15. L # K - + ~ + Energy ~ + ~ - - - Characteristic Radiation • Consists only of discrete x-ray energies • Energies correspond to difference between electron shells of target atom • Specific energies characteristic of target material

  16. L K - + ~ + ~ + ~ - - Bremsstrahlung • interaction of • moving electron from filament • nucleus of target atoms • + nucleus causes moving electron to change speed / direction • Electron slows down (loses kinetic energy) • Energy lost in form of Bremsstrahlung x-ray Electron from Filament -

  17. L K - + ~ + ~ + ~ - - - Bremsstrahlung (cont.) • Bremsstrahlung means braking radiation • Moving electrons have many Bremsstrahlung interaction • small amount of energy lost with each interaction

  18. Bremsstrahlung (cont.) • Random energy loss of moving electron • Depends on • distance from nucleus • charge (Z) of nucleus • Bremsstrahlung Energy Spectrum 0 - peak kilovoltage (kVp) applied to x-ray tube • most Bremsstrahlung photons have low energy • Don’t escape tube • easily filtered by tube or filters outside tube # Energy

  19. # Energy Output Beam Spectrum • Output photon beam consists of • Characteristic Radiation • characteristic of target material • several discrete energies • Bremsstrahlung • continuous range of energies • 0 - kVp setting • most photons have low energy • Spectrum • depicts fraction of beam at each energy # Energy

  20. Tube Current (mA) • rate of electron flow from filament to target • Electrons / second • Measured in milliamperes (mA) • Not the same as filament current +

  21. Exposure Parameters • kVp • High voltage applied to x-ray tube • mA • Rate of electron flow from cathode to anode during exposure • Time • Duration of x-ray exposure • mAs • Product of mA & time

  22. Beam Intensity • Product of • # photons in beam • Photon energy spectrum • Units • Roentgens (R) per unit time • Measure of ionization rate of air • Depends on • kVp • mA • target material • filtration

  23. Intensity & Technique • Beam intensity proportional to mA • Beam intensity ~ proportional to kVp2 filament voltage source + high voltage source

  24. + Focal Spot • Portion of anode struck by electron stream • Focal spot size affects & limits resolution

  25. Focal Spots • Most tubes have 2 filaments & thus 2 focal spots • only one used at a time • small focus • improves resolution • large focus • improves heat ratings • Electron beam strikes larger portion of target

  26. Focal Spot Size & Resolution The larger the focal spot the more it will blur a tiny place on the patient.

  27. Larger Focal Spot = Better Heat Ratings Electron beam applies huge amount of heat to target

  28. Larger Focal Spot = Better Heat Ratings The larger the area the electron beam hits, the more intense the beam can be without melting the target

  29. + Target Angle, Q Target Angle • Angle between target & perpendicular to tube axis • Typically 7 – 15 degrees

  30. + Target Angle Line Focus Principle • Actual (true) focal spot • as seen from filament • Apparent (effective, projected) focal spot • as seen from tube port or patient • Target angled 7-12o + Actual FS Apparent FS Patient

  31. Target Angle • Large • poorer heat ratings • better field coverage • Small • optimizes heat ratings • limits field coverage Large Target Angle (Small Actual Focal Spot) Small Target Angle (Large Actual Focal Spot) + + Same Apparent Focal Spot Size

  32. Heel Effect • Intensity of x-ray beam significantly reduced on anode side • beam goes through more target material exiting anode on anode side - x - - cathode side anode side

  33. Anodes • Stationary • Rotating • Target is annular track • spreads heat over large areaof anode • speeds • 3600, 9600 rpm • Faster = better heat ratings

  34. Rotating Anode • Advantages • better heat ratings • Disadvantages • More complex ($) • Rotor drive circuitry • motor windings in housing • bearings in insert

  35. Rotating Anode • Larger diameter • Better heat ratings • Heavier • $$$ • Materials • usually tungsten

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