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Resident Physics Lectures

Resident Physics Lectures. Christensen, Chapter 2A X-Ray Tube Construction. George David Associate Professor Department of Radiology Medical College of Georgia. *. X-Ray Tube Components. Housing Visible part of tube Glass Enclosure (insert) Vacuum Electrodes Cathode Filament Anode

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Resident Physics Lectures

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

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

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

  4. * Tube Housing • Shields against leakage radiation • lead lined • leakage limit • 100 mR / hour when tube operated at maximum continuous current for its maximum rated kilovoltage

  5. Tube Housing (cont.) • Shields against high voltage • electrically grounded • high voltage cable receptacles (wells) • housing filled with oil • cools • electrical insulation • all air removed • bellows • on end of tube • allows oil to expandwhen hot. Vacuum Oil Insert

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

  7. * X-Ray Tube Principle • Filament heated • electrons gain energy • electrons freed (“boiled” off) • Thermionic emission - -

  8. + * 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

  9. + keV = kilo-electron volt • energy of an electron • Kinetic energy • Higher energy electron moves faster • Electrons can be manipulated by electric fields • Accelerated • Steered

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

  11. Cathode (filament) • Coil of tungsten wire • similar to light bulb filament • Tungsten advantages • high melting point • little tendency to vaporize • long life expectancy • Tungsten disadvantages • not as efficient at emitting electrons as some other materials

  12. Cathode (filament) • Cathode is source of electrons • filament heated by electric current • ~ 10 volts • ~ 3-5 amps • filament current is not tube current

  13. X-Ray Production(cont.) • X-Rays are produced in the x-ray tube by two 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 corresponding to energy difference between electron shells of target atom • Specific energies characteristic of target material • for tungsten 59 keV corresponds to the difference in energy between K and L shells

  16. L K - + ~ + ~ + ~ - - Bremsstrahlung • interaction of moving electron from filament with nucleus of target atoms • Positive nucleus causes moving electron to change speed / direction • Kinetic energy lost • Emitted in form of Bremsstrahlung x-ray Electron from Filament -

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

  18. Bremsstrahlung (cont.) • Energy lost by moving electron is random & depends on • distance from nucleus • charge (Z) of nucleus • Bremsstrahlung Energy Spectrum 0 - peak kilovoltage (kVp) applied to x-ray tube • most Bramsstrahlung photons have low energy • lowest energy photons don’t escape tube • easily filtered by tube enclosures or added filtration # Energy

  19. # Energy Output Beam Spectrum • Output photon beam made up 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 value • combination of Bremsstrahlung and characteristic radiation # Energy

  20. Tube Current (mA) • rate of electron flow from filament to target • Electrons / second • Measured in milliamperes (mA) • Limited by • filament emission (temperature / filament current) • space charge (see next slide) +

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

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

  23. - - - * Space Charge • Electrons leave filament • filament becomes positive • Negative electrons stay close • Electron cloud surrounds filament • Cloud repels new electrons from filament • Limits electron flow from cathode to anode +

  24. + + - + - + + - Kilovoltage & Space Charge • raising kilovoltage gradually overcomes space charge • Higher fraction of electrons make it to anode as kilovoltage increases • At high enough kilovoltage saturation results • All electrons liberated by filament reach target • Raising kilovoltage further has no effect on # electrons reaching anode Tube Current (mA) Saturation Voltage kVp

  25. Saturation Voltage + + - + - + + • kilovoltage at which a further increase does not increase tube current • 100% of electrons already going to target • Tube current said to be emission limited • tube current can only be increasedby increasing filament temperature - Tube Current (mA) Saturation Voltage kVp

  26. + Focal Spot • portion of anode struck by electron stream • Focal spot sizes affects and limits resolution

  27. Focusing Cup • negatively charged • focuses electron stream to target • overcomes tendency of electrons to spread because of mutual repulsion + Focusing Cup

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

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

  30. Focal Spot Size & Heat The larger the area the electron beam hits, the more intense the beam can be without melting the target

  31. Filament (cont.) • Large Filament normally left on at low “standby” current • boosted before exposure (prep or first trigger) • With time tungsten from hot filament vaporizes on glass insert • thins the filament • filters the x-ray beam • increases possibilityof arcing • electrons attracted toglass instead of target +

  32. Cross Section of X-Ray Tube Dunlee Web Site: http://www.dunlee.com/new_tube_anatomy.html

  33. Cross Section of X-Ray Tube Dunlee Web Site: http://www.dunlee.com/new_target.html

  34. Line Focus Principle • Focal spot steeply slanted • 7-15 degrees typical • Focal spot looks small from patient’s perspective • Imaging size • Looks large from filament • better heat capacity + Actual FS Apparent FS Patient

  35. Line Focus Principle • Actual (true) focal spot • as seen from filament • Apparent (effective, projected) focal spot • as seen from tube port or patient + Actual FS Apparent FS Patient

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

  37. + Actual FS Apparent FS Target Angle, Q Line Focus (cont.) Apparent FS = Actual FS X sin Q

  38. 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!

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

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

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

  42. Rotating Anode • Larger diameter • Better heat ratings • heavier • requires more support • $$$ • Materials • usually tungsten • high melting point • good x-ray production • molybdenum (and now Rhodium) for mammography (sometimes) • low energy characteristic radiation

  43. Grid-controlled tubes • Grid used to switch tube on/off • grid is third electrode • relatively small voltagecontrols current flowfrom cathode to anode • Negative grid voltage repels electrons from filament • Grid much closer to filament than target • Applications • speedy switchingrequired • cine grid +

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