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The X-Ray Tube

The X-Ray Tube. DMI 50B Kyle Thornton. Evolution Of The X-Ray Tube. Roentgen experimented with a Crookes tube Evacuated tube containing two electrodes Could not control the number of electrons moving between the electrons (tube current)

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The X-Ray Tube

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  1. The X-Ray Tube DMI 50B Kyle Thornton

  2. Evolution Of The X-Ray Tube • Roentgen experimented with a Crookes tube • Evacuated tube containing two electrodes • Could not control the number of electrons moving between the electrons (tube current) • In 1913, W.D. Coolidge redesigned the tube, calling it a hot cathode tube • Replaced the negative electrode with a small spiral-wound tungsten wire • Current was applied to this wire creating heat to free electrons • Hot cathode tubes became the standard for x-ray tubes

  3. Cold Cathode X-Ray TubeEarly 20th century

  4. Cold Cathode Tube Fluorescence

  5. Coolidge Tube w/Stationary Anode

  6. Helpful website • http://www.ndt-ed.org/EducationResources/HighSchool/Radiography/xraygenerators.htm • http://www.youtube.com/watch?v=Bc0eOjWkxpU

  7. Common Components Of The X-Ray Tube • Glass envelope • Cathode • Anode

  8. Glass Envelope • A vacuum is created prior to sealing the tube • The glass envelope: • Provides support for the electrodes • Provides electrical insulation • Assists in the removal of heat • Is made of very thick glass to withstand heat • Is thinner at the exit window where the x-rays emerge from the tube

  9. Cathode • Negative electrode of the x-ray tube • Consists of a filament and a focusing cup • The filament provides a source of electrons • Electrons are freed when the filament is heated • The filament is a long thin tungsten wire shaped into a spiral coil

  10. Cathode • About 10 volts and 3 – 5 amperes are applied to the filament to heat it • Tungsten is used because of its high melting point - 3370° C • It is also malleable • Most modern tubes contain two filaments • The longer filament is used when large numbers of electrons are needed • The shorter filament is used when lower tube currents and maximum detail are needed

  11. The Focusing Cup • The filaments are mounted within a focusing cup • Generally made of nickel, stainless steel, or molybdenum • A negative charge is placed on the focusing cup • Focuses the electrons on a smaller spot of the anode • This improves detail on the film

  12. Benefit of the Focusing Cup

  13. X-Ray Tube Focusing Cup http://www.youtube.com/watch?v=LyWuvSZRSLc&feature=related

  14. The Anode • The anode is the positive electrode • It is struck by the electron stream • The area struck is the anode target • Usually made of tungsten • A great deal of heat is produced at the anode • Depends on the voltage, the current, and the length of time the anode is struck by electrons

  15. Stationary Anodes • Used when lower heat quantities are produced • Consists of a tungsten target and copper block and stem • Copper conducts the heat away from the tungsten target • The heat can damage the anode target, causing pitting • Results in x-ray beam of reduced intensity due to scattering and absorption in the uneven surface

  16. Rotating Anodes • The first was made by Philips Medical Systems in Holland in 1929 • Consists of a tungsten alloy disk on a molybdenum base • Rotating anodes range in size from about 5 cm to 12.5 cm • Disk sizes determine the thermal load • Anodes have an angle of about 7° to 20°

  17. Rotating Anodes • Rotating anodes assure that the same area of the target being struck over and over is rare • Heat energy is distributed more evenly over the anode face • More rapid exposures are possible • The rotating anode is driven by an induction motor

  18. Purposes Of The Anode • Serves as a positive electrode • Provides structural support for the target plate • Provides a means for of dissipating heat to the target

  19. Malleability High melting point - 3370° C High Z number Resists vaporization at high temperatures Ability to conduct heat away from area of heat production Its density Ability to absorb heat without raising the temperature of the conductor Its availability makes it cost-effective The Benefits Of Using Tungsten As A Target Material

  20. The Induction Motor • Works on the principle of Lenz’ law • Using opposing magnetic fields, the copper bar is made to rotate • Rotation of the anode is accomplished more efficiently this way • The anode is surrounded by electromagnets that are switched on and off in rapid sequence • Anodes utilize 60 Hz AC • Anodes usually rotate at 3600 rpm • Newer anodes may rotate at 10,000 rpm

  21. Induction Motor Diagram

  22. Target Plate Angulation • The area of the target struck by the electron stream is the focal spot • Image sharpness is improved when the focal spot is small • By angling the target, a small area can be struck, but at the same time provides a large space for heat dissipation

  23. Line-Focus Principle • By angling the target, the effective area is much smaller than the actual area of electron interaction • As the target angle decreases, so does the effective focal spot

  24. Dual Focus Anodes

  25. Heel Effect • A consequence of the line focus principle • The beam intensity on the cathode side is more intense than on the anode side • Patients must be positioned accordingly for some exams

  26. Heel Effect

  27. Off-Focus Radiation

  28. Off-Focus Radiation • Radiation produced when electrons bounce off the target area of the anode and strike other areas and produce x-rays • About 15% of the electrons do this • This reduces image contrast • Can be controlled by applying an additional diaphragm close to the focal spot

  29. Tube Exit Window • The useful beam is emitted from the tube exit window • This section of glass is generally much thinner than the rest of the tube

  30. The Tube Housing • Supports and houses the x-ray tube • Provides insulation • Prevents shock • Is lined with a lead tube shield to prevent leakage radiation • Oil surrounds the tube within the tube shielding

  31. The Tube Housing

  32. Causes Of X-Ray Tube Failure • A single excessive exposure • Long exposure times • Filament vaporization • The most common

  33. Safe Operation With Tube Rating Charts • Radiographic rating chart • Anode cooling chart • Housing cooling chart

  34. Tube Rating Charts

  35. Here’s Some Questions • Which of the following exposures are unsafe? • 95 kVp, 150 mA, 1 sec. 3400 rpm, .6mm focal spot • 80kVp, 400 mA, .5 sec. 3400 rpm, 1mm focal spot • 125 kVp, 500 mA, .1 sec 10,000 rpm 1 mm • 75 kVp, 700 mA, .3 sec, 10,000 rpm 1 mm focal spot • 88 kVp, 400 mA, .1 sec., 10,000 rpm .6mm focal spot

  36. Heat Units • The product of mA, T, and kVp • HU = mA X T X kVp • Used to determine thermal capacity of an anode or tube housing • The tube housing cooling chart is similar to that of the anode cooling chart

  37. Anode Cooling Chart

  38. Here’s Some More Questions • If 50,000 HU are delivered to the anode, how long will it take to cool completely? • How many heat units are produced if 6 films are exposed using a three phase, six pulse generator at 82 kVp and 120 mAs

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