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X ray tube

  1. 1. The X-Ray Tube S. Guilbaud Education Director School of Radiologic Technology
  2. 2. X-Ray Tube <ul><li>Electrical device used for the generation of x-rays. </li></ul><ul><li>This is accomplished by the acceleration of electrons and then suddenly decelerating them. </li></ul><ul><li>The energy of the x-rays is dependent on the kinetic energy of the electrons. </li></ul>
  3. 3. X-Ray tube components <ul><li>Glass envelope </li></ul><ul><li>Cathode </li></ul><ul><li>Anode </li></ul><ul><li>Protective housing </li></ul>
  4. 4. Glass envelope <ul><li>Made of Pyrex glass </li></ul><ul><ul><li>Able to withstand tremendous heat </li></ul></ul><ul><li>Tube maintains a vacuum. </li></ul><ul><li>Tube window </li></ul><ul><ul><li>A segment of glass that is thinner than the rest of the glass envelope. </li></ul></ul><ul><ul><li>Contributes to inherent filtration. </li></ul></ul><ul><ul><ul><li>0.5mm Al equivalency. </li></ul></ul></ul>
  5. 5. Cathode <ul><li>Negatively charged electrode. </li></ul><ul><li>Two primary parts: </li></ul><ul><ul><li>Filament </li></ul></ul><ul><ul><li>Focusing cup </li></ul></ul>
  6. 6. Cathode
  7. 7. Focusing cup <ul><li>Metallic shroud containing the two filaments. </li></ul><ul><ul><li>Usually made from nickel. </li></ul></ul><ul><li>Contains a negative charge. </li></ul><ul><ul><li>Designed to repel electrons. </li></ul></ul><ul><ul><li>Designed to condense electron beam to small area on on focal track. </li></ul></ul>
  8. 8. Focusing cup <ul><li>Four factors determine the effectiveness of the cup. </li></ul><ul><ul><li>Size & shape. </li></ul></ul><ul><ul><li>Charge </li></ul></ul><ul><ul><li>Filament size & shape. </li></ul></ul><ul><ul><li>Position of filament w/in cup. </li></ul></ul>
  9. 9. Filament <ul><li>Small coil of thoriated tungsten. </li></ul><ul><li>Modern x-ray tubes contain two filament. </li></ul><ul><ul><li>They correspond to the focal spot sizes. </li></ul></ul><ul><li>When machine is turned on, small amount of current flows through to heat filament. </li></ul><ul><li>Tube current is adjusted by controlling the filament current. </li></ul>
  10. 10. Anode <ul><li>Positively charged electrode. </li></ul><ul><li>Two types. </li></ul><ul><ul><li>Stationary anode. </li></ul></ul><ul><ul><li>Rotating anode. </li></ul></ul>
  11. 11. Stationary Anode <ul><li>Made of tungsten target embedded in a large copper bar. </li></ul><ul><li>Usually used in dental x-ray machine. </li></ul>
  12. 12. Rotating anode <ul><li>Constructed of tungsten target (focal track). </li></ul><ul><ul><li>High melting point 3400 0 Celsius. </li></ul></ul><ul><li>Molybdenum </li></ul><ul><ul><li>Surrounds tungsten target area. </li></ul></ul><ul><ul><li>Assists in dissipating heat. </li></ul></ul><ul><li>Graphite </li></ul><ul><ul><li>Serves as mount for molybdenum and tungsten target. </li></ul></ul><ul><ul><li>Also assists in dissipating heat. </li></ul></ul>
  13. 13. Rotating anode <ul><li>Provides greater target area and greater heat dissipation. </li></ul><ul><li>Affords the ability to attain greater exposure loads by providing a larger area for the electron beam to interact with the target. </li></ul>
  14. 14. Rotating anode <ul><li>The heating capacity is further enhanced with an increased RPM (3400). </li></ul>
  15. 15. Induction motor Responsible for driving the rotating anode. Consists of two parts separated by the glass envelope.
  16. 16. Induction motor <ul><li>Works on the principle similar to the transformer. </li></ul><ul><ul><li>Electromagnetic induction. </li></ul></ul><ul><ul><li>Current flowing in the stator develops a magnetic field. </li></ul></ul><ul><ul><li>Stator windings are sequentially energized so that the induced magnetic field rotates on the axis of the stator. </li></ul></ul><ul><ul><li>This causes the rotor to rotate. </li></ul></ul>
  17. 17. Line focus principle <ul><li>The area of the x-ray tube anode from which the x-ray photons are emitted. </li></ul><ul><li>This is called the actual focal spot </li></ul>
  18. 18. Line focus principle <ul><li>The projection perpendicular to the central ray, which is its apparent area from the position of the film, is the effective focal spot. </li></ul>
  19. 19. Line focus principle <ul><li>Was incorporated into x-ray tube targets to allow a large area for heating while maintaining a small focal spot. </li></ul><ul><li>The effective focal spot is the area projected onto the patient and film. </li></ul>
  20. 20. Line focus principle <ul><li>Focal spot sizes always make reference to the effective focal spot. </li></ul><ul><li>The lower the target angle, the smaller the effective focal spot size. </li></ul>
  21. 21. Line focus principle <ul><li>The advantage of the line-focus principle is that it provides the detail of a small focal spot while allowing a large amount of heat dissipation. </li></ul>
  22. 22. Line focus principle <ul><li>The unfortunate bi-product of the line-focus principle is the “anode heel effect” </li></ul>
  23. 23. Anode heel effect <ul><li>Construction phenomenon that causes the x-ray photons exiting the tube on the cathode side to have a greater energy value than those exiting the tube on the anode side. </li></ul>
  24. 24. Anode heel effect <ul><li>More energy absorption occurs at the anode heel resulting in less energy value from the incident photons at the anode heel. </li></ul>
  25. 25. Anode heel effect <ul><li>This is used to advantage when imaging anatomical parts that are unequal in thickness and densities throughout their respective lengths. </li></ul>
  26. 26. Using the anode heel effect <ul><li>The following anatomical parts may be imaged using the anode heel effect: </li></ul><ul><ul><li>Thoracic vertebrae </li></ul></ul><ul><ul><li>Humerus </li></ul></ul><ul><ul><li>Femur </li></ul></ul><ul><ul><li>Tibia & fibula </li></ul></ul><ul><ul><li>Forearm </li></ul></ul>
  27. 27. Using the anode heel effect
  28. 28. Anode heel effect <ul><li>The thicker portion of the anatomical part is placed beneath the cathode end of the x-ray tube. </li></ul>
  29. 29. Protective housing
  30. 30. Protective housing <ul><li>X-ray tube is always mounted inside a lead-lined protective housing that is designed to: </li></ul><ul><ul><li>Prevent excessive radiation exposure. </li></ul></ul><ul><ul><li>Prevent electric shock to the patient and operator (technologist). </li></ul></ul>
  31. 31. Protective housing <ul><li>Incorporates specially designed high-voltage receptacles. </li></ul><ul><li>Provides mechanical support for the x-ray tube and protects it from damage. </li></ul><ul><li>Some tube housings contain oil in which the tube is bathed. </li></ul><ul><li>Some tube housings contain a cooling fan to air-cool the tube. </li></ul>
  32. 32. Protective housing <ul><li>When properly designed, they reduce the level of leakage radiation to less than 100 mR/hr at 1 meter when operated at maximum conditions. </li></ul>
  33. 33. Tube rating charts
  34. 34. Tube rating chart <ul><li>A graph that indicates the maximum exposure values that may be made w/o damage to the tube. </li></ul><ul><li>Each chart contains a family of curves representing the various tube currents in mA. </li></ul>
  35. 35. Tube rating chart <ul><li>The X axis and Y axis show scales of the two radiographic parameters of kV and mA. </li></ul><ul><ul><li>For a given mA, any combination of kVp and time that lies below the mA curve is safe. </li></ul></ul>
  36. 36. Tube rating charts
  37. 37. Anode cooling chart <ul><li>Provides the thermal capacity of the anode and its heat dissipation characteristics. </li></ul>
  38. 38. Anode cooling chart <ul><li>Thermal energy is measured in British Thermal Units (BTU’s) where x-ray thermal energy is measured in Heat Units (HU). </li></ul><ul><li>Thus: </li></ul><ul><ul><li>1 kVp, 1 mA, 1 s = 1 HU. </li></ul></ul>
  39. 39. Calculating Heat Units <ul><li>For a single phase unit, </li></ul><ul><ul><li>HU = kVp x mA x s </li></ul></ul><ul><li>For a 3 phase 6 pulse unit, </li></ul><ul><ul><li>HU = 1.35 x kVp x mA x s </li></ul></ul><ul><li>For a 3 phase 12 pulse unit, </li></ul><ul><ul><li>HU = 1.41 x kVp x mA x s </li></ul></ul><ul><li>For a high frequency unit, </li></ul><ul><ul><li>HU = 1.44 x kVp x mA x s </li></ul></ul>
  40. 40. Anode cooling chart <ul><li>Determines the maximum heat capacity of the anode. </li></ul><ul><li>Determines the length of time required for complete cooling following any level of heat input. </li></ul>
  41. 42. References <ul><li>Bushberg, et al. The Essential Physics of Medical Imaging , Williams & Wilkins, 1994. </li></ul><ul><li>Bushong, S. Radiologic Science for Technologists, Physics, Biology, and Protection , 7 th Edition, Mosby, 2000. </li></ul><ul><li>Carlton et al. Principles of Radiographic Imaging an Art and a Science , 3 rd Edition, Delmar, 2001. </li></ul><ul><li>Selman, J. The Fundamentals of X-Ray and Radium Physics , 8 th Edition, Charles Thomas, 1994. </li></ul>