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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY. L 6: X Ray production. Introduction. A review is made of: The main elements of the X Rays tube: cathode and anode structure

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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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  1. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY L 6: X Ray production

  2. Introduction A review is made of: • The main elements of the X Rays tube: cathode and anode structure • The technology constraints of the anode and cathode material • The rating charts and X Ray tube heat loading capacities 6: X Ray production

  3. Topics • Basic elements of an X Ray source assembly • Cathode structure • Anode structure • Rating chart • X Ray generator • Automatic exposure control 6: X Ray production

  4. Overview • To become familiar with the technological principles of the X Ray production 6: X Ray production

  5. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 6: X Ray production Topic 1: Basic elements of an X Ray source assembly

  6. Basic elements of the X Ray source assembly • Generator : power circuit supplying the required potential to the X Ray tube • X Ray tube producing the X Ray beam 6: X Ray production

  7. X Ray tubes 6: X Ray production

  8. X Ray tube components • Cathode: heated filament which is the source of the electron beam directed towards the anode • tungsten filament • Anode (stationary or rotating): impacted by electrons, emits X Rays, > 99% of electron energy is dissipated as heat • Metal tube housing surrounding glass (or metal) X Ray tube (electrons are traveling in vacuum) • Shielding material (protection against extra-focal spot radiation from anode) 6: X Ray production

  9. housing cathode X Ray tube components 1: long tungsten filament 2 : short tungsten filament 3 : real size cathode 1: mark of focal spot 6: X Ray production

  10. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 6: X Ray production Topic 2: Cathode structure

  11. Cathode structure (I) • Cathode includes filament(s) and associated circuitry • tungsten material : preferred because of its high melting point (3370°C) • slow filament evaporation • no arcing • minimum deposit of W on glass envelope • To reduce evaporation the emission temperature of the cathode is reached just before the exposure • in stand-by, temperature is kept at ± 1500°C so that 2700°C emission temperature can be reached within a second 6: X Ray production

  12. Example of a cathode 6: X Ray production

  13. Cathode structure (I) • Modern tubes have two filaments • a long one : higher current/lower resolution • a short one : lower current/higher resolution • Coulomb interaction causes the electron beam to diverge on the way to the anode • larger area of target used • focal spot increased lower image resolution Focusing of electrons is crucial ! 6: X Ray production

  14. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 6: X Ray production Topic 3: Anode structure

  15. X Ray tube characteristics • Anode mechanical constraints • Material : tungsten, rhenium, molybdenum, graphite • Focal spot : surface of anode impacted by electrons • Anode angle • Disk and annular track diameter (rotation frequency from 3,000 to 10,000 revolutions/minute) • Thickness  mass and material (volume)  heat capacity • Anode thermal constraints • Instantaneous power load (heat unit) • Heat loading time curve • Cooling time curve 6: X Ray production

  16. Anode angle (I) • The Line-Focus principle • Anode target plate has a shape that is more rectangular or ellipsoidal than circular • the shape depends on : • filament size and shape • focusing cup’s and potential • distance between cathode and anode • Image resolution requires a small focal spot • Heat dissipation requires a large spot • This conflict is solved by slanting the target face 6: X Ray production

  17. Anode characteristic 1 : anode track 2 : anode pits caused by electron beam being stationery on the anode 6: X Ray production

  18. Anode angle (II) Angle  ‘ Angle  Actual focal spot size Actual focal spot size Incident electron beam width Incident electron beam width Increased apparent focal spot size Apparent focal spot size Film Film THE SMALLER THE ANGLE THE BETTER THE RESOLUTION 6: X Ray production

  19. Anode heel effect (I) • Anode angle (from 7° to 20°) induces a variation of the X Ray output in the imaging plane parallel to the anode-cathode axis • Absorption by anode of X photons with low emission angle • The magnitude of influence of the heel effect on the image depends on factors such as : • anode angle • size of film • focus to film distance 6: X Ray production

  20. Anode heel effect (II) • The heel effect is not always a negative factor • It can be used to compensate for different attenuation through parts of the body • For example: • thoracic spine (thicker part of the patient towards the cathode side of the tube) • mammography 6: X Ray production

  21. Focal spot size and imaging geometry • Focal spot finite size  image unsharpened • Improving sharpness  small focal spot size • For mammography focal spot size  0.4 mm nominal • Small focal spot size  reduced tube output (longer exposure time) • Large focal spot allows high output (shorter exposure time) • Balance depends on organ movement (fast moving organs may require larger focus) 6: X Ray production

  22. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 6: X Ray production Topic 4: Rating Chart

  23. Heat loading capacities • A procedure generates an amount of heat depending on: • kV used, tube current (mA), length of exposure • type of voltage waveform • number of exposures taken in rapid sequence • Heat Unit (HU) [joule] : unit of potentialxunit of tube currentx unit of time • The heat generated by various types of X Ray circuits are: • 1 phase units : HU = kV x mA x s • 3 phase units, 6 pulse : HU = 1.35 kV x mA x s • 3 phase units, 12 pulse: HU = 1.41 kV x mA x s 6: X Ray production

  24. X Ray tube rating chart (I) • Tube cooling characteristics and focal spot size • {mA - time} relationship at constant kV • intensity decreases with increasing exposure time • intensity increases with decreasing kV • Note: higher power reduced exposure time  reduced motion unsharpness 6: X Ray production

  25. X Ray tube rating chart (II) • Manufacturers combine heat loading characteristics and information about the limits of their X Ray tubes in graphical representations called Tube Rating Charts • Example: • Tube A: a 300 mA, 0.5 s, 90 kV procedure would damage the system operated from a 1-phase half wave rectified generator (unacceptable) • Tube B: a 200 mA, 0.1 s, 120 kV procedure comply with the technical characteristics of the system operated from a 3-phasefully rectified generator (acceptable) 6: X Ray production

  26. X Ray tube rating chart (III) X Ray tube A 1 f half-wave rectified 3000 rpm 90 kV 1.0 mm effective focal spot 700 600 500 400 300 200 100 70 kVp 50 kVp Tube current (mA) Unacceptable 90 kVp 120 kVp 0.01 0.05 0.1 0.5 1.0 5.0 10.0 Exposure time (s) 6: X Ray production

  27. X Ray tube rating chart (IV) 700 600 500 400 300 200 100 X Ray tube B 3f full-wave rectified 10.000 rpm 125 kV 1.0 mm effective focal spot 70 kVp 50kVp Tube current (mA) 90 kVp Unacceptable 125 kVp Acceptable 0.01 0.05 0.1 0.5 1.0 5.0 10.0 Exposure time (s) 6: X Ray production

  28. Anode cooling chart (I) • Heat generated is stored in the anode and dissipated by radiative cooling to the x-ray tube, oil, and housing • A typical cooling chart has: • input curves (heat units stored as a function of time) • anode cooling curve • The following graph shows that: • a procedure delivering 500 HU/s can go on indefinitely • if it is delivering 1000 HU/s it has to stop after 10 min • if the anode has stored 120,000 HU, it will take  5 min to cool down completely 6: X Ray production

  29. Anode cooling chart (II) Maximum Heat Storage Capacity of Anode 240 220 200 180 160 140 120 100 80 60 40 20 1000 HU/sec Imput curve 500 HU/sec 350 HU/sec Heat units stored (x 1000) 250 HU/sec Cooling curve 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Elapsed time (min) 6: X Ray production

  30. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 6: X Ray production Topic 5: X Ray generator

  31. X-ray generator (I) It supplies the X-ray tube with : Current to heat the cathode filament Potential to accelerate electrons Automatic control of exposure (power application time)  Energy supply  1000  X-ray beam energy (of which 99.9% is dissipated as thermal energy) 6: X Ray production

  32. X-ray generator (II) • Generator characteristics have a strong influence on the contrast and sharpness of the radiographic image • The motion unsharpness can be greatly reduced by a generator allowing an exposure time as short as achievable • Since the dose at the image plane can be expressed as: • D = k0 . Un . I . T • U: peak voltage (kV) • I: mean current (mA) • T: exposure time (ms) • n: ranging from about 1.5 to 3 6: X Ray production

  33. X-ray generator (III) • Peak voltage value has an influence on the beam hardness • It has to be related to medical question • What is the anatomical structure to investigate ? • What is the contrast level needed ? • For a thorax examination : 140 - 150 kV is suitable to visualize the lung structure • While only 65 kV is necessary to see bone structure 6: X Ray production

  34. Tube potential wave form (I) • Conventional generators • single  1-pulse (dental and some mobile systems) • single  2-pulse (double rectification) • three  6-pulse • three  12-pulse • Constant potential generators (CP) • HF generators (use of DC choppers to convert 50Hz mains into voltages with frequencies in the kHz range)  “Inverter technology” 6: X Ray production

  35. Tube potential wave form (II) Single phase single pulse kV ripple (%) 100% Single phase 2-pulse 13% Three phase 6-pulse 4% Three phase 12-pulse Line voltage 0.01 s 0.02 s 6: X Ray production

  36. The choice of the number of pulses (I) • Single pulse : low power (<2 kW) • 2-pulse : low and medium power • 6-pulse : uses 3-phase mains, medium and high power (manual or automatic compensation for voltage drop) • 12-pulse : uses two shifted 3-phase system, high power up to 150 kW 6: X Ray production

  37. The choice of the number of pulses (II) • CP : eliminates any changes of voltage or tube current • high voltage regulators can control the voltageANDswitch on and off the exposure • voltage can be switched on atany moment (temporal resolution) • HF : combines the advantages of constant potential and conventional generator • reproducibility and consistency of tube voltage • high frame rate possible 6: X Ray production

  38. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 6: X-ray production Topic 6: Automatic Exposure Control (AEC)

  39. Automatic exposure control • Optimal choice of technical parameters (kV, mA) to optimize patient dose and image quality • Radiation detector behind (or in front of) the film cassette (with due correction) • Exposure is terminated when the required dose has been integrated • Compensation for kVp at a given thickness • Compensation for thickness at a given kVp 6: X Ray production

  40. Automatic exposure control X Ray tube Collimator Beam Soft tissue Patient Air Bone Table Grid AEC detectors Cassette 6: X Ray production

  41. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 6: X-ray production Topic 7: X-ray equipment operation and mode

  42. X-ray equipment operation mode and application (II) • Radiography and Tomography • Single and 3  generators (inverter technology) • output : 30 kW at 0.3 focus spot size • output : 50 - 70 kW at 1.0 focus spot size • selection of kV and mAs , AEC • Radiography and Fluoroscopy • Under couch equipment, three  generator (invertertechnology) - continuous output of 300 - 500 W • output : 50 kW at 1.0 focus size for spot film • output : 30 kW at 0.6 for fluoroscopy (high resolution) • capable of pulsing at 30, 15, 7.5 fps or less • priority given to contrast • automatic settings of kV 6: X Ray production

  43. X-Ray equipment operation mode and application (III) • Radiography and Fluoroscopy • Over couch equipment, three phase generator (inverter technology) - continuous output of at least 500 W • output : 40 kW @ 0.6 focus size for spot film • output : 70 kW @ 1.0 for fluoroscopy (high resolution) • priority given to contrast • automatic settings of kV • Cardiac angiography • Three phase generator - continuous output  1kW • output : 30 kW @ 0.4 focus size • output : 80 kW @ 0.8 focus size • frame rate : up to 120 fr/s 6: X Ray production

  44. Summary • The x-ray system: • provides the required source of power • delivers an appropriate X Ray spectrum • assures the optimum adjustment of exposure to optimize image quality 6: X Ray production

  45. Where to Get More Information • The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Boone. Lippincott Williams & Wilkins, Philadelphia, 2011 6: X Ray production

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