IAEA International Atomic Energy Agency RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L 6: X Ray production IAEA Training Material on.

Présentation au sujet: "IAEA International Atomic Energy Agency RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L 6: X Ray production IAEA Training Material on."— Transcription de la présentation:

1 IAEA International Atomic Energy Agency RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L 6: X Ray production IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

2 IAEA 6: X Ray production2 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

3 IAEA 6: X Ray production3 Topics Basic elements of an X Ray source assembly Cathode structure Anode structure Rating chart X Ray generator Automatic exposure control

4 IAEA 6: X Ray production4 Overview To become familiar with the technological principles of the X Ray production

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

6 IAEA 6: X Ray production6 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

7 IAEA 6: X Ray production7 X Ray tubes

8 IAEA 6: X Ray production8 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)

9 IAEA 6: X Ray production9 X Ray tube components 1: long tungsten filament 2 : short tungsten filament 3 : real size cathode 1: mark of focal spot housingcathode

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

11 IAEA 6: X Ray production11 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

12 IAEA 6: X Ray production12 Example of a cathode

13 IAEA 6: X Ray production13 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 ! Cathode structure (I)

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

15 IAEA 6: X Ray production15 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

16 IAEA 6: X Ray production16 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

17 IAEA 6: X Ray production17 Anode characteristic 1 : anode track 2 : anode pits caused by electron beam being stationery on the anode

18 IAEA 6: X Ray production18 THE SMALLER THE ANGLE THE BETTER THE RESOLUTION Anode angle (II)  Angle Incident electron beam width Apparent focal spot size Actual focal spot size Film  Angle Incident electron beam width Increased apparent focal spot size Actual focal spot size Film ‘

19 IAEA 6: X Ray production19 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

20 IAEA 6: X Ray production20 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 Anode heel effect (II)

21 IAEA 6: X Ray production21 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)

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

23 IAEA 6: X Ray production23 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 potential x unit of tube current x 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

24 IAEA 6: X Ray production24 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

25 IAEA 6: X Ray production25 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-phase fully rectified generator (acceptable) X Ray tube rating chart (II)

26 IAEA 6: X Ray production26 0.010.050.10.51.05.010.0 700 600 500 400 300 200 100 50 kVp 70 kVp 90 kVp 120 kVp Unacceptable Exposure time (s) Tube current (mA) X Ray tube A  half-wave rectified 3000 rpm 90 kV 1.0 mm effective focal spot X Ray tube rating chart (III)

27 IAEA 6: X Ray production27 0.010.050.10.51.05.010.0 700 600 500 400 300 200 100 50 kVp 70 kVp 90 kVp 125 kVp Acceptable Exposure time (s) Tube current (mA) X Ray tube B 3  full-wave rectified 10.000 rpm 125 kV 1.0 mm effective focal spot X Ray tube rating chart (IV) Unacceptable

28 IAEA 6: X Ray production28 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

29 IAEA 6: X Ray production29 240 220 200 180 160 140 120 100 80 60 40 20 Elapsed time (min) Heat units stored (x 1000) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 500 HU/sec 1000 HU/sec 350 HU/sec 250 HU/sec Imput curve Cooling curve Maximum Heat Storage Capacity of Anode Anode cooling chart (II)

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

31 IAEA 6: X Ray production31 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)

32 IAEA 6: X Ray production32 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 = k 0. U n. I. T U: peak voltage (kV) I: mean current (mA) T: exposure time (ms) n: ranging from about 1.5 to 3 X-ray generator (II)

33 IAEA 6: X Ray production33 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 X-ray generator (III)

34 IAEA 6: X Ray production34 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”

35 IAEA 6: X Ray production35 100% 13% 4% Line voltage Single phase single pulse Single phase 2-pulse Three phase 6-pulse Three phase 12-pulse 0.02 s 0.01 s kV ripple (%) Tube potential wave form (II)

36 IAEA 6: X Ray production36 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

37 IAEA 6: X Ray production37 CP : eliminates any changes of voltage or tube current high voltage regulators can control the voltage AND switch on and off the exposure voltage can be switched on at any moment (temporal resolution) HF : combines the advantages of constant potential and conventional generator reproducibility and consistency of tube voltage high frame rate possible The choice of the number of pulses (II)

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

39 IAEA 6: X Ray production39 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

40 IAEA 6: X Ray production40 Automatic exposure control X Ray tube Collimator Beam Soft tissue Bone Air Patient Table Grid Cassette AEC detectors

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

42 IAEA 6: X Ray production42 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 (inverter technology) - 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

43 IAEA 6: X Ray production43 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

44 IAEA 6: X Ray production44 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

45 IAEA 6: X Ray production45 Where to Get More Information The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Boone. Lippincott Williams & Wilkins, Philadelphia, 2011