X-ray Tube (Cathode)

X-ray Tube (Cathode)

1. TOPIC 1 1 23/8/2018 X-RAY IMAGING SYSTEM Dr. Nik Noor Ashikin Nik Ab Razak School of Physics Universiti Sains Malaysia [email protected]

2. 1.2 CATHODE 1.2.1 Filament 1.2.2 Focusing Cup 2 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

3. 8/23/2018 3 2.3 Cathode NNANAR

4. 1.2.1 Filament 5 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

5. 1. Negative Electrode 2. Source of electron heated by an electric current 3. Mou n t ed wi th i n a ne g a t i v e l y c ha r g ed f o c u si ng c up 4. Made from tungsten wire of 0.2–0.3 mm diameter and operates at around 2700 k FILAMENT : Properties 1.2.1

6. FILAMENT : ADVANTAGES OF TUNGSTEN High melting point (3370 o C). Little tendency to vaporize. Ductility & Stability Malleability and strength A trace of thorium in the tungsten wire increases the efficiency of the electron emission and prolongs the life of filament) Good thermionic emitter Rugged and able to be drawn into the thin wire required Disadvantages:- Not an efficient electron emitting material. 1.2.1

7. A very high current (few amperes ) is passed through the filament and heats the metal causing the outer electrons of the tungsten atoms boiled off, and ejected from the surface of the coil. Emission of electrons resulting from absorption of thermal energy is k/a thermionic emission. The electrons are liberated at a rate that increases with the filament current EDISON EFFECT The electron cloud surrounding the filament, which is produced by thermionic emission FILAMENT : THERMIONIC EMISSION 1.2.1

8. 9 2.3.1a Thermionic Emission Disadvantages Thermionic Emission NNANAR 2.3.1.1 Thermionic Emission FILAMENT : THERMIONIC EMISSION 1.2.1

9. 2.3.1 Filament 10 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak FILAMENT : Space charge 1.2.1

10. FILAMENT : EQUILIBRIUM STATE As the electrons leave the filament , it acquires a positive charge attracting some electrons back to itself. Number of electrons returning to filament is equal to number of electrons being emitted . As a result, space charge remains constant with actual number depending on filament temperature. 1.2.1

11. Two electrical currents flow in an x-ray tube : The filament current = flow of electrons through the filament to raise its temperature and release electrons. The tube current = flow of released electrons from the filament to the anode across the x-ray tube (varies from a few to several hundred milliamperes) FILAMENT : Filament Current & Tube Current 1.2.1

12. FILAMENT : TUbe Current Is the number of electrons flowing from cathode to anode per second. Measured in milli amperes(mA) The tube current is unidirectional - from cathode to anode . x-ray tubes do not exceed 1000 mA because of the space charge effect 1.2.1

13. FILAMENT : saturation Current At a given filament current, x-ray tube current rises w/ increasing kVp to max value. further increases kVp does not result in higher mA since all available e- were used, not reached at low kVp b/c space charge limitation . *at levels about the saturation current, an increase in kVp will not increase the tube current as all available electrons have been used 1.2.1

14. 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 15 1.2.1

15. 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak 16 1.2.1

16. Contain single filament/ double filaments/ sometimes 3 filaments DOUBLE FILAMENT ARRANGEMENT:- They are placed side by side or one above the other. Only one filament is used for any fixed x ray exposure. FILAMENT : MODERN DAY X-RAY TUBES 1.2.1

17. 2.3.1.2 Dual Focal Tube 18 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak Modern tubes have two filaments: examples: 1.2 mm and 2.0 mm 0.6 mm and 1.2 mm 0.3 mm and 2.0 mm Long One : higher current, lower resolution , larger exposure Short One : lower current , higher resolution 1.2.1

18. Tube with 3 filaments Stereoscopic angiographic tube -In this tube, 2 focal spots are widely separated producing stereoscopic film pair when 2 films are exposed. -Used in angiography. FILAMENT : HIGHLY SPECIALISED X-RAY TUBES 1.2.1

19. When x-ray is turned on and no exposure is made, (as in fluoroscopy) stand by current heats the filament at low current (5mA). When exposures are needed, automatic circuit will raise filament current to required value and lower it to stand by after exposure. FILAMENT : AUTOMATIC FILAMENT BOOSTING CURCUIT 1.2.1

20. 1.2.2 Focusing Cup 21 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

21. FOCUSSING CUP : Properties The filament is embedded in a metal shroud called the focusing cup Made of NICKEL High melting point (due to tremendous amount of heat that is generated at the cathode) Relatively poor thermionic emitter It is maintained at same negative terminal as that of filament 1.2.2

22. The specially designed cup cause the electron stream to converge to the target area on the anode Prevents bombardment of unacceptably large target area All electrons accelerated from cathode to anode are electrically negative, the beam tends to spread out due to electrostatic repulsion, and some electrons can even miss the anode completely FOCUSSING CUP : Functions 1.2.2

23. 1.3 Glass Envelope 24 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

24. Is a sealed evacuated tube made up of borosilicate (to withstand the tremendous heat generated & maintains vacuum inside the tube) mounted from the anode and cathode. The glass must be a good electrical insulator , or a substantial current will flow through it when a potential difference is applied between the anode and cathode GLASS ENVELOPE : Properties 1.3

25. Enclosed the tube with the vacuum Accelerated electrons  collide with gas molecules  secondary electrons (less speed) wide variation in tube current and energy of x ray produced. The purpose of the vacuum in the modern x-ray tube is to allow the number and speed of accelerated electrons to be controlled independently . The shape and size of these x-ray tubes are specially designed to prevent electric discharge between electrodes . GLASS ENVELOPE : Functions 1.3

26. On long term use, tungsten vaporizes and form thin coat on inner surface of glass wall of x-ray tube. This alters the electric potential of the tube, allowing tube current to stray and interact with the glass envelope; the result is arcing and tube failure If tube becomes glassy , x-ray production will fall off and tube will fail A failing vacuum , resulting from leakage or degassing of the materials , causes increased ionization of the gas molecules , which slows down the electrons . further, a current of positive ions flowing back could impair or destroy the cathode filament GLASS ENVELOPE : problems 1.3

27. 2.4 Glass Envelope (SOLUTION) 28 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak Improvement in tube design incorporates metal rather than glass - Metal maintain a constant electric potential between the electron of the tube current and the envelope. Therefore they have longer life and less likely to fail GLASS ENVELOPE : solution 1.3

28. Made of aluminum oxide. They insulate high voltage parts of x-ray tube from metal envelope . Less off focus radiation Longer tube life with high tube currents Higher tube loading Adequate electrical safety Compact size GLASS ENVELOPE : CERAMIC INSULATORS 1.3

29. 1.4 Tube Housing 30 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

30. Consists of metal case made up of Aluminum alloy lined on the inside by a layer of lead which protects and supports the glass x-ray tube insert. The tube housing is packed with industrial grade oil to provide electrical and thermal insulation. TUBE HOUSING : properties 1.4

31. 2.5 Tube housing 2.5 Tube Housing Tube housing provides an efficient radiation barrier where in the x-rays produced in the x-ray tube are attenuated in all the directions except at the tube port . Provides shielding for the high voltages required to produce x rays TUBE HOUSING : function 1.4

32. * X-rays that escape through protective housing are they contribute no diagnostic info and result in unnecessary exposure *Leakage radiation must not exceed 100mR/hr (1 Gy t) at 1 meter TUBE HOUSING : LEAKAGE RADIATION TUBE HOUSING 1.4

33. It is due to electron back scatter from anode interacting with metal other than the focal track and striking anode a second time to produce X-rays. Decreased by : Placing collimator Lead diaphragm as close to X-ray tube as possible. Using a metal enclosure – attracts off focus radiation to the grounded metal tube . USEFUL OFF-FOCUS TUBE HOUSING : OFF FOCUS RADIATION 1.4

34. 1.4

35. COOLING MECHANISM OF X-RAY TUBE Almost all energy put into x-ray tube is converted into heat and <1% is converted into x-rays. 2.5 Tube Housing 1.4

36. * most heat generated in the x-ray tube is dissipated by radiation from the anode * RADIATION is the transfer of heat by emission of infrared radiation * CONDUCTION is transfer of heat by touching from one area to another * CONVECTION is the transfer of heat by movement of liquid or gas COOLING MECHANISM OF X-RAY TUBE 2.5 Tube Housing 1.4

37. Due to time allowed for rotor to accelerate to designed rpm; filament current increases to get correct x-ray tube current 2. What happens when the exposure switch is first pressed ? 1. Why is there a small delay when the rad tech pushes the exposure button of the imager? Phase 1 Some of the electricity is diverted to the induction motor of the x-ray tube to bring the rotor up to speed. Approximately 3400 RPM Phase 2 The second phase actually initiates the x-ray production process

38. 1.5 Power Ratings For X-ray Tubes 40 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

39. 41 Dr. Nik Noor Ashikin Bt Nik Ab Razak 2.6.2 Maximum Energy 2. Maximum-energy ratings are provided for the target, anode, and housing of an x-ray tube. These ratings are expressed in heat units : SINGLE-PHASE THREE-PHASE 1. The heat storage capacity of a tube component is the total number of heat units that may be absorbed without damage to the component. (diagnostic x-ray tubes range from several hundred thousand to over a million heat units) 1.5 Power Ratings For X-ray Tubes

40. The area above each curve reflects combinations of tube current and exposure time that overload the x-ray tube and might damage the target . 1.5 Power Ratings For X-ray Tubes

41. 44 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak EXAMPLE An exposure of 80 kVp , 250 mA and 100 msec results in following HU deposition on the anode: Single-phase generators: 80 × 250 × 0.1 = 2000 HU Three-phase generators: 80 × 250 × 0.1 × 1.35 = 2700 HU Constant-potential generators: 80 × 250 × 0.1 × 1.4 = 2800 HU For continuous x-ray production (fluoroscopy), the HU/sec is defined HU/sec = kVp × mA

42. How many heat units are generated by an exposure of 70 kVp , 300 mA, and 0.1 second on a single phase full-wave rectified unit? How many heat units are generated by an exposure of 70 kVp , 400 mA and 0.1 second on a 3 phase 6-pulse unit? How many heat units are generated by an exposure of 100kVp, 400mA and 0.01 second on a 3 phase 6-pulse unit? How many heat units are generated by 3 exposures of 100kVp, 400mA and 0.01 second on a 3 phase 12-pulse unit?

43. QUESTION ANSWER SIX SEQUENTIAL SKULL FILMS ARE EXPOSED WITH A THREE-PHASE GENERATOR OPERATED AT 82 KvP , 120 mAs . WHAT IS THE TOTAL HEAT (IN HU) GENERATED? Number of heat units/film = 1.35 ×82×120 = 13,284 HU total HU = 6 × 13,284 HU = 79,704 HU

44. SUMMARY 47 23/8/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

45. Summary of X-ray tube component