Modern x-ray tube

History of x-ray tube development
Types of x-ray tube
Components of x-ray tube

1. History of Development of Modern X-ray Tube Presented by: Sachidanand G iri JR-2

2. CONTENT: A.Introduction B.History of development of x-ray tube C.Types of x-ray tube D.Components of modern x-ray tube: 1.cathode 2.anode 3.glass envelop 4.oil insulation 5.tube shield

3. INTRODUCTION: X-RAYS were discovered by Wilhelm Conrad Röntgen in November, 1895, whilst he was experimenting with the passage of electricity through a gas at very low pressure. The vital piece of his apparatus was a long glass vessel from which as much air as possible had been removed and into each end of which a short platinum electrode was sealed.

4. When an electric discharge at high voltage was passed through the almost evacuated tube, Röntgen noticed a glow on a piece of glass , covered with zinc sulphide , which was lying a short distance from the tube. The glow persisted even when the discharge tube was shrouded in black paper, and Röntgen was quickly able to establish that the cause was a undiscovered radiation.

5. To it he gave the name X-rays , X being the established symbol for the unknown quality.

6. Although a modern X-ray tube bears no very obvious resemblance to the discharge tube of Rontgen's apparatus, the basic mechanism of X-ray production remains the same. X-rays are produced whenever high-speed electrons are suddenly brought to rest, some of their kinetic energy, at least, being converted into the electromagnetic radiation.

7. In the original apparatus the source of the electrons was the residual gas in the tube. Accelerated by the applied voltage they were brought to rest by the glass end of the tube, whence the X-rays were emitted. Nowadays the electrons come, by thermionic emission, from an electrically heated filament of tungsten.

8. History of development of x-ray tube Sir William Morgan (1785), while investigating the discharge of high tension current in perfect vacuum, obtained a vacuum so high that there was no discharge. In one of his experiments, the glass cracked and Morgan observed a display of colors, beginning with yellow-green and followed by red, violet and blue. Unknown to him he was the first man to produce X-rays .

9. In 1821, Michael Faraday conducted his first experiment on electric discharges in partially evacuated glass vessels using a vacuum pump built in 1650 by Otto von Geuricke . He described that the ‘voltaic arc’ was accompanied by fluorescence of gas remaining within the vessel. He called the fluorescence as ‘ radiant matter ’ and considered it as the fourth state of matter.

10. Vacuum tube and pump used by Michael Faraday

11. Julius Plucker (1859) was the first to observe Green Glass Fluorescence in partially evacuated discharge tubes.

12. Wilhelm Hittorf (1870) improved vacuum pumps. He observed that the fluorescent discharge increased in size as the tube was evacuated and identified the source of the phenomenon as cathode and termed it as ‘ cathode rays .’

13. He found that these rays travelled in straight lines, produced heat. Caused fluorescence on glass where they impinged, cast shadow of the object placed in their way and were deflected by a magnet. His work was subsequently verified by Eugen Goldstein (1879).

14. In 1880s, Sir William Crookes described additional changes that took place in the fluorescence. He considered ‘radiant matter’ to be the ‘ultra gaseous state.’ He found that the freshly opened photographic plates were strangely fogged and blackened.

15. He referred to a ‘molecular’ and ‘emissive’ ray from his tube which could only be seen when a fluorescent screen was placed in the ray’s path beyond the tube. He had unconsciously and unknowingly generated X-rays. He subsequently redesigned the tube.

16. Philip Lenard showed that cathode rays would pass through a special aluminum window built into the wall of his discharge tube and retained enough energy outside the tube also, to cause fluorescent screen to glow. These rays caused air to glow in front of the window. This glow extended in all directions for about five centimeters in air and became known as ‘Lenard’s Ray.’

17. Lenard proposed the ‘ Inverse Square Law.’ In 1895, Jead Perrin stated that cathode rays were negatively charged particles. In 1896, John Joseph Thomson discovered the ‘electron’.

18. Diagram of the cathode ray tube used by JJ Thomson when he discovered the electron

19. Roentgen, while experimenting and searching for the invisible light rays turned on a low pressure Crooke’s tube , completely enclosed in heavy black paper and applied power to the electrodes with a Ruhmkorff induction coil . Immediately he was surprised to see, a fluorescent screen, covered with barium platinocyanide standing on a table, at some distance away, started to glow brightly .

20. When he interposed objects between the tube and the screen, shadows were cast on the screen. These rays could not be reflected or refracted. They were unaffected by a magnetic or electric fields. He termed these rays “X-rays ”, Since ‘X’ was considered the American way to term the unknown, he ultimately, called them ‘ Roentgen Rays .’

21. Hittorff -Crooke’s tubes, of the kind used by Roentgen to discover X-rays

22. Ruhmkorff induction coil used to power early tubes

23. Types of x-ray tube : 1. Gas discharge tube/Crookes tube 2. Regulator Tubes 3. Vacuum Tube 4.Coolidge tube 5. Shockproof Dental X-ray Unit

24. Gas discharge Tubes/Crookes tube Invented by Brittish physicist William Crookes, in early 1870s. Crookes tubes also called   cold cathode  tubes, meaning that they do not have a heated  filament  in them that releases  electrons  . Early gas tubes depended upon the incomplete vacuum to provide the source of electrons at the cathode.

25. Components of Crookes tube 1. Tube envelop: Partially evacuated glass bulb of cerium. Low Air pressure 10 −6  to 5×10 −8   atmosphere . 2.Anode: made of platinum with atomic number 74 and mp-1768*c 3. Cathode :made of aluminium with atomic number 13 and mp-660*c 4. Anticathode : made of copper plate DC current of high voltage (100kvp)

26. When high  voltage  is applied to the tube, the  electric field  accelerates the small number of electrically charged  ions  and free  electrons . The electrons collide with other gas molecules , knocking electrons off them and creating more positive ions in a chain reaction called a  Townsend discharge .

27. As the tube was used, the gas molecules combined with or were trapped by vaporized residues from the anode and cathode which gradually increases the vacuum. When the vacuum became too high, no X-rays were produced and the tube was considered to be ‘ cranky .’

28. This ‘cranky’ tube could be heated by an alcohol lamp to drive gas molecules from its walls which maintained continued production of X-rays.

29. Crookes tube

30. Regulator Tubes: To increase the longevity of X-ray tube, automatically self-regulating and regenerative tubes were developed in 1896 by Queen and Company. It utilized the principle that certain chemicals ( caustic potash and potassium permanganate ) liberated gases upon heating and absorbed them upon cooling.

31. When the vacuum in the tube became high, resistance increased and the current supplying the tube was diverted to the low vacuum accessory bulb by means of adjustable wire. This resulted in heating of the caustic potash, which produced gas and caused the vacuum in the main tube to be lowered sufficiently to produce X-rays again.

32. Gas Regulated Tube (1902) in comparison with the modern X-ray tube

33. Vacuum Tube In an effort to eliminate the gas and stabilize the operation of X-ray tubes, J.E. Lilienfeld , an Austrian developed a tube in 1911 based on Field Current principles. The electrons were extracted from the cathode by using a high potential across the tube.

34. The operation of such a cold cathode tube was described as “ticklish.” Due to the use of a curved cathode, charges became so crowded on the curved part that they easily leaked ( Lilienfeld Effect). So, to increase the drain of electrons from the cathode the electrons were ejected from a pointed cathode.

35. Coolidge tube The real breakthrough in the tube design was the development of the hot cathode tube by William David Coolidge, (1913). Coolidge used a coil of tungsten as the source of electrons (as a filament cathode)in the new 2 tubes which out-performed Lilienfeld’s cathode tube.

36. It permitted: • Greater flexibility in the quality and quantity of X-rays produced. • Greater tube stability during the production of X-rays. • Smaller tube size. • Longer tube life. • Direct operation from a transformer .

37. An early problem with this new hot cathode tube was the conduction of heat away from the tungsten target. A tungsten anode backed by copper was found to be the most satisfactory method of dissipating heat rapidly; the heat was conducted to the radiation fins at the end of the tube or by circulating cold water through the anode stem.

38. Until 1918, all X-ray tube cooling was provided by means of air and water. Hirsh patented the idea of submerging the X-ray bulb in oil to effect greater cooling of the anode and tube. In 1919, Harry Waite submerged the tube and transformer as a single unit in the same oil bath.

39. Shockproof Dental X-ray Unit In 1918-1919, Coolidge and General Electric Co. introduced the Victor CDX Shockproof Dental X-ray Unit , which eliminated the exposed high tension wires. The principle of this design was to place the tube and high voltage components in an oil filled grounded compartment which acted as an electric insulator, coolant and radiation shield .

40. Victor CDX – shockproof tube housing (1919)

41. The advantage of this tube was that the electrical and fire hazard was eliminated. The anode and the tube length was reduced thereby permitting more rapid removal of heat.

42. Components of Modern x-ray tube 1.Cathode 2.Anode 3.Glass envelop 4.Oil insulation 5.Tube shield

43. Cathode: Has two main components: a)Filament b)Focusing cup FILAMENT: The filament is the source of electrons within the x-ray tube. It is a coil of tungsten wire about 2mm in diameter and 1cm or less in length . Filament typically contain about 1% of thorium , which greatly increases the release of electrons from the heated wire. It is mounted on two stiff wires that support it and carry the electric current.

44. These two mounting wires lead through the glass envelope and connect to both the high- and low-voltage electrical sources. The filament is heated by the flow of current from the low-voltage source and emits electrons at a rate proportional to the temperature of the filament. The filament has its own circuit powered by a relatively low voltage (10-15 volts) and 4 –6 amperes .

45. Most diagnostic X-ray tubes have two focal spots of different sizes and these are obtained by having two filaments, each in its own focusing cup, side by side in the filament assembly. By appropriate external switching either one or the other of the filaments ( not both ) is selected for use. The small filament is designed to be used with relatively small parts while the large filament is used when larger body parts are being x-rayed.

46. Focusing Cup The filament lies in a focusing cup, a negatively charged concave reflector made of molybdenum. The focusing cup electrostatically focuses the electrons emitted by the incandescent filament into a narrow beam directed at a small rectangular area on the anode called the focal spot .

47. The electrons move in this direction because they are repelled by the negatively charged cathode and attracted to the positively charged anode. The x-ray tube is evacuated to prevent collision of the moving electrons with gas molecules, which would significantly reduce their speed. This also prevents oxidation and burnout of the filament.

48. It is designed and shaped so that when the x-ray machine is powered up, electrons will literally “boil” off the filament. It glows white hot and the electrons hover around the filament in a “space charge” until the moment of exposure and then they accelerate very rapidly towards the anode which is not very far away. This process is known as “thermionic emission”/Edition effect.

49. Anode The anode consists of a tungsten target embedded in a copper stem. The purpose of the tungsten of the target in an x-ray tube is to convert the kinetic energy of the electrons generated from the filament into x-ray photons. This is an inefficient process with more than 99% of the electron kinetic energy converted to heat.

50. The target is made of tungsten, a material that has several characteristics of an ideal target material. It has a high atomic number (74), high melting point(3380*c), high thermal conductivity, and low vapor pressure at the working temperatures of an x-ray tube. The tungsten target is typically embedded in a large block of copper to dissipate heat.

51. Copper, a good thermal conductor, dissipates heat from the tungsten, thus reducing the risk of the target melting. In addition, insulating oil between the glass envelope and the housing of the tube head carries heat away from the copper stem. This type of anode is a stationary anode.

52. The focal spot is the area on the target to which the focusing cup directs the electrons from the filament. The sharpness of the radiographic image increases as the size of the focal spot decreases. The heat generated per unit target area, however, becomes greater as the focal spot decreases in size.

53. To take advantage of a small focal spot while distributing the electrons over a larger area of the target, the target is placed at an angle to the central electron beam. The projection of the focal spot perpendicular to the central electron beam ( the effective focal spot) is smaller than the actual size of the focal spot. This is known as the “ Line Focus Principle ’.

54. Typically, the target is inclined about 20 degrees to the central ray of the x-ray beam. This causes the effective focal spo t to be almost 1x 1mm , as opposed to the actual focal spot, which is about 1 x 3mm . The effect is a small apparent source of x rays and an increase in sharpness of the image with a larger actual focal spot for heat dissipation.

55. The angle of the target to the central ray of the x-ray beam has a strong influence on the apparent size of the focal spot. The projected effective focal spot is much smaller than the actual focal spot size .

56. Another method of dissipating the heat from a small focal spot is to use a rotating anode . In this case the tungsten target is in the form of a beveled disk that rotates when the tube is in operation. As a result, the electrons strike successive areas of the target, widening the focal spot by an amount corresponding to the circumference of the beveled disk and distributing the heat over this expanded area.

57. As a consequence, small focal spots can be used with tube currents of 100 to 500 miliamperes ( mA ), 10 to 50 times that possible with stationary targets. The target and rotor (armature) of the motor lie within the x-ray tube, and the stator coils (which drive the rotor at about 3000 revolutions per minute ) lie outside the tube. Such rotating anodes are not used in intraoral dental x-ray machines but may be used in cephalometric units and in medical x-ray machines requiring higher radiation output.

58. X-ray tube with a rotating anode, which allows heat at the focal spot to spread out over a large surface area

59. Glass Envelop The whole cathode and anode assembly are contained in a evacuated glass envelop. It is a leaded glass vacuum tube that prevents X-rays from escaping in all directions ( radiation leakage ). One central area of the leaded glass tube has a “ window ” that permits the X-ray beam to exit the tube and directs the X-ray beam towards the aluminum disk, lead collimator and PID.

60. Oil Insulation Expedite cooling and to insulate the tube, it is immersed in oil to ensure that it is electrically insulated and so that the oil will also help cool down the tube during operation. The cooling of the oil is sometimes assisted with a cooling fan. The oil serves two major purposes: One, it helps cool the x-ray tube just like oil in your car helps cool the engine.

61. Additionally, the oil helps insulate electrically for safety. And one additional thing it does is if you notice where the primary beam exits the housing, the x-rays must pass through a small thickness of oil before they emerge from the tube and this contributes to minimal filtration or absorption of the x-ray beam when you make an exposure.

62. It is important that the oil should not become too hot since insulators progressively lose their insulating properties as their temperature rises. Metal bellows which extends as the heated oil expands. If the bellows expand beyond a certain amount (i.e., the oil has exceeded a certain temperature) then they operate a micro-switch which prevents operation of the tube until the oil has cooled sufficiently.

63. Tube Shield The tube housing function to containing and supporting the X-ray tube and oil, and protecting them from external damage. The metal tube shield has two other very important functions to perform. Firstly, it provides a completely encircling metallic shield which, because it is firmly connected electrically to earth potential, protects the user from any possibility of electrical shock.

64. The second purpose of the tube shield is to afford protection to the radiographer and patient against unwanted X-radiation. It is arranged that any X-rays (both primary and secondary) which are not within the wanted beam are attenuated by the shield. This is usually achieved by lining the steel shield with appropriate thicknesses of lead , the actual thickness depending upon the likely intensity of radiation reaching that portion of the shield.

65. There is, an aperture opposite the target through which the maximum size of useful beam can emerge. This beam is reduced to the size required, i.e., that needed to just cover the film size in use, by a set of collimating diaphragms or by a cone.

66. References: 1.Textbook of Dental and Maxillofacial Radiology Freny R Karjodkar , Second Edition 2.Fundamental physics of radiology Meredith/Massey , Third Edition 3.ORAL RADIOLOGY Principles and Interpretation White and Pharoah , Sixth Edition