XRAY TUBE and production of x ray in india

X ray tube

1. X-RAY TUBE Mr. Subhash Chandra Yadav Associate Professor Department of Radiodiagnosis & Medical Imaging UCMS, Bhairahawa

2. Contents Introduction Types of Anode Components of x-ray tube Line focus principle and anode heel effect Actual vs. effective focal spot. Tube Failure Tube rating and charts Advances Types of X-ray Tube

3. Introduction An X-ray tube is a vacuum tube . Contains a pair of electrodes i.e. a Cathode and an Anode . Cathode is a filament (tungsten) that releases electrons when high voltage is applied. Anode is made up of tungsten, which attracts the electrons. When the electrons released from the cathode come in contact with the target (anode), they release energy in the form of photons.

4. Types of X-ray Tube According to the cathode Two types Cold cathode tube (Crookes tube) Hot cathode tubes (all other types) According to the anode Two types Stationary anode x-ray tube Rotating anode x-ray tube

5. Crookes Tube

6. Gas Tube (Crooke’s Tube) Historically, X-rays were discovered radiating from experimental discharge tubes called Crooke’s tube . Invented by British Physicist Sir William Crookes. Known as Cold cathode tube or Discharge tube . Early X-ray tube known as Gas tube because its action depends upon the presence of small residual amount of gas present in it.

7. Crooke’s tube consists of: Cathode is on the right, the anode is in the center with attached ‘heat sink’ at left. An Aluminum cathode plate (Z:13 MP: 660 o C) having curve shape to created the beam of electrons. A platinum anode target ( Z: 78, MP: 1768 o C). The device at top is a Softener used to regulate the gas pressure. Glass bulb with around 10 -6 to 5 x 10 -8 atm. pressure

8. How X-rays produced in Crookes Tube During operation, the residual gas in the tube would be absorbed by the glass and the metal electrodes leading to a reduction in the x-ray output. The side tube contained a few mica sheets which would release small amount of gas when heated by an electric heater wire. A high voltage was made between the electrodes (cathode and anode), this induces an ionization of the residual air, and thus an electron flow or "discharge" from the cathode to the anode. When these electrons hit the target, they are slowed down, producing the X-rays (Bremsstrahlung).

9. Limitations: Relatively low intensity of x-ray (not more than 5 mA) was unreliable and unstable as x-ray production depended upon the gas content which was a very variable factor. This tube can not produce X-rays continuously. We can not operate the kVp and the mAs independently as there is presence of gases. Presence of gas produced blackening of tube, thus shorting its life. Over heating due to heavy use.

10. The Coolidge Tube (Hot Cathode Tube) The Coolidge tube was improved by William Coolidge in 1913. The Coolidge tube, also known as hot cathode tube, is the widely used. It works with a very good quality vacuum (about 10 -4 Pa, or 10 -6 Torr ). In the Coolidge tube electrons are produced by Thermionic effect i.e. (on heating metal element emit electrons). Coolidge tube is the prototype for modern X-ray tubes being used today

11. X-Ray Tube Components Protective housing Glass envelope Cathode assembly Anode assembly

12. 1 . Tube/Protective Housing 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 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.

13. 2 . Glass Envelope Made up of Pyrex glass (borosilicate). to withstand tremendous heat generated during x-ray. Envelope maintains a vacuum inside the tube. to control the number and speed of the accelerated electrons independently. Tube window A 5cm square segment of glass that is thinner than the rest of the glass envelope. Contributes to inherent filtration. 0.5mm Al equivalency . 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.

14. Leakage Radiation: X-rays that escape through protective housing are they contribute no diagnostic info and result in unnecessary exposure. Leakage radiation must not exceed 100 mR / hr (1 Gy ) at 1 meter

15. Cathode Assembly A negative electrode of an x-ray tube. Contains two primary parts: Filament Focusing cup

16. Filament Cathode (filament) is made of thin tungsten wire which is a source of electrons . It works on the phenomenon of Thermionic emission . Tungsten is used as filament material because; High melting point (3370 degree Celsius), Low vapor pressure, High thermal conductivity and specific heat Tungsten filament wire consists of: 0.2mm in diameter that is coiled to form a vertical spiral Diameters of spiral - 0.2 cm Length - 1.0 cm

17. Thermionic emission 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.

18. Connecting Wires Used to supply voltage and current to the filament which are attached with the filament heating circuit and also H.T. Transformer. Cathode filament is mounted on two supporting or connecting wire. One wire is connected to low voltage for filament current and another is connected to high voltage current to produce high voltage b/w anode and cathode. Current 3-5 amperes and 10-12 volts are used in filament supply by filament heating transformer .

19. Filament Current & Tube Current 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 )

20. Some modern x-ray tubes have two filaments of different sizes placed side by side. This is known as a dual filament – one smaller and one larger. These two filaments produce two different sizes of electron foci on the anode, one for general use and the other (smaller) for fine focus applications requiring less geometric unsharpness.

21. Focusing Cup It is the device surrounding the cathode filament in an X-ray tube. This is actually a third electrode in the tube. It is usually made up of Ni because of: Light wt. Poor thermal conductivity High melting point . It is maintained at same negative terminal as that of filament.

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 (large bombardment area on anode). If this electrode is not present, the electrons would hit the anode over a very large area.

23. Space Charge It is inability to operate the current and voltage of the x-ray tube independently . Electrons emitted from the tungsten filament form a small cloud in front of the filament. This collection of negatively charged electrons is known as space charge. Tendency of space charge to limit emission of other electrons from filament, called space charge effect. Remedy: high voltage is applied → electrons moved towards anode → space charge effect is diminished. ↑ KVp = space charge effect ↓

24. Space Charge

25. Anode The anode (target) is the positive electrode of the tube. Usually made up of tungsten . Small amount of rhenium ( approx. 10%) is added to prevent development of crack on anode surface. Tungsten is used as target material because; high atomic number to increase the X-ray production efficiency, high melting point to withstand high temperature, high thermal conductivity to dissipate heat quickly, low vapor pressure at high temperature to prevent the evaporation of target material

26. Types Two types of anode: Stationary anode Rotating anode

27. Stationary Anode Stationary anodes are used in dental x-ray and some portable x-ray machine where high tube current and power are not required i.e. used when lower heat quantities are produced. Consists of a small plate of tungsten alloy of 2 or 3 mm thick that is embedded with a large copper block to facilitate heat dissipation. Copper prevents excessive raise in temperature because of high thermal conductivity. Square or rectangular in shape with each dimension usually greater than 1cm. Anode angle is usually 6-20 deg.

28. The heat can damage the anode target , causing pitting while projectile electrons strike the some particular area of an anode (target) . Once a target has been damaged in such a way, the x-rays produced from that are a scatter in undesirable directions.

29. Rotating Anode The Rotating anodes are used for all the other applications where high tube output and high tube ratings are required 125 – 150 KVp upto 1200 mA The rotating anode allows the electron beam to interact with a much larger target area. The heating of the anode is not confined to a small area.

30. Rotating Anode Construction Consists of tungsten or alloy of tungsten with rhenium. Rotating anodes range in size from about 7.5 cm to 12.5 cm Anodes have an angle of about 6° to 20°. Speed of anode rotation is 3000 to 3600 rpm practically. A tungsten disc rotates during exposure, thus effectively increasing the area bombarded by the electrons. The energy is dissipated to much larger volume as it is spread over the anode disc.

31. Components Rotating Target Anode steam Stator of induction coil Rotor of induction coil Ball bearings Safety circuit

32. Anode Steam: The stem of the anode is the shaft between anode and the rotor. Usually made up of molybdenum. It has high melting point but poor conductor of heat; thus it protects ball bearing from undesirable heat. The length of the molybdenum steam should be as short as possible ( ↑ length = ↑ inertia = ↑ load on the bearings) Bearings: Increases life of the tube. Lubricant used is silver. Silver is suitable in vacuum

33. Rotor and Motor System The rotating anode is driven by an electromagnetic induction motor, which consists of two principal parts separated from each other by the glass envelope. STATOR: Located outside the glass envelope, consists of a series of electromagnets equally spaced around the neck of the tube. ROTOR: Inside the glass envelope is a shaft made of bars of copper and soft iron fabricated into one mass.

34. Induction Motor Provides magnetic field necessary for induction of current. The anode stator motor must accelerate the anode to working speed rapidly ready for an x-ray exposure then must bring it back to stationary equally rapidly to prevent wear and wobble on slowing down. The “motor” consists of electromagnet coils round the glass to provide a rotating magnetic field to induce the currents and produce the forces needed to rotate the copper rotor. The induction motor is energized for about 1 second before high voltage is applied to the x-ray tube . This delay ensures that electrons do not strike the target before the anode reaches its maximum speed of rotation.

35. Rotor The anode disc needs to be rotate at high speed and this is achieved by attaching the stem to a large copper rotor, which forms the armature of a motor. The magnetic field provided by stator induces current in copper rotor. This current provides power for rotation of anode assembly. The rotor bearing are special as they need to operate in a vacuum, conduct a high voltage and reach high temperatures (500°C)

36. Why increased speed of rotation ↑ speed of rotation = ↑ ability of anode to withstand heat Modifications to increase speed of anode Decrease anode-stem length ( ↓ inertia) Use of two sets of ball bearings. Decrease weight of anode ( ↓ inertia) compound anode disc molybdenum or graphite

37. Focal Spot Focal spot is the area on the anode, which is bombarded by the electrons and x rays are produced. Focal spot size determines amount of x-rays falling on image receptor and resolution of image

38. Dual focus x-ray tube It has two different focal spot - Large focal spot Small focal spot “Small focal spot arise from a small filament” Focal spot small , the heating of the target is concentrated in a small area and damages the target area of X-ray production.

39. Large actual focal size : more x-ray yield better heat dissipation suitable for High Intensity exposure Small effective focal size: for image sharpness, short exposure. Steeper anode angle restricts the field size. Heel effect.

40. Line-focus Principle Relation between anode angle and focal spot. Area of the target that interacts with the electron beam is called actual focal spot. Angle of the target surface with respect to the central ray in the x-ray field called effective focal spot.

41. Effective focal length = Actual focal length × sin θ Where, θ is the anode angle. Anode angle causes the effective focal spot length to be smaller than the actual focal spot length This foreshortening of the focal spot length, as viewed down the central ray, is called the lined focus principle.

42. The advantages of line focus principle is: the heat to be spread over an area about 3 times larger than the effective focus minimizing the temperature rise in the target (requiring a large focal area) minimizing the size of the X-ray source (requiring a small focal area). provides the sharpness of image of a small focal spot

43. Heel Effect Intensity of x-rays depends on the anode angle at which the x-rays are emitted from the focal spot. The intensity of x-ray beam towards anode side is less than that towards cathode side. Intensity of the beam towards the anode side of the tube is less because of absorption of some of the x-ray photons by the target itself.

44. Heel Effect: to Compensate A FILTER may be installed in the tube housing near the exit port of the x-ray beam. The thickness of filter increases from the anode to the cathode side of the x-ray beam. Positioning thicker portions of a patient near the cathode side of the x-ray beam also helps to compensate for the heel effect.

45. Breast

46. Remedy Used for obtaining balanced densities in radiographs of body parts of different thickness, i.e. thicker parts towards cathode When FFD is increased, heel effect is reduced . For smaller films, less heel effect . Larger the anode angle, less heel effect .

47. Causes of x-ray tube failure All causes of tube failure relate to the thermal characteristics of the tube. When the temperature of the anode during a single exposure is excessive, localized melting and pitting occurs. These surface irregularities lead to variable and reduced radiation output. If the temperature of the anode increases to rapidly, the anode can crack and then become unstable in rotation.

48. X-ray tube rating charts It is essential for the radiographer to understand how to use tube rating charts. There are three types of x-ray tube rating charts: X-ray/Radiographic Rating Chart Anode Cooling Chart Tube Housing Cooling Chart

49. Radiographic Rating Chart Most important of the three charts . Conveys which radiographic exposures are safe and which are unsafe. Chart shows a family of curves for different mA Two axis X & Y show scales of Time and kV respectively For a given mA, any combination of kVp and time that lies below the curve is safe Any combination that lies above the curve of desired mA is unsafe Modern x-ray systems have a microprocessor control that does not allow unsafe exposure to be made.

50. Use of Series of radiographic rating charts Important to use the correct rating chart e.g Rating charts for different filament sizes (focal spot sizes) For different anode rotation speeds For different anode angles For the type of high voltage rectification Radiographic rating charts can be used to check the proper operation of microprocessor control protection circuit

51. Anode cooling chart Anode cooling charts contain the information about the thermal capacity of an anode and its heat dissipation characteristics. It does not depend on the filament size and the speed of rotation Usually the cooling is rapid at first and slows as the anode cools In addition to knowing the maximum heat capacity the chart is used to determine the length of time required for complete cooling after any level of heat input.

52. Housing Cooling Chart The cooling chart for the housing of the x-ray tube has a similar shape as the anode cooling chart. The maximum heat capacity of the housing is in the range of several million heat units. Complete cooling after maximum heat capacity requires from 1 to 2 hours.

53. Advancement in Rotating X-ray Tube Metal ceramic x-ray tube Grid controlled x-ray tube Stereoscopic x-ray tube

54. Advancements in Rotating X-ray Tube Main features of rotating X-ray tube: The shape of the glass envelope has been modified to accommodate the different types of the electrode and the rotating assembly. Dry lubricants are used for rotation like silver , lead Rotation of anode is achieve by the process of electro-magnetic induction in rotor and stator assembly. Cathode cup and filament are offset the target track near the periphery of the beveled anode track (angled at 6° to 20°). Disc is connected to anode stem made up of Mo .

55. Metal-ceramic Tubes A more recent development in X-ray tube construction is the metal ceramic tube, which is made from a steel (ferrochrome ) cylinder brazed to alumina ceramic (aluminum oxide ) insulators at each end. These insulators carry the anode and cathode assemblies. The metal-ceramic tubes are smaller and more robust than their glass equivalents. They have another advantage, in that they enable more flexibility in the electrical circuitry associated with the tube. Offers greater heat dissipation results less load to the x-ray tube. Metal envelope grounded offers no chance of arcing of x-ray tube.

56. Features of Metal Ceramic X-ray Tube Offer greater heat dissipation results less load to the x-ray tube. Metal envelope grounded offers no chance of arcing of x-ray tube.

57. Advantages of Metal-ceramic Tube Higher Tube Loading: Allows higher tube currents to be used because of larger heat storage capacity of anode Longer Tube Life Deposition of tungsten on the glass wall acts as electrode causing arcing bet. Glass and filament shortening tube life. When metal enclosure is grounded, this deposition will not alter grounding thus increasing its life Reduced off-Focus Radiation Electrons back scattered from the anode may strike anode again producing x-rays from areas other than focal spot. The metal enclosure decreases off focus radiation by attracting off focus electrons to the grounded metal wall relatively Positive as compared to electrons. Low atomic no. of metal may produce few and low energy x-rays.

58. Grid Controlled X-ray Tube A third electrode called grid is used. Focusing cup surrounding the filament cup is used as third electrode to control the flow of electron. A negative bias voltage around 1500 V is applied to the cup relative to the filament to punch off the flow of electron. Thus focusing cup acts as exposure switch to turn the current ON or OFF when required. Application in capacitor discharged mobile radiographic equipment , pulsed-fluoroscopy and cardio-angiography and vascular angiography.

59. Different type of x-ray tubes Depending Upon Applications Mammography tube CT X-ray tube X-ray tube for angiography Radiotherapy X-ray tube Stereographic X-ray tube Field emission X-ray Tube

60. Mammography X-ray Tube For maximum visualization of soft tissues of the breast having similar ability to absorb x-rays a beam of soft radiation (longer wavelength) is required. Longer wavelength can be produced by selecting x-ray tube which operate at low KVp (25-32)

61. Features of a Mammography Tube Use of target made of molybdenum. Closer spacing of cathode and anode. Beryllium window : is used as it has low atomic no.(4) & lower absorption of x-rays. Use of molybdenum filter in place of aluminum filter. Focal spot size-0.1-0.3mm. Heat storage capacity- 0.3-0.5 M.H.U .

62. Latest Developments in Mammography Units Introduction of the dual metal x ray tubes (having dual track of molybdenum/vanadium & rhodium). Rhodium track & filter produces a slightly higher x ray spectrum for superior penetration of the dense breast tissue in the younger women and in those who have undergone radiation treatment or on hormone therapy. Mo/tungsten dual track with a high emission flat emitter cathode with different k-edge filters Mo & rhodium meant for normal and dense breast. But with an increase in x-ray tube voltage from 25 to 32Kv simultaneously replacing Mo with a rhodium filter the x-ray spectrum for a tungsten anode is clearly shifted and higher energy especially advantageous for the radiography of large subject/dense breasts.

63. Stereographic X-ray Tube Similar to conventional rotating anode x-ray except:- Rotating anode is bombarded simultaneously by two beams of electrons from two independent cathode assemblies. Used for stereographic and stereo-fluoroscopic x-ray examination. Two Filaments With focusing Anode target disk

64. CT X-ray Tube Since CT require longer continuous exposure time at higher KV and mA than needed for general radiography. These have been charged with heavy duty rotating anode tube with higher thermal capacity and smaller focal spot (up to 0.6mm). These tubes are air cooled with current value up to 600mA. It Should supply monochromatic X-ray beam for accurate reconstruction. Earlier model used were oil cooled , Fixed anode relatively large (2 x 16mm) focal spot operated at 120 KVp & 30 mA & heavily filtered as those use in radiotherapy.

65. Developments in CT Tubes One of the more interesting developments is the Siemens Straton x-ray tube, which is currently available as an option on Sensation 16 scanners, The tube itself is a radical new design, where the entire tube body rotates, rather than just the anode, as is the case with conventional designs. This change allows all the bearings to be located outside the evacuated tube, and enables the anode to be cooled more efficiently. The Straton has a low inherent heat capacity of 0.8 MHU, but an extremely fast cooling rate of 5 MHU / min

66. This compares with typical figures of 7-8 MHU and up to 1.4 MHU / min for existing tubes. The heat capacity and cooling rate combine to produce a tube which Siemens claim is ‘0 MHU', implying that tube cooling considerations are a thing of the past. Sensation 16 scanners fitted with the Straton tube now have a fastest scan time of 0.37 seconds.

67. Thank You