Production of x rays

ORAL RADIOLOGY

1. PRODUCTION OF X-RAYS

2. Contents X-ray machine Production of X-rays Factors controlling the X-ray beam Tube current Tube voltage Exposure time Filtration Collimation Inverse square law Interaction of X-rays with matter Coherent scattering Photoelectric effect Compton effect References

3. QUESTIONS X-ray machine ( Manipal 2011 ) Draw the labeled diagram of x-ray tube & expalin the functions of each component (DA 2014) Filtration & collimation of diagnostic x-ray machine ( Amrita 2014 ) Production of x-rays. (PAHER 2016 , Manipal 2010 ) Factors controlling the X-ray beam (DA 2016 )

4. X-rays are produced by the sudden deceleration or stoppage of a rapidly moving stream of electrons at a metal target in a high vaccum tube. X-ray tube is an important part of any X-ray machine. Dental X-ray machine is made up of three parts or components: Control panel Extension arm Tube head

5. CONTROL PANEL Control panel of the dental X-ray machine contains- An on and off switch & an indicator light. An exposure button . Control device( time, kvp , mA selectors) to regulate the X-ray beam.

6. Extension arm It contains electrical wires that extend from control panel to the tube head. It also allows the movement and positioning of the tube head .

7. Tube Head It is a tightly sealed, heavy metal housing that contains: -Metal body that surrounds the x-ray tube -Insulating oil -Tube head seal -X-ray tube - Aluminium disks -Lead collimator -Position indicating device

8. Metal Housing: This is the metal body of the tube head that surrounds the X-ray tube and transformer and is filled with oil, it protects the X-ray tube and ground the high voltage component. Insulating Oil : It is that which surrounds the X-ray tube and transformer inside the tube head, it prevents over heating by absorbing the heat created by the production of X-rays

9. Tube Head Seal: Aluminum or leaded glass of the tube head that permits the exit of X-rays from the tube head, it seals the oil in the tube head and acts as a filter to the X-ray beam X-ray Tube : It is the main X-ray generating system.

10. Aluminum disks – the sheet of 0.5mm thick aluminum is placed in the path of the X-ray beam. It filters the non-penetrating, longer wavelength X-rays which results in a higher energy & more penetrating useful beam, which is less harmful to patients. 2 types of filtration - inherent filtration - added filtration

11. Inherent Filtration It takes place when the primary beam passes through the glass window of the x-ray tube, the insulating oil and the tube head seal. It is approximately 0.5 to 1mm thickness of aluminum .

12. Added filtration Placement of aluminum disks in the path of the x-ray beam between the collimator and the tube head seal. Aluminum disks may be added in 0.5 mm increments . Total filtration (inherent + added filtration) is regulated by the state and federal law (in USA).

13. Dental machines operating: At or below 70 kvp require a minimum total filtration of 1.5 mm of aluminum thickness. Above 70 kvp require a minimum total filtration of 2.5 mm of aluminum thickness.

14. Lead collimator It is a lead plate with a central hole that fits directly over the opening of the metal housing where the x-ray exits. Used to restrict the size and shape of the x-ray beam and thus reduce exposure to the patients. Collimator is of two types: Fixed Adjustable

15. In the dental X-ray machine usually the fixed collimators are used, they may either have a round or rectangular opening. A rectangular collimator restricts the size of the X-ray beam to an area slightly larger than a size of 2 intraoral (normal adult intraoral periapical films) and thus significantly reduces the patient exposure.

16. A circular collimator produces a cone shaped beam that is 2.75 inches in diameter and is considerably larger than the size of two intraoral periapical films, and thus leads to an increased skin dose to the patient.

17. Position indicating device Open end lead cylinder Extend from the opening of metal housing of the tube head also called as “ cone”. It appears as an extension of the tube head; it aims and shapes the X-ray beam. There are three types of PIDs- Conical Rectangular round

18. Both rectangular and round PIDs are available in two lengths: Short(8 inches) Long(16 inches)

19. X-RAY TUBE Design introduced by W.Coolidge in 1913 An x-ray tube is composed of a cathode and an anode situated within an evacuated glass envelope or tube Electrons stream from a filament in the cathode to a target in the anode, where they produce x rays.

20. For the x-ray tube to function, a power supply is necessary to Heat the cathode filament to generate electrons Establish a high-voltage potential between the anode and cathode to accelerate the electrons toward the anode.

21. CATHODE It is principally composed of two parts: Filament Focussing cup It is the source of electrons in the tube, made up of a tungsten wire Diameter – 0.2cm Width – 1-2 cm Thickness- 0.1-0.2 mm Length – 7- 15 mm

22. It is mounted on two stiff wires that support it and carry the electric current. These two mounting wires lead through the glass envelope and connect to both the high- and low-voltage electrical sources. The filament is heated to incandescence by the flow of current from the low-voltage source and emits electrons at a rate proportional to the temperature of the filament

23. Focusing cup It is a negatively charged concave reflector cup of molybdenum or nickel and houses the filament. The focal spot - The focusing cup parabolic shape electrostatically focuses the electrons emitted by the filament into a narrow beam, directed at a small rectangular area in the anode.

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

25. Anode It consist of a wafer thin tungsten plate embedded in a solid copper stem. Target: Function is to convert the kinetic energy of electrons into x-ray photons Inefficient process & more than 99% of energy is converted into heat. Copper – Good thermal conductor, dissipates heat from tungsten , reduces risk of target melting

26. Why is Tungsten used? High atomic number (74) High melting point (3422 ̊ C) – can be raised to high temperature without detriment High thermal conductivity(173 W ) thus readily dissipating its heat into the copper stem. Low vapor pressure - low vapor pressure of tungsten at high temperatures helps maintain the vacuum in the tube at high operating temperatures.

27. Anode - Types

28. Stationary Anode Small plate of tungsten embedded in a large mass of Copper Helps in heat dissipation from small focal spot.

29. Rotating Anode A large disc of tungsten/alloys of tungsten, which theoretically rotates at speed of about 3000 rpm Used in tomographic or cephalometric units Used in medical x-ray machines requiring high radiation outputs

30. Focal spot Focal spot is the area on target to which the focusing cup directs the electrons from the filament It is the region where X-rays & the heat are produced Heat generated per unit target area becomes greater as the focal spot decreases in size. To take advantage of small focal spot along with proper heat dissipation, target is placed at an angle with respect to electron beam in the tube.

31. Line focus principle The Benson line focus principle developed in 1918 is a method of reducing the effective focal spot size. Target is inclined about 17°-20° to central ray of x-ray beam Use of an anode with the target angulated such that the effective focal spot is smaller than the actual focal spot size.

32. Projection of focal spot perpendicular to electron beam is effective focal spot Actual focal spot is projected perpendicular from target Actual focal spot = 1x 3 mm, Effective focal spot = 1x 1 mm

33. Heel Effect: Intensity of the beam is not uniform across the exposure field Cathode side of the beam is more intense than the anode side because of self-absorption of some of photons in the target itself.

34. Effect arises because x-rays are generated at a small depth inside the target and target surface gets rough with use, therefore attenuation.

35. Electrical Circuit of the X-Ray Unit Electrical Current : Refers to the flow of electrons through the conductor Direct Current : When the electrons flow in one direction through the conductor Alternating Current : Electrical current in which electrons flow in two, opposing directions

36. Rectification is the process of converting AC to DC A rectifier essentially eliminates the – ve phase of the AC, leaving the + ve phase to behave as DC

37. Most dental x-rays, the amount of heat produced at the anode does not give rise to excessive electrons, when the current changes its direction, there are no electrons at the anode to travel back to the cathode, this half of the cycle is called inverse voltage, hence the dental x-ray tube is called self or half wave rectifying.

38. Amperage- is the measurement of the number of electrons moving through a conductor. Current- is measured in amperage or milli amperage. If the mA increases, the number of electrons passing through the cathode filament increases. Voltage : Measurement of electrical force that causes electrons to move from negative cathode to positive anode.

39. After entering the x-ray machine housing, the electrical current is directed into 2 transformers. One of these transformers, step-up, increases the incoming voltage into the range of thousands of volts or KV range (high-voltage circuit) Other transformer, step-down, decreases the incoming voltage to 10V (low voltage circuit or filament circuit)

40. Filament circuit, using 10V, regulates the flow of electrical current to the filament of the x-ray tube, & is controlled by the mA settings Filament current regulates the tube current which consists of electrons flowing from negative cathode to positive anode High voltage circuit then provides the high voltage required to accelerate the electrons & to generate the x-rays & is regulated by the kilovoltage settings

41. Power supply Primary functions are to : Provide low-voltage current to heat filament by use of step-down transformer Generate high potential difference between anode and cathode by use of high-voltage transformers

42. TRANSFORMERS Is a device that is used to either increase or decrease the voltage in an electrical circuit 3 transformers are used to adjust the electrical circuit -step-down transformer -step-up transformer -autotransformer

43. Step-down Transformer: Used by the filament circuit. It is used to decrease the voltage from the incoming 110-220 line voltage to 3-4V as required for the filament circuit. (This transformer has more coils in the primary coil than in the secondary coil).

44. Step-up Transformer: : It is used to increase the voltage from the incoming 110-220 line voltage to 65,000- 1,00,000 volts as required by the high voltage circuit.

45. Autotransformer : Converts primary voltage from the input source into secondary voltage Secondary voltage is regulated by kvp selector dial , selects voltage from different levels on autotransformer and applies it across primary winding of high-voltage transformer Accelerates electrons from cathode to anode and generate x-rays

46. Timer A timing control device to control x-ray exposure time is included in the primary circuit of the high voltage supply

47. Tube Rating And Duty Cycle X- ray tubes produces heat at the target while in operation. Heat buildup at the anode is measured in heat units(HU), HU= kVp × mA × second Heat storage capacity for anodes of dental dignostic tubes is approx. 20 kHU . Heat is removed by copper anode and then to the surrounding oil and housing tube and by convection to the atmosphere.

48. Production of X-rays The kinetic energy of electrons is the tube current is converted into x-ray photons at the focal spot of an X-ray tube by two mechanisms : Bremsstrahlung radiation Characteristic radiation

49. Bremsstrahlung Radiation Bremstrahlung radiation, the primary source of X-ray photons from an X-ray tube, is produced by 1 .Direct hit of electron on nucleus in target – during this type of collision all the kinetic energy of high speed electron will be converted into single X-ray photon with maximum energy.

50. The energy of the resultant photons in units of kilo electron volts is numerically equal to the energy of the electron that is in turn equal to the kilovoltage applied across the x-ray tube at the time of its passage.

51. 2. By passage of electron near nucleus , which results in electron being deflected and decelerated. This deceleration causes it to lose some of its kinetic energy. This energy is given off in the form of a photon of electromagnetic radiation, which has an energy equal to that lost by the deflected electron i.e. photon of lower energy.

52. Characteristic Radiation : Characteristic radiation occurs when a bombarding electron of the tube current displaces an electron from a shell of a target atom, thereby ionizing the atom. Incident electron ejects photoelectron from inner orbit, creating vacancy. Inner vacancy is filled with electron from outer orbit.

53. Photon is emitted with energy equal to difference in energy levels of two orbits. Electrons from various orbits may be involved, giving rise to other photons. Energies of photons thus created are characteristic of target atomic composition (i.e. energy levels of various electron orbital levels).

54. Factors Controlling the X-ray Beam Exposure T ime (Timer) Tube current ( mA ) Tube Voltage Peak ( kVp ) Filtration Collimation Inverse square law

55. Exposure time Exposure time(Timer) : As exposure time increases, so does the total number of photons, but the mean energy and maximal energy of the beams the uncharged.

56. Tube current Tube current (MA) : Quantity of radiation produced by an x-ray tube (i.e. number of photons that reach the patient and film) is directly proportional to tube current and time the tube is operated.

57. Tube voltage Tube Voltage ( kvp ) : As kvp is increased (with mA held constant), a corresponding increase occurs in mean energy of the beam, the total number of photons emitted, and maximal energy of photons Quantity of X-ray radiation is directly proportional to tube current and exposure time Quality of X-ray radiation is directly proportional to tube voltage

58. Filtration : The purpose of conventional filtration is to remove low energy X-ray photons selectively from x-ray beam. This results in decreased patient exposure with no loss of radiologic information .

59. Total Filtration: Is sum of the inherent filtration plus any added external filtration. Governmental regulations require the total filtration in path of a dental x-ray beam to be equal to equivalent of 1.5 mm of aluminium to 70 KVp 2.5 mm of aluminium for all higher voltages

60. Inherent Filtration : Consists of materials that X-ray photons encounter as they travel from the focal spot on the target to form the usable beam outside the tube enclosure . Inherent filtration of most X-ray machine ranges from equivalent of 0.5 to 2 mm of aluminium .

61. Examples of inherent filtration are : Glass wall of the X-ray tube. Insulating oil that surrounds many dental tubes. Barriers material that prevents oil from escaping through the x-ray port.

62.   External Filtration : Aluminium disks placed over the port in the head of the x-ray machine. Aluminium wedge filter is a part of cephalostat .

63. Collimation : A collimator is a metallic barrier with an aperture in the middle used to reduce the size of the x-ray beam and therefore the volume of irradiated tissue within the patient lese of collimation also improves image quality. Dental x-ray beams are usually collimated to a circle 2¾ inches (7cm) in diameter.

64. Types of Collimators : Round Collimator : is a thick plate of radiopaque material with a circular opening centered over the port in the x- ray head .Round collimator are built into open – ended aiming cylinders.

65. Rectangular Collimator : limits the size of the beam to just larger than the x- ray film, thereby reducing patients exposure

66. Intensity of a x-ray beam is inversely proportional to the square of distance between the source and the point of measure The relationship is follows I1/I2 = (D2)2/(D1) Where I is Intensity and D is distance Inverse Square Law :

67. Interaction of X-rays with Matter Intensity of an X-ray beam is reduced by interaction with the matter it encounters. X-ray photons are attenuated by absorption and scattering of X-ray photons out of the absorber as a result of interactions with the orbital electrons of component atoms. There are three mechanisms – Coherent scattering Photoelectric effect Compton effect

68. Coherent Scattering (Thompson effect or classic scattering) Coherent scattering (also known as classical, elastic , or Thompson scattering ) may occur when a low-energy incident photon (less than 10 keV ) passes near an outer electron of an atom. The incident photon interacts with the electron by causing it to become momentarily excited at the same frequency as the incoming photon .

69. The incident photon ceases to exist. The excited electron then returns to the ground state and generates another x-ray photon with the same frequency (energy) as in the incident beam . Usually the secondary photon is emitted at an angle to the path of the incident photon. The net effect is that the direction of the incident x-ray photon is altered.

70. Coherent scattering accounts for only about 7% of the total numberof interactions in a dental exposure Coherent scattering contributes little to fi lm fog because the number of scattered photons is small and their energy is too low for many of them to reach the film or sensor.

71. COMPTON SCATTERING: Or inelastic scattering is an interaction of photons with free or loosely bound outer shell electron. The photon gives some of its energy to the electron and it, itself continues in a new direction, but with reduced energy and hence with increased wavelength.

72. The ejected outer shell electron is called compton or recoil electron. If scattered through a small angle, very small amount of energy is lost to the outer electron.

73. The recoil electrons further ionizing interactions with the tissues, and gradually lose energy along their tracts by causing secondary radiations and consequent biological damage. Due to their energy, rays can emit photoelectrons from metals, when allowed to fall on them.

74. PHOTOELECTRIC EFFECT: It is a process of interaction of the incident photon and the bound electron leading to emission of characteristic radiation. It occurs when an incident photon collides with a bound electron in the atom of the absorbing medium. The incident photon ceases to exit and its energy helps to eject a bound electron from its shell to become a recoil electron or a photo electron.

75. The kinetic energy imparted to the recoil electron is equal to the energy of the incident photon minus that required to overcome the electron binding energy. The orbital vacancy caused by the electron reshuffle and the neutrality is obtained by attracting an electron from outside. During this rearrangement characteristic radiation is emitted.

76. References Text book of oral radiology -white & Pharoah 6 t h edition Text book of oral radiology - Freny Karjodkar 1 st edition Text book of diagnostic radiology – cristensen’s 4 th edition Text book of oral radiology - Ghom

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