1. Medical Equipment Technology Department 1 Introduction to Biomed. Imaging Systems Lecture No.5-6 Dr. Yousif Mohamed Y. Abdallah

2. X-Ray Production

3. Objectives:  X-ray tube interactions  Characteristic and Bremsstrahlung X-rays  X-ray emission spectrum

4. X-ray Imaging System PRINCIPAL PARTS –Operating Console –High-voltage generator –X-ray tube PRIMARY FUNCTION –The system is designed to provide a large number of e- at cathode with high kinetic energy focused to a small target at anode.

5. X-ray Tube Construction Radiographic Equipment G F E D C A B

6. How “X-rays” are created  Power is sent to x-ray tube via cables  mA (milliamperage) is sent to filament on cathode side.  Filament heats up – electrons are produced  Negative charge

7. How “X-rays” are created Positive voltage (kVp) is applied to anode Negative electrons are attracted across the tube to the positive anode. Electrons slow down and finally come to rest Electron beam is focused from the cathode to the anode target by the focusing cup

8.  The distance between filament and the x-ray tube target is 1 cm.  Velocity of electron is raised from zero............half the speed of light

9. E- traveling from cathode to anode  Projectile electron interacts with the orbital electron of the target atom.  This interaction results in the conversion of electron kinetic energy into thermal energy (heat) and electromagnetic energy in the form of infrared radiation (also heat) and x-rays.

10. Tube Interactions  Heat (99%)  x-rays (1%)  X-rays = Characteristic Bremsstrahlung

11. Heat Most kinetic energy of projectile e- is converted into heat – 99% Projectile e- interact with the outer-shell e- of the target atoms but do not transfer enough energy to the outer-shell e- to ionize Outer shell electrons are simply raised to an excited/ higher energy level.

12. Heat production  Outer shell electrons immediately drop back to their normal energy level with the emission of infrared radiation.  The constant excitation and return of outer shell electrons are responsible for most of the heat generation

13. Heat is an excitation rather than an ionization

14. Heat production  Production of heat in the anode increases directly with increasing x-ray tube current  Doubling the x-ray tube current doubles the heat produced  Increasing kVp will also increase heat production

15.  Efficiency of x-ray production is independent of the tube current  Efficiency of x-ray production increases with increasing kVp.  At 60 kvp.........0.5%  At 100 kVp.......1%  At 20 MV..........70%

16. Characteristic Radiation Projectile electron interact with inner shell electron Projectile e- with energy high enough to totally remove an inner-shell electron of the target atom e.g. tungsten Characteristic x-rays are produced when outer- shell e- fills an inner-shell

18. Only K-characteristic x-rays of tungsten are useful for imaging

19. Bremsstrahlung Radiation  Bremsstrahlung is produced by projectile e- interacting with the nucleus of a target atom

20. Bremsstrahlung Radiation A projectile e- that completely avoids the orbital e- as it passes through a target atom may come close enough to the nucleus of the atom to come under the influence of its electric field projectile e- kinetic energy to EM energy electrostatic force

21. Bremsstrahlung Radiations  As the projectile electro passes by the nucleus, it is slowed down and changes its course, leaving with reduced kinetic energy in a different direction.  This loss of kinetic energy reappears as an x- ray.

22. Bremsstrahlung is a German word meaning “slowed-down Radiation”

23. X-ray energy  Characteristic x-rays have very specific energies. K-characteristic x-rays require a tube potential of a least 70 kVp  Bremsstrahlung x-rays that are produced can have any energy level up to the set kVp value. Brems can be produced at any projectile e- value

24. Discrete spectrum  Contains only specific values

25. Characteristic X-ray Spectrum  Characteristic has discrete energies based on the e- binding energies of tungsten  Characteristic x-ray photons can have 1 of 15 different energies and no others

26. Characteristic x-ray emission spectrum

27. Continuous Spectrum  Contains all possible values

28. Bremsstrahlung X-ray Spectrum  Brems x-rays have a range of energies and form a continuous emission spectrum

29. Factors Affecting the x-ray emission spectrum Tube current, Tube voltage, Added filtration, Target material, Voltage waveform The general shape of an emission spectrum is always the same, but the position along the energy axis can change

30. Quality  The farther to the right the higher the effective energy or quality

31. Quantity  The more values in the curve, the higher the x- ray intensity or quantity

32. mAs  A change in mA results in the amplitude change of the x-ray emission spectrum at all energies  The shape of the curve will remain the same

33. mA increase from 200 to 400

34. kVp  A change in voltage peak affects both the amplitude and the position of the x-ray emission spectrum

35. Filtration Adding filtration is called hardening the x-ray beam because of the increase in average energy Filtration more effectively absorb low-energy x- rays than high energy x-rays Characteristic spectrum is not affected & the maximum energy of x-ray emission is not affected

36. Filtration  Adding filtration to the useful beam reduces the x-ray beam intensity while increasing the average energy (higher quality)  Lowering the amplitude and shifting to the right

37. What A does this graph indicate?

38. Target Material  The atomic number of the target affects both the quantity and quality of x-rays  Increasing the target atomic number increases the efficiency of x-ray production and the energy of characteristic and bremsstrhlung x-rays

39. Target material

40. Voltage Waveform  5 voltage waveforms: half-wave rectification, full- wave rectification, 3-phase/6-pulse, 3-phase/12- pulse, and high-frequency.  Maintaining high voltage potential

41. Voltage generators

42. Factors affecting X-Ray beam quality and quantity An increase inResults in Current(mAs)An increase in quantity; no change in quality Voltage (kVp)An increase in quantity and quality Added filtrationA decease in quantity and an increase in quality Target atomic number(Z)An increase in quantity and quality Voltage rippleA decrease in quantity and quality

44. kVp KINETIC ENERGY OF ELECTRONS

45. CATHODE -------- MADE OF TUNGSTEN + 1%-3% THORIUM

46. TUNGSTEN Z # 74 MELTING POINT- 3,410 DEG. CELSIUS

47. THORIUM Z # 90

48. THERMIONIC EMISSION CATHODE HEATED UP TO AT LEAST 2,200 DEG. CELSIUS

49. ANODE +++++ TUNGSTEN TARGET

50. TUNGTEN AS TARGET HIGH Z# - 74------EFFICIENCY OF X-RAY PRODUCTION HIGH MELTING POINT –3,410 ° C– TARGET HEATED TO 2,000 ° C

51. X-RAY PRODUCTION  BREMSSTRAHLUNG RADIATION  CHARACTERISTIC RADIATION

52. BREMSSTRAHLUNG RADIATION  If an incoming free electron gets close to the nucleus of a target atom, the strong electric field of the nucleus will attract the electron, thus changing direction and speed of the electron.  The Electron looses energy which will be emitted as an X- ray photon. The energy of this photon will depend on the degree of interaction between nucleus and electron, i.e. the passing distance.  Several subsequent interactions between one and the same electron and different nuclei are possible. X-rays originating from this process are called bremsstrahlung.  Bemsstrahlung is a German word directly describing the process: "Strahlung" means "radiation", and "Bremse" means "brake

53. Bremsstrahlung Radiation

54. BREMS

55. DIFFERENT DEGREES OF DECCELERATION X-RAYS HEAT

56. BREMS RADIATION IS: POLYENERGETIC ! 90% OF X-RAYS ARE PRODUCED THROUGH BREMS INTERACTIONS WHEN 80-100 KVP APPLIED

57. BREMS EMISSION- CONTINUOUS

58.  The high energy electron can also cause an electron close to the nucleus in a metal atom to be knocked out from its place.  This vacancy is filled by an electron further out from the nucleus.  The well defined difference in binding energy, characteristic of the material, is emitted as a monoenergetic photon.  When detected this X-ray photon gives rise to a characteristic X-ray line in the energy spectrum. Characteristic X-rays

59. Characteristic Radiation KE OF PROJECTILE ELECTRON > BINDING ENERGYORBITAL ELECTRON

60. CHARACTERISTIC CASCADE

61. TUNGSTEN-74 K-70 KEV L-12 KEV M-2.8 KEV BINDING ENERGIES OF DIFFERENT SHELL ELECTRONS

62. CHARACTERISTIC X-RAYS L K 70-12 = 58 keV MK 70-3 = 67 keV M L 12-3 = 9 keV

63. TRANSFER OF ELECTRONS BETWEEN OUTER SHELLS RESULTS IN: HEAT PRODUCTION !

64. EACH CHARACTERISTIC RADIATION ( ex. K TO L TRANSFER) IS: MONOENERGETIC ! THERE ARE MANY CHARACTERISTIC RADIATION PRODUCED IN ONE ATOM THEREFORE CHARACTERISTIC RADIATION IS ALSO POLYENERGETIC !

65. CHARACTERISTIC EMISSION- LESS POLYENERGETIC!

66. X-RAYS PRODUCTION AN INEFFICIENT PROCESS 80 KVP 99.4% HEAT 0.6% X-RAYS

67. % EFFICIENCY OF X-RAY PRODUCTION % EFFICIENCY= K * Z * kVp K = 1 * 10 -4

68. X-RAY EMISSION  ISOTROPICAL- IN EVERY DIRECTION

69. USEFUL RADIATION – PROJECTED TOWARD THE PATIENT

70. LEAKAGE RADIATION HOUSING

71. OFF-FOCUS RADIATION USEFUL OFF-FOCUS

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