CHAPTER-7 - QUALITY OF X-RAY BEAM

CHAPTER-7 - QUALITY OF X-RAY BEAM

1. Ch 7. Quality of X-ray Beams A Radiation Oncologist’s perspective Dr Kanhu Charan Patro Mr A C Prabu Mr A Srinu 1

2. Faiz M. Khan, PhD Faiz M. Khan, PhD Professor Emeritus Department of Radiation Oncology University of Minnesota Medical School Minneapolis, Minnesota 2 John P. Gibbons, PhD Chief of Clinical Physics Mary Bird Perkins Cancer Center Baton Rouge, Louisiana

3. Slow my flow Basic wordings Parameters of X-Ray beam quality Half value layer Peak voltage Mean energy Effective energy Energy spectrum Depth dose distribution Measurement of energy spectrum Specification of clinical beam quality 3

4. Define the term quality of X-Ray? The penetrating ability of the radiation is often described as the quality of the radiation 4

5. Specification of beam quality The quality of clinical photon beam can be usually described by penetrability of x ray beam HVL kVp MV PDD Ratio of depth doses under reference conditions etc. 5

6. Superficial orthovoltage X Ray HVL and kVp Gamma Rays The energy of nuclide origin Like Cobalt 1.33 Mev and 1.17Mev Avg. 1.25 Mev Megavoltage beam The kinetic energy of electron on target As X Ray beam is heterogeneous Maximum energy designated by MV[megavolts] As if the beam were generated by applying that voltage on X Ray tube 6 Specification of beam quality

7. The mechanism 7

8. Noooo ….. 8

9. The opposite attraction 9

10. The catalyst 10

11. Thermionic emission 11

12. Kilovoltage peak 12

13. The running electron 13

14. The energizing factor 14

15. Voltage Determines the speed of electrons that travel from cathode to anode. When voltage is increased, the speed of the electrons is increased. When the speed of the electrons is increased, the electrons strike the target with greater force and energy. Resulting in a penetrating x-ray beam with a short wavelength. Measured in volts or kilovolts ( kV’s ) This is adjusted according to individual diagnostic needs of the patient. 15

16. Bremsstrahlung X-ray generation The X-rays produced by the primary beam electrons comprise mostly a large (almost infinite) number of low-energy X-rays. Beam hardening  is the phenomenon that occurs when an x-ray  beam  comprised of polychromatic energies passes through an object, resulting in selective attenuation of lower energy photons. 16

17. The X-Ray production 17

18. The X-Ray chamber 18

19. Production of X-Ray 19

20. X-ray Beam Quality The beam of radiation we use in dentistry may be described in two ways: 1. Quality = penetrating ability - energy of the photon beam 2. Quantity = amount - # of photons in beam 20

21. Kilovoltage Kilovoltage is controlled by the kilovoltage peak dial, or kVp . (Only on some machines) Determines the wavelength (quality) or speed and energy of the wave. Density= overall darkness or blackness of an image. Adjustment in the kVp = a change in the density. 21

22. Kilovoltage Quality = penetrating ability kVp is the electrical force needed to move the electrons from the –cathode to the +anode kVp setting is the only factor influencing the penetrating ability (quality, power) of beam 22

23. - shorter wavelength, more penetrating- also more X Rays produced Increased kVp 23

24. -longer wavelength, less penetrating, fewer X-Rays produced Decreased kVp 24

25. Soft xrays Hard xrays KVP 25

26. Types of radiation Gamma Ray Cobalt X Ray KV 4,6,10,15 MV 26

27. Which is important? Quality Spectral distribution Quantity Fluence 27

28. 28

29. Define it? An ideal way to describe the quality of an x-ray beam is to specify its spectral distribution , that is, energy fluence in each energy interval However, spectral distributions are difficult to measure and, furthermore, such a complete specification of the beam quality is not necessary in most clinical situations. Since the biologic effects of x-rays are not very sensitive to the quality of the beam, in radiotherapy one is interested primarily in the penetration of the beam into the patient rather than its detailed energy spectrum. Thus, a crude but simpler specification of the beam quality is often used, namely the half-value layer 29

30. Describing radiation beam Quality Spectral distribution Quantity Fluence Measurements HVL 30

31. Quantity Fluence Fluence  can be defined as the total number of particles (typically Gamma Ray  Photons ) crossing over a sphere of unit cross section which surrounds a Point Source of Ionizing Radiation. . 31

32. Penetration and spectral distribution? 32

33. Because all x-ray beams produced by radiation generators are heterogeneous in energy (i.e., possess continuous energy spectra that depend on the Peak voltage, Target material Beam filtration Factors effecting energy spectra? 33

34. Inherent filtration   is the  filtration  or reduction in radiation energy due to absorption by the material necessary to provide the vacuum, the electrical insulation, and mechanical rigidity of the X-ray tube. This so-called inherent filtration is equivalent to approximately 1-mm Al in most x-ray tubes Inherent filtration? 34

35. Crude but simpler specification of the beam quality is often used, namely the half-value layer-HVL Single parameter specifying the overall penetrating ability of the beam Specification of beam quality? 35

36. In the case of low-energy x-ray beams (below megavoltage range), it is customary to describe the quality in terms of HVL together with kVp , although HVL alone is adequate for most clinical applications. On the other hand, in the megavoltage x-ray range , the quality is specified by the peak energy and rarely by the HVL. The reason for this convention is that in the megavoltage range the beam is so heavily filtered through the transmission-type target and the flattening filter that any additional filtration does not significantly alter the beam quality or its HVL Describing beam quality? 36

37. 37

38. Before Marriage - you are choosing[filtering] Social status Education Profession In laws Etc. 38

39. After Marriage - you are blocking Social status Education Profession In laws Etc. 39

40. 40 Half-value Layer Filters Measurement of Beam Quality Parameters Measurement of Megavoltage Beam Energy Measurement of Energy Spectrum

41. Introduction Ideal way to describe radiation quality Specify the spectral distribution of x-ray beam e.g. energy fluence in each energy interval Difficult to measure Not necessary in most clinical situations HVL is crude but simpler way More interest in penetration of the beam into the patient The penetrating ability of radiation is often described as the quality of the radiation 41

42. HVL-Half value layer Thickness of an absorber of specified composition required to attenuate the intensity of the beam to half its original value Although HVL can described the quality of all beams, it usually used in x-ray beams produced by radiation generators HVL of x-ray beam depends upon energy spectrum which is a function of primarily peak voltage and the filtration [inherent and added] The HVL is related to the linear attenuation coefficient by following equation HVL= 0.693/ μ 42

43. Half-value Layer Definition Thickness of an absorber of specified composition required to attenuate the intensity of the beam to half its original value Although HVL can described the quality of all beams, it usually used in x-ray beams produced by radiation generators γray beam is usually stated in terms of the energy A known emission spectrum e.g. 60 Co → 1.17 and 1.33 MeV (average 1.25 MeV) Cobalt-60 beam X-ray beams are usually describe by HVL Heterogeneous in energy 43

44. Half-value Layer Low-energy x-ray beams (< MV) HVL + kVp Megavoltage x-ray range The quality is specified by the peak energy and rarely by HVL The beam is so heavily filtered through the transmission type target and flattening filter Additional filtration do not significantly alter the beam quality or its HVL The average energy = 1/3 of the peak energy 44

45. HVL 45 HVL ↑ Filter thickness ↑ Beam “harder” ↑ Choice of the filter Suitable HVL Acceptable beam output

46. 46

47. HVL HVL- l/l =1/2 HVL must be measured under narrow beam and good geometry Subsequent HVL are more than before It explains about overall penetrating power of beam Given by 0.693/ μ As filter thickness increases the average energy of the transmitted beam increases or the beam becomes increasingly harder By increasing the filtration in such an X-Ray beam one increases the penetrating power or the HVL of beam 47

48. 48 Half-value Layer Filters Measurement of Beam Quality Parameters Measurement of Megavoltage Beam Energy Measurement of Energy Spectrum

49. Defining the filters An X-ray filter is a material placed in front of an X-ray source in order to reduce the intensity of particular wavelengths from its spectrum and selectively alter the distribution of X-ray wavelengths within a given beam. For medical purposes, X-ray filters are used to selectively attenuate, or block out, low-energy rays during x-ray imaging ( radiography ). Low energy x-rays (less than 30 keV) contribute little to the resultant image as they are heavily absorbed by the patient's soft tissues (particularly the skin). Thus, it is favorable to remove these low energy X-rays from the incident light beam. X-ray filtration may be inherent due to the X-ray tube and housing material itself or added from additional sheets of filter material. The minimum filtration used is usually 2.5 mm aluminium (Al) equivalent, although there is an increasing trend to use greater filtration. Manufacturers of modern  fluoroscopy  equipment utilize a system of adding a variable thickness of copper (Cu) filtration according to patient thickness. This typically ranges from 0.1 to 0.9 mm Cu. 49

50. Various filters in practice Molybdenum  - Used in Mammography Rhodium  - Used in Mammography with Rhodium anodes Aluminium  - Used in general radiography x-ray tubes Copper  - Used in general radiography - especially in pediatric applications. Silver  - Used in Mammography with tungsten anode Tantalum  - Used in fluoroscopy applications with tungsten anodes Niobium  - Used in radiography and dental radiography with tungsten anodes Erbium  - Used in radiography with tungsten anodes 50

51. Schematic graph showing changes in spectral distribution of 200 kVp x-ray beam with various filters 51 Energy of x-ray beam 200 kVp Filters Curve A → Al Curve B → Sn + Al Curve C → Sn + Cu + Al (58 – 69 keV) (29.2 keV) (30 – 70 keV) K edge of Cu → 9 keV Bremsstrahlung pinching is due to the atomic mass. The denser the atom, the higher the X-Ray Absorption. Only the higher energy X-Rays pass through the filter, appearing as if the Bremsstrahlung continuum had been pinched

52. 52 Filters The character of the energy spectrum of x-ray beam The effect of x-ray beam filtered by 1-mm-thick aluminum filter The effect of so-called inherent filtration The effects of attenuation The glass envelope of the x-ray tube The surrounding oil The exit window of the tube housing Equal to about 1-mm Al K characteristic x-rays produced in the tungsten target 58 and 69 keV K characteristic x-rays produced in the tin target 29.2 keV

53. Filters Combination filters Containing plates of Sn, Cu, Al Increase HVL of the orthovoltage beams Without reducing the beam intensity Thoraeus filters 53 Table7.1. Thoraeus Filter Used with Orthovoltage X-Rays Filter Composition Thoraeus I 0.2 mm Sn + 0.25 mm Cu + 1 mm Al Thoraeus II 0.4 mm Sn + 0.25 mm Cu + 1 mm Al Thoraeus III 0.6 mm Sn + 0.25 mm Cu + 1 mm Al

54. Application of filter in clinical radiation machine 54 Machine Filter Diagnostic & superficial x-ray energy range Primary aluminum filter (mmAl) Orthovoltage range Combination filter (range: 1-4 mm Cu) Cesium & Cobalt teletherapy machines No filter (Monoenergetic) Megavoltage x-ray beam Inherent filtration of the transmission target Flattening filter (Primary purpose: make beam intensity uniform in cross-section)

55. The flattening flatter The bremsstrahlung generated when high energy electron strikes the target is forwardly peaked. To make the beam intensity uniform across the field, a flattening filter is inserted in the beam. The filter is usually made of lead, although tungsten, uranium, steel, aluminum or a combination. 55

56. Bow-tie filter in CBCT Varian uses two custom-designed aluminium filters called “Bow-tie” that equalize x-ray intensity laterally across the detector for two different modes of acquisition. In Elekta, the kV panel can be positioned at three different field of view (FOV) positions, namely S (small FOV), M (medium FOV) and L (Large FOV). The bow-tie filter (F1) is inserted between the source and the patient to reduce intensity variations across the detector 56

57. Filters in CBCT 57

58. Filters in Elekta CBCT 58

59. 59 Half-value Layer Filters Measurement of Beam Quality Parameters Measurement of Megavoltage Beam Energy Measurement of Energy Spectrum

60. The effective energy Because x-ray beams used in radiology are heterogeneous in energy, it is convenient sometimes to express the quality of an x-ray beam in terms of the effective energy. The heavily filtered X-Ray beam effective or average energy approximately one third of peak energy A 6 MV of linear accelerator of an average energy is 2Mev 60

61. Direct measurement of Peak Voltage Voltage Divider Several high resistances are connected in series to form a resistance tower Placed across the high tension leads V = I x R Sphere-Gap Method High voltage lead of the x-ray tube is connected to a polished metallic sphere by a cable adaptor The distance between the two sphere is reduced until an electric spark passes between them Calculation of peak voltage Critical distance Correction for air density and humidity 61

62. Indirect Measurement of Peak Voltage Fluorescence Method Two principles Peak photon energy is given by peak potential ( hv max in keV = kVp ) K edge absorption occurs when the photon energy ≧ binding energy of the K shell electron Using materials of several different K absorption edges , one can calibrated the kVp dial on the machine Attenuation Method. Penetrometer 62

63. Experimental arrangement for measuring tube voltage by K fluorescence method. 63 measures radiation transmitted through the attenuator measures characteristics as well as scattered x-rays. Absorbs most of scattered radiation from attenuator

64. 64 Half-value Layer Filters Measurement of Beam Quality Parameters Measurement of Megavoltage Beam Energy Measurement of Energy Spectrum

65. 65 Measurement of Megavoltage Beam Energy Measure the complete spectrum of a MV x-rays beam Calculation of thin target bremsstrahlung spectra Scintillation spectrometry Photoactivation Practical method of determining the MV beam energy by measuring Percent depth dose distribution Tissue-air ratios Tissue-maximum ratios Comparing the data with the published data (e.g. Hospital Physictist’s Association)

66. 66 Measurement of Megavoltage Beam Energy Photoactivation ratio (PAR) method Sensitive method of monitoring x-ray beam spectral quality Process Irradiating a pair of foils Activated by the photodisintegration process Use scintillation counter to measure the induced radioactivity in the foil Ratio of induced activities → PAR → peak photon energy More sensitive method than the conventional method measuring HVL in water

67. 67 Half-value Layer Filters Measurement of Beam Quality Parameters Measurement of Megavoltage Beam Energy Measurement of Energy Spectrum

68. An apparatus used for recording and measuring spectra, especially as a method of analysis. A spectrometer is any instrument used to probe a property of light as a function of its portion of the electromagnetic spectrum, typically its wavelength, frequency, or  energy . The property being measured is usually intensity of light , but other variables like polarization can also be measured. Spectrometer and spectrometry 68

69. Scintillation Scintillation  is the  process  in which the energy from a certain radiation interacting with a volume of sensitive material (called a  scintillator ) is converted into electromagnetic waves. The frequency of the emitted electromagnetic waves is within or near the visible spectrum 69

70. 70

71. 71 Measurement of Energy Spectrum Scintillation spectrometer Fig 7.6. Energy spectrum of an x-ray beam determined by scintillation spectrometer

72. 72

73. 73 Sort out electronically different-size pulses by multichannel pulse height analyzer e- ejected in the crystal Produce ionization and excitation of crystal atoms Produce light photon Striking the photosensitive surface of photomultiplier tube Eject low-energy photoelectrons Collected and multiplied by photomultiplier dynodes Measurement of Energy Spectrum

74. Summary Quality of x-ray beams is specified by kVp , filtration, and HVL (for diagnostic, superficial, and orthovoltage beams); and MV and percent depth dose in water (for megavoltage x-rays). Quality of cobalt-60 beams is designated simply as cobalt-60 because it is known that they have the same energy, namely g rays of 1.17 and 1.33 MeV. Peak voltage ( kVp ) applied to an x-ray generator can be measured directly (e.g., voltage divider, sphere-gap method) or indirectly (e.g., fluorescence, attenuation, or a penetrameter device such as an Adrian-Crooks cassette). Peak energy (MV) of a megavoltage x-ray beam can be measured directly by scintillation spectrometry or by photoactivation of appropriate foils (e.g., PAR method). Effective or equivalent energy of an x-ray beam is the energy of a monoenergetic photon beam that has the same HVL as the given beam. Energy spectrum of an x-ray beam can be measured by scintillation spectrometry. 74

75. The following table used in problem 1 to 3 describes the measured beam intensity as a function of aluminum filter thickness:

76. 1. What is the first HVL for this beam ? a) 1.7mm Cu b) 2.0mm Al c) 2.7mm Al d) 0.5mm Al e) 0.5mm Cu

77. 2 . What is the attenuation coefficient m for this beam in aluminum? a) 0.26mm-1 b) 0.26mm-1 c) 0.41mm-1 d)1.4mm-1 e) 2.0mm-1

78. 3. The homogenecity coefficient is defined as the first HVL divided by the second . What is the homogenecity coefficient for this beam? a) 0.15 b) 0.18 c) 1 d) 5.6 e) 6.5

79. a) Unfiltered X ray beam b) Filtered X ray beam c ) Monoenergetic beam d) All three will have the same HVL e) Not enough information is given 4.Suppose there were three photon beam with the same peak energy ; one from an X ray machine without filtration, one from an Xray machine with 2.0mm Al filtration, and one monoenergetic. Which beam would have the largest HVL?

80. a) 0.029cm-1 b ) 0.039cm-1 c) 0.049cm-1 d) 0.059cm-1 e) 0.069cm-1 5.The beam quality for megavoltage photon beams is sometimes specified in terms of their ionization ratio, or the ratio of the doses at depth of 20 to 10 cm for a fixed source detector distance and meter setting . If a nominal 6MV photon beam has an ionization ratio of 0.68, what is the approximate attenuation coefficient of this beam , neglecting scatter radiation?

81. 6.HVL should be measured under good geometry conditions that require: A narrow beam with negligible scatter Abroad beam with full scatter Detector positioned far away from the absorber to avoid scatter Chamber imbedded in a phantom at a sufficient depth to provide full build-up matter

82. 7 .A Thoreaus filter for orthovoltage beams must be inserted with : Aluminium filter facing the patient Tin filter facing the patient Copper filter facing the patient Lead filter facing the X-ray tube

83. 8 .The following instrument(s) can be used to measure kVp directly if the high tension leads of the x-ray tube are accessible: Penetrameter Voltage divider Sphere-gap apparatus Wisconsin test cassette

84. 9.HLV measured for a 6 MV beam turned out to be13.3 mm Pb .For this beam: The mass attenuation coefficient is 4.59 x 10-3 m2 kg-1 in lead The mass energy absorption coefficient in lead is the same as in water The mass attenuation coefficient is greater than the mass energy absorption coefficient. The effective energy is approx. 2 Mev .