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08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
08 chap 07 quality of x-ray beams
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08 chap 07 quality of x-ray beams

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  • 1. Chapter 7 Quality of X-Ray BeamsHalf-value layer is used to describe the quality of a beam, theability to penetrate materials of known composition.It is used for bremsstrahlung photon beams, which has aspectrum of energies, but not for γ–ray beams, whose energy isdiscrete and unique (for example, Co-60 has energies 1.17 &1.33 MeV photons).The half-value layer is defined as the thickness of an absorberof specified composition required to attenuate the intensity ofthe beam to half its original value. 1
  • 2. 7.1 Half-Value LayerFor low-energy x-ray beams, beam quality is described interms of HVL together with kVp. This is because the beam isusually further filtered.For mega-voltage x-ray beams, the quality is specified by thepeak energy alone (e.g. 6-MV). This is because the beam isalready filtered through the target and flattening filter so thatno additional filtration is used. TMR (d = 20cm,10 × 10cm 2 )(Sometimes TMR ratio such as TMR =20 10 TMR (d = 10cm,10 × 10cm 2 ) is also used.)The average energy is approximately 1/3 of its peak energy. 2
  • 3. 7.2 Filters K-characteristic radiation of Tungsten (58-69 keV) Energy fluence per energy interval Al (inherent) filtration K-edge of Tin (29 keV) Al + Sn filtration Al + Sn + Cu filtration 50 100 150 200 Photon energy (keV) 3
  • 4. 7.2 Filters (cont’d)Thoraeus filters (combination filters) used with orthovoltage x-raysFilter target (tungsten) compositionThoraeus I 0.2 mm Sn + 0.25 mm Cu + 1 mm AlThoraeus II 0.4 mm Sn + 0.25 mm Cu + 1 mm AlThoraeus III 0.6 mm Sn + 0.25 mm Cu + 1 mm Al Note the order is important: W (Z=74), Sn (Z=50), Cu (Z=29), Al (Z=13) 4
  • 5. 7.2 Filters (cont’d)• For cesium and cobalt machines (γ-rays), filters are not needed because the beams are almost monoenergetic (thus no preferential attenuation by the filters).• For megavoltage x-ray beams, the beam is hardened by the target (transmission type) and the flattening filter, thus no additional filtration is needed. 5
  • 6. 7.3 Measurement of Beam Quality Parameters (HVL) transmitted Incident photon fluence photon fluence scattered detector collimator photons 100 50Transmitted intensity (%) 50 narrow beam 25 ‘good’ geometry 12.5 10 1st HVL 2nd HVL 3rd HVL 1 0 1 2 3 4 5 6 6 Absorber thickness (mm Al)
  • 7. 7.3 Measurement of Beam Quality Parameters (peak voltage) Direct Methods n R R R R ••• I Voltage V=R×I divider Vtot = n×V Sphere gap d 7
  • 8. 7.3 Measurement of Beam Quality Parameters (peak voltage) –sphere-gap method Sphere Diameter (cm) gap (cm) 5 10 15 25 50 100 200 0.5 17.5 (kv) 16.9 16.5 1 32.2 31.6 31.3 31 1.5 46.1 45.8 45.5 45 2 58.3 59.3 59.2 59 2.5 69.4 72.4 72.9 73 3 79.3 84.9 85.8 86 4 107 111 113 112 5 128 134 138 138 137 137 8
  • 9. 7.3 Measurement of Beam Quality Parameters (peak voltage)Indirect Methods (fluorescent method) K-edge of the attenuator = kVp scattered radiation X-ray Transmitted/ source filter Tube voltage attenuator Chamber 2 attenuation coefficient Chamber 1 Photon energy 9
  • 10. 7.3 Measurement of Beam Quality Parameters (effective energy) X-ray beams are always heterogeneous because they are produced by bremsstrahlung. The effective energy of an x-ray beam is the energy of a monoenergetic beam that has the same attenuation coefficient or HVL. (for illustration only) 180 160 140 copper attenuation coefficienteffective energy 120 copper 0.693 (keV) 100 µ= 80 HVL 60 40 aluminum aluminum 20 1 2 3 4 5 energy HVL(mm 10 )
  • 11. Half-value layer in lead is not the best choice to express beamquality for megavoltage beams. Low atomic number materialssuch as water are more sensitive to changes in spectral qualityin megavoltage x-rays. Half-value layer (mm Pb or mm H2O) 30 water 20 lead 10 0 10 20 30 40 Peak photon energy (MeV) 11
  • 12. 12
  • 13. 13
  • 14. 7.3 Measurement of Beam Quality Parameters (mean energy)The mean energy of a beam may be calculated in two different ways: Emax ∫0 Φ E EdE E = Emax ΦE = photon fluence ∫0 Φ E dE or Emax ∫0 ΨE EdE E = Emax ΨE = photon energy fluence ∫0 ΨE dE 14
  • 15. 7.4 Measurement of Megavoltage Beam EnergyThe maximum (peak) photon energy can be estimated bymeasurements such as percent depth dose, tissue air ratios,or tissue maximum ratios. But these methods arerelatively insensitive to the maximum energy.Alternatively, it can also be measured with thephotoactivation ratio method. The procedure involvesphotoactivation of a pair of foils which react differently todifferent energies.As the maximum energy itself is not the most importantfactor for beam quality, it is not routinely measured in theclinic. 15
  • 16. 7.5 Measurement of Energy SpectrumEnergy spectrum is needed for certain applications such asaccurate dose calculations. It can be measured with ascintillation spectrometer. Scintillation crystal photomultiplier X-ray Energy fluence per energy interval source Lead shield Pulse height analyzer scaler Photon energy 16

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