DosismetingenComputer Tomografie    Koos Geleijns
First dosimetric evaluations in CT in 1971• Dr. BJ Perry – Head of  Medical Physics  SGH/AMH (radiation)• First paper in B...
Standard: 10 cm long chamber; 15 cm long PMMA phantom
Standard equipment: 10 cm ionization chamber and two phantoms                                 100 mm pencil               ...
De geometrie van een 64 slice CT scanner
Definition of CTDI                          +7T                    1CTDI FDA         =                   N .T   ∫ D( z )dz...
No big problems until 2004    1x5    4x1   16 x 0.5   64 x 0.5          1998    2001       2004
CTDI100 efficiencyThe CTDI100 efficiency is the ratio of theaccumulated dose integral measuredusing the 100 mm chambermeas...
Body phantom, CTDI not efficient, but up to 40 mm consistentJohn Boone, The trouble with CTDI100, Medical Physics, 2007
Head phantom, CTDI not efficient, but up to 40 mm consistentJohn Boone, The trouble with CTDI100, Medical Physics, 2007
Tails of scattered radiation within the CT dose phantomJohn Boone, The trouble with CTDI100, Medical Physics, 2007
Eerste pragmatische aanpassing door IEC ….                                         +50 mm                    1CTDI100   = ...
Tweede, meer solide, aanpassing door IEC …. Voorgesteld: Apply up to a beam width of 40 mm the ‘old’ methodology:         ...
For example: measure CTDI free in air in three steps                       3 x 10 cm
CTDI free in air in drie stappen bij brede bundels                        160 mm                        3 x 10 cm
Inefficiency remains, but consistent for broad beams
AAPM rapport 2010, first proposal small chamber
A new phantom …                  Quick-fill                  water                  phantom:                  30-cm       ...
A small ionization chamber …
Integration of dose profile by movement of the table(Left figure) Phantom assembly and centered ionization chambermoving t...
Also free in air, small ionization chamber
ICRU report, expected 2011ICRU CT phantom     contrast     spatial     resolution                                 contrast...
ICRU report, expected 2011                             ICRU proposes                             polyethylene             ...
Conclusie• Dosimetrie bij CT is gebaseerd op meting van de CTDI  met een 10 cm lange ionisatiekamer, in standaard CT  dosi...
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8. ct dosimetrie, geleijns

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8. ct dosimetrie, geleijns

  1. 1. DosismetingenComputer Tomografie Koos Geleijns
  2. 2. First dosimetric evaluations in CT in 1971• Dr. BJ Perry – Head of Medical Physics SGH/AMH (radiation)• First paper in BJR: Dosimetry and image quality, measurements and methods
  3. 3. Standard: 10 cm long chamber; 15 cm long PMMA phantom
  4. 4. Standard equipment: 10 cm ionization chamber and two phantoms 100 mm pencil chamber
  5. 5. De geometrie van een 64 slice CT scanner
  6. 6. Definition of CTDI +7T 1CTDI FDA = N .T ∫ D( z )dz −7T +50 mm 1CTDI100 = N .T ∫ D( z )dz −50 mm
  7. 7. No big problems until 2004 1x5 4x1 16 x 0.5 64 x 0.5 1998 2001 2004
  8. 8. CTDI100 efficiencyThe CTDI100 efficiency is the ratio of theaccumulated dose integral measuredusing the 100 mm chambermeasurement to the total dose integralJohn Boone, The trouble with CTDI100, Medical Physics, 2007
  9. 9. Body phantom, CTDI not efficient, but up to 40 mm consistentJohn Boone, The trouble with CTDI100, Medical Physics, 2007
  10. 10. Head phantom, CTDI not efficient, but up to 40 mm consistentJohn Boone, The trouble with CTDI100, Medical Physics, 2007
  11. 11. Tails of scattered radiation within the CT dose phantomJohn Boone, The trouble with CTDI100, Medical Physics, 2007
  12. 12. Eerste pragmatische aanpassing door IEC …. +50 mm 1CTDI100 = ∫mm min{N × T ,100mm} − 50 D( z )dz
  13. 13. Tweede, meer solide, aanpassing door IEC …. Voorgesteld: Apply up to a beam width of 40 mm the ‘old’ methodology: +50 mm 1 CTDI100 = N .T ∫ D( z )dz −50 mm If the beam width exceeds 40 mm: - Apply the ‘old’ methodology to a beam width of about 20 mm, this gives the ‘reference value’ (100 mm chamber, dose phantoms) - Scale to the beam width > 40 mm by measuring CTDI free in air 1 ⎛ + 50 mm ⎞ ⎛ CTDI free − in − air , N ×T ⎞CTDI100 = × ⎜ ∫ Dref ( z )dz ⎟ × ⎜ ⎟ ( N ref × Tref ) ⎜ − 50 mm ⎝ ⎟ ⎜ CTDI ⎠ ⎝ ⎟ free − in − air , ( N ref ×Tref ) ⎠
  14. 14. For example: measure CTDI free in air in three steps 3 x 10 cm
  15. 15. CTDI free in air in drie stappen bij brede bundels 160 mm 3 x 10 cm
  16. 16. Inefficiency remains, but consistent for broad beams
  17. 17. AAPM rapport 2010, first proposal small chamber
  18. 18. A new phantom … Quick-fill water phantom: 30-cm diameter, 50 cm long body phantom. A Farmer chamber is shown on the central axis.
  19. 19. A small ionization chamber …
  20. 20. Integration of dose profile by movement of the table(Left figure) Phantom assembly and centered ionization chambermoving together from right to left over scanning length L3 (rightfigure), so that the chamber integrates values over the area indicatedin green under the dose profile.
  21. 21. Also free in air, small ionization chamber
  22. 22. ICRU report, expected 2011ICRU CT phantom contrast spatial resolution contrast resolution
  23. 23. ICRU report, expected 2011 ICRU proposes polyethylene phantom 30 cm diameter 30 cm length ~14 kg
  24. 24. Conclusie• Dosimetrie bij CT is gebaseerd op meting van de CTDI met een 10 cm lange ionisatiekamer, in standaard CT dosis fantomen• De CTDI onderschat de dosis (flanken van verstrooide straling buiten het bereik van de 10 cm lange ionisatiekamer), maar door aanpassingen (IEC) blijft het concept van CTDI wel toepasbaar (ook voor toepassingen zoals bij DRN´s), zelfs voor bundels breder dan 10 cm.• Er zijn ontwikkelingen om te komen tot aangepaste methodes voor de meting van de dosis bij CT (AAPM, ICRU), nieuwe fantomen, en kleinere dosismeters.

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