Dosismetingen

Computer Tomografie

Koos Geleijns

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

Standard: 10 cm long chamber; 15 cm long PMMA phantom

Standard equipment: 10 cm ionization chamber and two phantoms

100 mm pencil chamber

De geometrie van een 64 slice CT scanner

Definition of CTDI

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1998 2001 2004

No big problems until 2004

CTDI100 efficiency

The CTDI100 efficiency is the ratio of the accumulated dose integral measured using the 100 mm chamber measurement to the total dose integral

John Boone, The trouble with CTDI100, Medical Physics, 2007

Body phantom, CTDI not efficient, but up to 40 mm consistent

John Boone, The trouble with CTDI100, Medical Physics, 2007

Head phantom, CTDI not efficient, but up to 40 mm consistent

John Boone, The trouble with CTDI100, Medical Physics, 2007

Tails of scattered radiation within the CT dose phantom

John Boone, The trouble with CTDI100, Medical Physics, 2007

Eerste pragmatische aanpassing door IEC ….

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Apply up to a beam width of 40 mm the ‘old’ methodology:

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

For example: measure CTDI free in air in three steps

3 x 10 cm

CTDI free in air in drie stappen bij brede bundels

3 x 10 cm

160 mm

Inefficiency remains, but consistent for broad beams

AAPM rapport 2010, first proposal small chamber

A new phantom …

Quick-fill water phantom: 30-cm diameter, 50 cm long body phantom. A Farmer chamber is shown on the central axis.

A small ionization chamber …

Integration of dose profile by movement of the table

(Left figure) Phantom assembly and centered ionization chamber moving together from right to left over scanning length L3 (right figure), so that the chamber integrates values over the area indicatedin green under the dose profile.

Also free in air, small ionization chamber

ICRU report, expected 2011

ICRU CT phantom

spatialresolution

contrast resolution

contrast

ICRU report, expected 2011

ICRU proposes

polyethylene phantom30 cm diameter30 cm length~14 kg

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.