1. Dosismetingen
Computer Tomografie
Koos Geleijns

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

4. Standard equipment: 10 cm ionization chamber and two phantoms
100 mm pencil
chamber

5. De geometrie van een 64 slice CT scanner

6. Definition of CTDI
+7T
1
CTDI FDA =
N .T ∫ D( z )dz
−7T
+50 mm
1
CTDI100 =
N .T ∫ D( z )dz
−50 mm

7. No big problems until 2004
1x5 4x1 16 x 0.5 64 x 0.5
1998 2001 2004

8. 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

9. Body phantom, CTDI not efficient, but up to 40 mm consistent
John Boone, The trouble with CTDI100, Medical Physics, 2007

10. Head phantom, CTDI not efficient, but up to 40 mm consistent
John Boone, The trouble with CTDI100, Medical Physics, 2007

11. Tails of scattered radiation within the CT dose phantom
John Boone, The trouble with CTDI100, Medical Physics, 2007

12. Eerste pragmatische aanpassing door IEC ….
+50 mm
1
CTDI100 = ∫mm
min{N × T ,100mm} − 50
D( z )dz

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. For example: measure CTDI free in air in three steps
3 x 10 cm

15. CTDI free in air in drie stappen bij brede bundels
160 mm
3 x 10 cm

16. Inefficiency remains, but consistent for broad beams

17. AAPM rapport 2010, first proposal small chamber

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. A small ionization chamber …

20. 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 indicated
in green under the dose profile.

21. Also free in air, small ionization chamber

22. ICRU report, expected 2011
ICRU CT phantom
contrast
spatial
resolution
contrast resolution

23. ICRU report, expected 2011
ICRU proposes
polyethylene
phantom
30 cm diameter
30 cm length
~14 kg

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.