1. Implementation of an Audit and
Dose Reduction Program for CT
Y. Matyagin and P.J. Collins
Department of Nuclear Medicine, PET and Bone
Densitometry
Royal Adelaide Hospital

2. CT exposure
 Increasing concern worldwide regarding the
relatively high level of CT radiation exposure
 National Council on Radiation Protection and
Measurements: US average dose from medical
procedures increased 6x between 1980 and
2007
 60% due to CT
F.A Mettler Jr., et al. Radiology 2009 Nov;253(2):520-31.

3. CT exposure
 CT now constitutes:
 15% of total number of radiological
imaging procedures
 50% of population’s medical radiation
exposure
 25% of total radiation exposure
F.A. Mettler Jr., et al Health Phys. 95(5), 502-507 (2008).

4. Radiation induced health risk
 A 2009 study estimates (using LNT
model) 29,000 cases of cancer could be
due to CT scans that were performed in
USA during 2007
 There is potential for significant dose
reduction in many studies as part of an
image/dose optimization process
B.A. de Gonzales, et al. Arch Intern Med 2009 Dec 14;169(22):2071-7.

5. Spiral CT
CT machine
Motorized table
Rotating
X-ray
detectors
X-ray beam
Rotating
X-ray
source

6. Weighted Computed Tomography
Dose Index (CTDIw)
http://www.impactscan.org/slides/impactcourse/principles%20of%20ct%20dosimetry/index.html
• Weighted CTDI represents the average dose in scan
plane of Perspex phantom
• CTDIW = 1/3 CTDICentre + 2/3 CTDIPeriphery
• Phantom diameter:
32 cm or 16 cm
• Units: mGy

7. Volume CTDI (CTDIvol)
 CTDIvol: average dose over scanned volume
 CTDIvol: accounts for non-contiguous exposure along z-axis
 CTDIvol = CTDIw / pitch
Pitch 1.5
Pitch 1.0
Pitch 0.75

8. Dose-length product (DLP)
 Total dose: CTDIvol integrated along the scan
length, ie.
 DLP = CTDIvol • L (where L = scan length)
 Units: mGy • cm

9.  Different tissues of the body have
different radiosensitivity
 Unit of Effective dose (E) is the Sievert
where wT = weighting factor for organ, or tissue T
HT = equivalent dose to organ/tissue
Effective dose
E w HT T
T
 

10. Automatic Exposure Control (AEC)

11. Aim
 Compare CT doses (adults) at RAH with
an International benchmark
 Modify protocol settings to optimize
dose/image quality

12. Dose reference levels
 RANZCR have adopted EC1999 values
 European Guidelines (EC 2004) were
used in this study
 Address multi-slice scanners
 Includes data from 8 countries
EC 2004. G. Bongartz, et al. European Guidelines for Multislice Computed
Tomography. FIGM-CT2000-20078-CT-TIP. March 2004

13. Methods
 CT configured to display relevant dose
information (CTDIvol, DLP etc.) in PACS
 Baseline audit performed
 CT scanner exposure settings were
reduced in step-wise fashion so as not
to compromise image quality

14. Results
CT Doses (% of EC2004)
Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"

15. Brain perfusion
 Major International concern regarding
brain perfusion CT
 Cedars-Sinai Medical Center in USA:
 excessive CT doses in 250 patients
 Some studies 8 times expected level
 FDA reported significant CT
overexposure at several other medical
centers in 2009

16. Brain perfusion overdose
Hair loss in patients who
received radiation overdoses
W. Bogdanich, New York Times, July 31, 2010

17. Discussion
CT Doses (% of EC2004)
Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"

18. CT Doses (% of EC2004)
Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"
Discussion

19. CT Doses (% of EC2004)
Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"
Discussion

20. CT Doses (% of EC2004)
Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"
Discussion

21. Dose parameters - PACS
Store in separate dose information page:
 CTDIvol
 DLP
 Dose/image quality reference settings
 Scan time
 Patient’s size information

22. Toshiba dose page 1

23. Toshiba dose page 2

24. Siemens dose page

25. Automatic exposure control (AEC)
 AEC switched on
 Dose references level set
 Effect of other CT parameters on
functioning of AEC evaluated
 Min and max tube current
 Tube voltage
 Beam collimation and reconstructed slice
thickness
 Image reconstruction filters and algorithms

26. Large/small patients
A separate protocol should be employed:
 Higher tube voltages for large and
(possibly) lower tube voltages for small
patients
 Rotation time and pitch adjusted to
ensure tube current is within acceptable
limits
 Somewhat higher image noise is
acceptable for large patients

27. Future work
 More detailed analysis of image quality
 eg. using phantoms
 Investigate other key parameters
 Slice thickness
 Inter-slice intervals
 Streak artefacts

28. Acknowledgement
 Radiology Department
 George Kourlis (CT senior)