implementation of an audit and dose reduction program for ct matyagin
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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
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.
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).
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.
Spiral CTCT machine
Motorized table
Rotating X-ray detectors
X-ray beam
Rotating X-ray source
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
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
Dose-length product (DLP)
Total dose: CTDIvol integrated along the scan length, ie.
DLP = CTDIvol • L (where L = scan length)
Units: mGy • cm
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 TT
Automatic Exposure Control (AEC)
Aim
Compare CT doses (adults) at RAH with an International benchmark
Modify protocol settings to optimize dose/image quality
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
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
ResultsCT Doses (% of EC2004)
Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"
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
Brain perfusion overdose
Hair loss in patients who received radiation overdoses
W. Bogdanich, New York Times, July 31, 2010
DiscussionCT Doses (% of EC2004)
Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"
CT Doses (% of EC2004)Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"
Discussion
CT Doses (% of EC2004)Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"
Discussion
CT Doses (% of EC2004)Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"
Discussion
Dose parameters - PACS
Store in separate dose information page:
CTDIvol
DLP
Dose/image quality reference settings
Scan time
Patient’s size information
Toshiba dose page 1
Toshiba dose page 2
Siemens dose page
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
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
Future work
More detailed analysis of image quality
eg. using phantoms
Investigate other key parameters
Slice thickness
Inter-slice intervals
Streak artefacts
Acknowledgement
Radiology Department
George Kourlis (CT senior)