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Implementation of an audit and dose reduction program for ct matyagin



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  • 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) • 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)