1 / 20

CT Imaging and Dose Optimization: How medical physicists could help?

CT Imaging and Dose Optimization: How medical physicists could help?. Prof. Francis R. Verdun. Summary. Medical physicists and CT dose optimization ? Context of our experience Results obtained from last year In search to be useful in patient dose optimization framework

zamir
Download Presentation

CT Imaging and Dose Optimization: How medical physicists could help?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CT Imaging and Dose Optimization:How medical physicists could help? Prof. Francis R. Verdun

  2. Summary • Medical physicists and CT dose optimization ? • Context of our experience • Results obtained from last year • In search to be useful in patient dose optimization framework • What has been learned • Where are we going to put our efforts

  3. Quality assurance in Switzerland before 2010 • Manufacturers are authorized by FOPH to: • Installand test the system before the 1st patient • Check a list of IECrequirements • Do the preventive maintenances • FOPH organizes audits in centers • Advantages • Simple, relatively homogeneous, in a way, cost efficient • Main limitations • Performances are detector oriented (not patient oriented) • Manufacturers are both judge and judged • Does not comply with Euratom 97/43 requirements

  4. Towards legal compliance Certainly not perfect but a decent working basis 2011 : Working group proposes a concept “Requirements for medical physicists in Nuclear Medicine and Radiology” (June 2011)”

  5. Outcomes from our 2013 experience • 45 units tested • About15% of the CT units in use in Switzerland • Problems found • QC: • 4 CT : Laser poorly adjusted • 21 CT : Hounsfield unit not calibrated if kV ≠ 120 • Use of the unit: • Tube current modulation is not used optimally • Penumbra and over-ranging phenomena are poorly handled • Dose Reference Level often misunderstood

  6. Use of the unit on patient Mean dosimetric results for CT examinations. The CTDI and DLP are given per phase, whereas E is given per examination Minimum and maximum patient effective doses according to ICRP 103. All values are given per examination

  7. CT abdominal: Largest variation of practice

  8. Proposal for 2014 • Sampling of examinations • Use common indications • A more practical QC evaluation • Use of an objective way to assess low contrast resolution • Assess the actual benefit of iterative recons • Benchmark a few clinical protocols • Be more on the image quality side than dose

  9. Focus on the way the CT unit is used • Choice of protocol indications • Head • Trauma • Sinus • Willis polygon • Chest • Pulmonary embolism suspected • Seek for a primary tumor • Abdomen • UroCT • Seek for a primary liver tumor

  10. Challenges to CT image evaluation • CT systems are not linear and not shift-invariant • Resolution/sharpness depends on object size, contrast, location, and noise level • There isn’t one MTF that describes the system transfer • Use of the concept of the Task Transfer Function (TTF) • Noise is colored • Pixel variance doesn’t tell the whole story • Noise is non-stationary • Noise texture or NPS depends on location • Iterative reconstruction introduces more difficulties

  11. Task Transfer Function Phys Med Biol. 2014 Aug 7;59(15):4047-64

  12. Main challenge • Dose reduction and loss of low contrast resolution • Iterative reconstructions remove some signs of high noise level • One can have a reasonable looking image while low contrast structures are not transferred • Use of an objective way to assess low contrast resolution

  13. Observer’s template Image Principle of linear model observers The LMO computes the decision variable l for each image

  14. Image noise Image with signal x x = = Response ls Response ln

  15. ROC curve construction From the values of ln and ls : observer response Comparison with human observers Area under Curve : AUC dA

  16. Results

  17. Correlation: model observer/human observer

  18. Expected outcomes • Ensure that dose reduction reduces low contrast resolution in a safe way • Adequacy of the protocol with the level of image quality required • Iterative is not a magic solution, radiologists need to know what is lost and judge if it is OK or not • Promote the use of new technological progresses while being critic

  19. Conclusion • How medical could help in CT optimization? • Focus on image quality assessments in CT • We should never speak about dose without noticing image quality • Help radiologists in setting minimum low contrast resolution requirements • Protocol optimization indication based • Ideally it would be nice to link a dose report to an image quality performance

  20. Thankyou for your attention Acknowledgements Nick Ryckx, Julien Ott, Damien Racine

More Related