Institut de radiophysique 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 What has been learned Where are we going to put our efforts

Quality assurance in Switzerland before 2010 Manufacturers are authorized by FOPH to: Install and test the system before the 1 st patient Check a list of IEC requirements 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

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

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

Use of the unit on patient 6 ProtocolCTDI [mGy]DLP [mGy cm]# phasesE [mSv] Head 51.2 ± 13.9 DRL: ± DRL: ± ± 1.3 Survey 08 : 3.0 Chest 8.5 ± 5.4 DRL: ± DRL: ± ± 3.5 Survey 08 : 5.0 Abdomen 12.2 ± 7.0 DRL: ± DRL: ± ± 12.7 Survey 08 : 9.0 ProtocolE min [mSv]E max [mSv]Max/Min ratio Head Chest Abdomen 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

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

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

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

Task Transfer Function Phys Med Biol Aug 7;59(15):

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

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

Image with signalImage noise xx = = Response n Response s 14

ROC curve construction From the values of n and s : observer response Area under Curve : AUC Comparison with human observers 15 dAdA

Results

Correlation: model observer/human observer

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

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

Thank you for your attention Acknowledgements Nick Ryckx, Julien Ott, Damien Racine