Quantitative Quality Assurance in Hull & East Yorkshire

1. Quantitative Quality Assurance in Hull & East Yorkshire Dr. Craig Moore & Dr. Tim Wood Radiation Physics Department

2. What imaging modalities do we have in Hull & East Yorks? • Lots of Computed Radiography (CR) • Some Direct Radiography (DR) • CT • MRI • Full Field Digital Mammo (FFDM) • Cone Beam CT • Dental • Radiotherapy imaging • Digital Fluoro labs • In this talk I’ll try to give you a flavour of the quantitative QA measurements we do on some of the above (with some results!!)

3. What do we measure for CR and DR? • We (try to!!) conform to the requirements of IPEM Report 32 part 7 • MTF • NNPS • SNR • Variance as a function of dose

4. Computed Radiography • Have been measuring MTF and NNPS of our CR systems for approx 5 years • Only have Agfa CR in Hull • Originally wrote own software in Matlab • But have since moved to IQWorks

5. Measurement of CR MTF • First need to derive the system transfer properties (STP) of the system, i.e. flat field images at doses of: • 1 uGy • 4 uGy • 12 uGy • 50 uGy • Always with 1 mm Cu filtration and 70 kVp

6. Measurement of CR MTF • Acquire ‘MTF image’ using a tungsten metal edge tool, 50 x 50 mm and 1 mm thick 50mm 50mm

7. Measurement of CR MTF tube 1.5m CR cassette MTF edge tool

8. Typical results…

10. Measurement of CR NNPS • Use the 4 uGy STP image • Use IQWorks (or Matlab) to do the number crunching

11. Typical Results…

13. Measurement of SNR and Variance • Use STP images • Linearise • Chop out central 5 cm square portion of image • Calc mean, standard deviation and variance • SNR = mean/SD • Plot STP corrected variance against dose

14. STP Corrected Variance If only quantum noise the exponent should be unity Find relationship and quote the exponent

16. CR results over time… • Over the last 5 years, we have seen no significant changes in • MTF • NNPS • SNR • Variance • STP • DDI calibration (Agfa’s ‘SAL log’ number) • Do these quantitative QA tests take any extra time? • No! • only one extra exposure required (MTF edge) as STP images would be taken anyway for detector response

17. Getting the images off the system…… • For Agfa CR we set up a DICOM node on each system and send the image to our laptops • Must have DICOM server running on laptop • We use K-PACS • Images must always be acquired as: • System diagnosis • Flat field • Speed class 200

18. General DR measurements • We only have one general Philips DR system • Measure MTF, NNPS etc in the same manner as for CR • Grid removed MTF edge on detector housing

19. Typical DR results… • STP is logarithmic (usually linear for a DR system)

20. DR Variance • Variance as function of dose

21. Hot off the press… • Last week we found significant difference in MTF and NNPS from baseline of our DR system • Service engineer attending this Friday • Will be interesting to see if we find any genuine issues!!!

22. DR image transfer • Not easy to transfer images • Same method as Agfa CR but Philips were very reluctant to help us with this

23. FFDM Philips MicroDose • Photon counting full field digital mammo system • STP measured with 2 mm Al filter at the end of the collimator • MTF measured in a similar way to CR/DR • Edge tool placed on breast platform • No extra filtration • 32 kV/12 mAs • Approx 4cm from chest wall edge • Easy to take images off the system directly to an external hard drive

24. Philips MicroDose MTF

25. We did have an issue with one of our MicroDose systems!!! • Strange shape to the MTF • Corresponding dip in LSF • No issues with basic TOR MAX line pair test • We went back to test the system but it passed

26. MicroDose Noise • We also measure detector response with dose • And quantum and structure variance

27. kV tube imager Radiotherapy 2D on-board imaging • We work very closely with our RT colleagues wrt QA and testing of kV imaging systems • 2D and 3D • STP images • 1 mm Cu at tube port • 70 kVp • 1 to 20 uGy • MTF edge tool placed on imager • Approx 70 uGy • Same analysis and reporting as for diagnostic CR/DR • Get images off the system with the help of RT physicists

28. RT 2D Imaging • Huttner image for this detector scored 12 groups (1.8 lp/mm) • This is much worse than this MTF suggests • Possible image processing on the acquisition monitor not applied to image sent for MTF analysis • Does this demonstrate the usefulness of doing quantitative QA?

29. RT 2D Imaging - Variance Genuine due to structure noise?

30. RT 3D Imaging • Use Catphan phantom • Measure the MTF with the bead tool • Measure CT numbers etc

31. Cone Beam Dental Imaging • Tests conform to HPA-CRCE-010 • Guidance on safe use of dental cone beam CT equipment • We use a specially designed phantom shaped like a jaw to hold Sedentex inserts

32. CBCT Dental MTF

33. Dental CBCT

34. MRI Quantitative QA • Use Leeds Test Object MagIQ phantom • Measure: • SNR • Linearity • Uniformity • MTF

35. Future Work • Want to start quantitative QA on our digital fluoro labs • Just recently found out (we think!!) how to take images off our Siemens and Philips systems • Thorough CT analysis via automated Catphan measurements • Non-CBCT digital dental • Do we want to start measuring effective MTF, NNPS, DQE etc???? • Similar to clinical set up with an appropriate phantom so not detector centric • But would be much more time consuming

36. Conclusions • We have implemented routine quantitative testing (e,g. MTF, NNPS, SNR, Variance) into our QA programme, specifically for the following modalities: • CR • DR • FFDM • CBCT (RT and Dental) • MRI • No significant time penalty in doing these quantitative tests if software is already set up • We haven’t seen any significant changes in results over the years – but is this to be expected??? • We rarely see gross changes in Leeds Test Objects either • Vital we get cooperation from manufacturers to ensure we can easily acquire images in correct format and easily get them off the system