Standardization of Parameters in Radiography for radiation protection in digital radiology

1. Joint Annual Scientific Meeting, Annual General Meeting & Annual Dinner 2011 Quality, Standard & Safety in Radiography 16 April, 2011 Standardization of Parameters in Radiography for radiation protection in digital radiology Marco LO, Physicist M. HTA&M, CEng MIET

2. Three different principles are used for radiation protection in ICRP • Justification - should only be used where it brings more good than harm • Optimization - doses should be kept as low as reasonable achievable (ALARA) • Dose limits to the individual

3. In radiation protection of the patient in x-ray diagnosis, the three principles introduced by the ICRP for occupational radiation protection should be applied also; it should be recognized, however, that in applying these principles a higher flexibility, compared to occupational radiation protection, is needed in order not to adversely affect the care for the patient in special situations. THE ROLE AND DETERMINATION OF PATIENT DOSE IN X-RAY DIAGNOSIS Flexibility in choice of exposure techniques

4. EU Council Directive 97/43 EURATOM on health protection of individuals against the dangers of ionizing radiation in relation to medical exposure “The optimization process shall include the selection of equipment, the consistent production of adequate diagnostic information as well as the practical aspects, quality assurance including quality control and the assessment and evaluation of patient doses.”

5. The imaging decision

7. H P Busch and K Faulkner What image quality (or diagnostic information) is needed for a medical imaging task?

8. Levels of image quality in term of speed class H P Busch and K Faulkner simple variable speed (tailor exposure to exam) …. but more difficult to correctly use since the energy sensitivity of DR and CR is quite different than that of FS

9. MHRA keynote notice, “Radiation Dose Issues with Digital Radiography Systems” is more specific and states that a supplier should provide the following information with a digital radiography system: • kVp compensation curves or set-up methods recommended for automatic exposure control • recommended receptor dose for optimised images • http://www.mhra.gov.uk/home/idcplg?IdcService=SS_GET_PAGE&nodeId=263

10. TG116 recommends avoiding the concept of “speed class” when referring to DR system performance. KTGT (Target Equivalent Air Kerma) values should be used to describe how one system may vary from another with respect to radiographs of a particular body part and view. Recommended Exposure Indicator for Digital Radiography Report of AAPM Task Group #116

11. mGy mSv Speed (Receptor dose uGy) Speed and dose related metrics

12. The receptor dose needed to produce a specified display response (film density) as a measure of system speed are • appropriately independent of many details of use, such as the body part being examined and choice of collimation • proven useful for classifying and comparing the detector choices available to the radiologist • straight forward for the physicist in estimating the effect of a proposed detector change in dose to the patient population

13. The speed class concept is widely used in CR and DR literatures • The speed concept is the starting point in transition from film/screen to digital radiology • Digital detectors are variable speed systems • Speed class can be conceptually used as the sensitivity of CR image receptor

14. Why digital radiography standardization?

15. Quanta III/IOS 400 Quanta Fast Detail/IOS 200 Quanta Detail/IOS 60 Latitude 3.0 2.5 1000 2.0 750 Gamma Pixel Value O.D. 1.5 500 1.0 250 0.5 0.0 0 0.1 1 10 100 1000 0.1 1 10 100 1000 Receptor Dose (uGy) Receptor Dose (uGy) Dynamic range - contrast relationship

16. Quanta III/IOS 400 Quanta Fast Detail/IOS 200 Quanta Detail/IOS 60 Screens with phosphors that have the same conversion gain will have similar total noise levels, irrespective of their actual thickness. non-quantum limited region SNR SNR 0.1 1 10 100 1000 0.1 1 10 100 1000 Receptor Dose (uGy) Receptor Dose (uGy) Dose - noise relationship

17. Maximization of image contrast can be independent of exposure dynamic range • More direct and efficient control of the trade-off between radiation dose and noise • The choice of pixel size for each application can be tailored to the tradeoff between noise and contrast resolution

18. DR Speed

19. Photon detected in resolution area Visibility Detectability Signal contrast ratio, S Detectability and dose creep in digital X-ray Motz J W and Danos M. Image information content and patient exposure Med. Phys. 5 8-22, 1978

20. The reasons behind dose creep • The direct relationship between dose and film density, which is familiar from film/screen exposures, no longer exists in digital radiography • No consistent feedback to technologists concerning the use of optimal acquisition techniques

21. The wide dynamic range of a digital systems allow a high tolerance for variations in exposure techniques • Digital radiography could be seen as offering far greater opportunity for patient dose increase than decrease overall • Optimal / standard exposure techniques are needed to ensure the appropriate image quality at the lowest possible patient exposure

22. film screen CR imaging plate Bacher K, Smeets P, Bonnarens K, De Hauwere A, Verstraete K, et al. Dose reduction in patients undergoing chest imaging: digital amorphous silicon flat-panel detector radiography versus conventional film-screen radiography and phosphor-based computed radiography. AJR Am J Roentgenol 2003;181:923–9 amorphous silicon flat-panel detector Frequency distribution of measured mAs for PA chest acquired on three imaging systems

23. Optimize for human vision • signal contrast • latitude • dynamic range • (acquired vs. displayed) • sharpness • noise • Optimize for consistency • cassette erasure difficulties • CR reader problems • processing algorithm issues • display monitor deviations • Optimize for machine vision • CAD • subtraction • segmentation • Optimize for distribution • image compression • memory utilization • network efficiency Medical Image Processing – Many Goals

24. Optimized spatial frequency filtering TP Pathological 1.0 0.8 0.6 Unsharply edged lung nodules 0.4 0.2 0.0 AZ, area under the ROC curves in the studies 0.0 0.2 0.4 0.6 0.8 1.0 FP ROC results of the optimization of post processing Hoeschen, C., Reissberg, S. and Dohring, W. The importance of optimizing the image processing for different digital x-ray detectors to get as much information as possible from the radiographs. Proc. SPIE 4682, 828–838 (2002)

25. The detectable information in radiographs produced with digital systems is strongly dependent on the speed class and image processing used. Inappropriate speed class would violate the ALARA principle while suboptimum image processing may lead to suppression of diagnostic information.

26. Various aspects to the optimization of radiation protection in digital radiology ICRP 93 • Equipment design considerations and technical methods of reducing patient dose • Operational approaches to reduce unnecessary patient doses by the appropriate selection of radiological examination and technical parameters

27. Specify the medical imaging task Determine the quality criteria Propose parameters in display-ready image to met quality criteria of the imaging task Standardize techniques and optimize processing in terms of the exposure required to produce the specified response in the displayed image Evaluate displayed image inconsistencies subquality Standardization in radiography