Correction and Preprocessing Methods

1. Correction and Preprocessing Methods Veselin Dikov Moscow-Bavarian Joint Advanced Student School19-29. March 2006 Moscow, Russia

2. Outline • Beam hardening • Scattered radiation • Ring artefacts • References

3. Beam hardening • Monochromatic vs. Polychromatic X-ray source • Absorption of low energy photons • Additive detector • Mean energy higher - beam hardening

4. Thickness measurement • Monochromatic • Polychromatic for homogenous medium

5. Thickness measurement • “Non-linear” deviation of the polychromatic signal

6. Beam hardening artefacts • Cupping • Simulated skull

7. Beam hardening artefacts • Streaks • Hard (e.g. bone) tissues

8. Correction Strategies • Preprocessing • On projection data • Postprocessing • On reconstructed data • Dual-energy imaging • Considered most elegant

9. Preprocessing • Select “appropriate” absorber • Measured value A is corrected to A’ • Assumes “homogeneous” medium • Fails if bone present

10. Postprocessing • Iterative approach: • first: preprocessing step • second: threshold to identify hard structures • third: forward-project the contribution of the hard structures into projection data • Good results

11. Dual-energy image • Modeling of the attenuation coefficient • Substitute in • Gives

12. Dual-energy image • Two energies - two integral equations • Reconstruct and • Now can be reconstructed for any E • Two scans?

13. Outline • Beam hardening • Scattered radiation • Ring artefacts • References

14. Scattered radiation • Scattered radiation • Components of scatter • Off-focus radiation (OFR) • Scattered radiation in the patient • Veiling glare

15. Scatter measure • SPR – Scatter-to-Primary-Ratio • Broad beam model estimation for SPR

16. Scatter artefacts – loss of contrast • Loss of contrast • Contrast without scatter • Contrast with scatter

17. Scatter artefacts – loss of sharpness • Loss of sharpness • Transmitted radiation • - blurring kernel • - 2D convolution

18. Scatter artefacts – loss of sharpness • Loss of sharpness • Veiling glare contribution

19. Scatter reduction methods • Anti-scatter grids • 4G vs. 3G • Higher voltage • Air gaps • Magnifying effects • Increased blurring • Beam collimation • Complicated systems reminder:

20. Measurement of scatter • Opaque disc techniques • Scatter in the “shadow” • Arrays of discs with decreasing size • Aperture techniques • Narrow beam “clean” of scatter • Arrays of apertures • Hybrid techniques • One vs. Two scan techniques

21. Correction schemes - Convolution Filtering • Detected image • Use as an estimation for • Spatial vs. frequency domain filtering • Analytical vs. empirical approaches for filtering schemes

22. Correction schemes – Scatter sampling • Sampling of the scatter measurement • Opaque discs arrays • Aperture grid • Interpolation of the smooth surface • -6 dB

23. Correction schemes – Scatter sampling • Interpolation schemes • 2D least squares fitting • Filtration with sinc and jinc • 2D polynomial fitting • 2D bicubic splines • Sampling rate depends on surface frequencies

24. Results

25. Outline • Beam hardening • Scattered radiation • Ring artefacts • References

26. Ring artefacts

27. Ring Artefacts • Reason • 4G vs. 3G • Postprocessing correction only

28. Correction scheme • ROI • I P • Artefact pattern • Artefact subtraction • P I

29. ROI • Select ROI with morphological operations

30. Find artefact pattern • Scan for irregularities • Use variance of signal • or Use median of signal • etc. • Extract ring pattern

31. Results

32. References • T.Buzug, Einfürung in die Computertomographie, Springer-Verlag (2004) • A.Kak and M.Slaney, Principles of Computerized Tomographic Imaging, IEEE Press (1988) • K.P.Maher and J.F. Malone, Computerized scatter correction in diagnostic radiology, Contemporary Physics 38, 131-148 (1997). • J.Sijbers and A.Postnov, Reduction of ring artefacts in high resolution micro-CT reconstructions, Physics in Medicine and Biology Vol. 49 (07/2004) • M.Zellerhoff et al, Low contrast 3D-reconstruction from C-arm data, Medical Imaging SPIE, 646-655 (04/2004) Thank you for your attention!