1. The Characterization of Uncertainties and Artifacts in X-Ray Microtomography Tony Evershed Dental Biophysics Group, Institute of Dentistry

2. What is XMT? Tomography, from Greek tomos (‘section’) and graphos (‘to write’). 2-3D representation based on a large number of projections. Tens-of-microns spatial resolution. Attenuation coefficient resolution sufficient for mineral-content analysis.

3. XMT at QMUL MuCAT Systems 1 and 2. Cone-beam XMT with time-delay integrating detectors. Based on COTS infrastructure with in-house software and detector hardware.

4. Cone Beam Tomography Image: Wikipedia (released into Public Domain)

5. Cone Beam Tomography Image: Wikipedia (released into Public Domain)

6. QMT at QMUL - TDI Means of averaging pixel sensitivity. Charge-coupled devices move charge in ‘steps’ by switching voltage at each pixel. Synchronization of step frequency to sample movement. Animation: Michael Schmidt (released under GFDL.)

7. Applications of XMT Examining decayed or damaged scrolls. Video: Dr G R Davis

8. Applications of XMT Analysis of biomaterial and artificial structures. Image: F Ahmed

9. Applications of XMT Mineralization studies in hard tissue. Video: Dr G R Davis

10. Sources of Artifacts Geometrical artifacts Centre-of-rotation errors. Specimen motion errors. Focus: grayscale artifacts. Beam Hardening Scattering.

11. Artifacts: Beam Hardening Arises from use of polychromatic radiation. Materials do not follow Beer’s law: I = I 0 e -μx. Materials absorb ‘soft’ X-rays preferentially. Beam becomes ‘harder’ and more penetrating. Image: Dr G R Davis

12. Artifacts: Beam Hardening Observed LAC ‘Cupping’ artifact from beam hardening.

13. Artifacts: Beam Hardening Correction: compare ideal Beer-law case with a known material added to the sample. Multi-mode samples complicate matters. Image: Dr G R Davis

14. Artifacts: Scatter Instead of being absorbed, photons may be deflected. Compton (incoherent) scattering, from outer electron shells, largely responsible. Increases level of noise in the reconstruction, particularly near high-attenuation regions. Decreases contrast ratio in the reconstruction.

15. Artifacts: Scatter Correction: none at present at QMUL. Beam-hardening correction also corrects for some scatter. Level of scatter can be determined from projection borders (outside cone beam.) Monte Carlo modelling of virtual phantoms using Geant4 transport code.

16. Conclusion: Research Outcomes for QMUL Reconstruction developments fed into existing MuCAT systems. MuCAT 3 next-generation scanner. Improved spatial resolution. Larger sample capacity. In tender, for delivery during 2012.

17. Acknowledgements Supervisors: Dr Graham Davis (Institute of Dentistry) Dr Andrea Cavallaro (School of Electronic Engineering and Computer Science). Post-doc: Dr David Mills. Engineering and Physical Sciences Research Council grant EP/G007845/1.