The Characterization of Uncertainties and Artifacts in X-Ray Microtomography
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Transcript of The Characterization of Uncertainties and Artifacts in X-Ray Microtomography
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The Characterization of Uncertainties and
Artifacts in X-Ray Microtomography
Tony EvershedDental Biophysics Group, Institute of Dentistry
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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.
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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.
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Cone Beam Tomography
Image: Wikipedia (released into Public Domain)
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Cone Beam Tomography
Image: Wikipedia (released into Public Domain)
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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.)
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Applications of XMT
Examining decayed or damaged scrolls.
Video: Dr G R Davis
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Applications of XMT
Analysis of biomaterial and artificial structures.
Image: F Ahmed
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Applications of XMT
Mineralization studies in hard tissue.
Video:Dr G R Davis
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Sources of Artifacts
• Geometrical artifacts
• Centre-of-rotation errors.
• Specimen motion errors.
• Focus: grayscale artifacts.
• Beam Hardening
• Scattering.
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Artifacts: Beam Hardening
•Arises from use of polychromatic radiation.
•Materials do not follow Beer’s law: I = I0 e-μx.
•Materials absorb ‘soft’ X-rays preferentially.
•Beam becomes ‘harder’ and more penetrating.
Image: Dr G R Davis
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Artifacts: Beam Hardening
Ob
serv
ed
LA
C
• ‘Cupping’ artifact from beam hardening.
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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
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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.
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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.
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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.
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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.