The Characterization of
Uncertainties and Artifacts in
X-Ray Microtomography
Tony Evershed
Dental Biophysics Group, Institute of Dentistry
What is XMT?
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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.
XMT at QMUL
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MuCAT Systems 1 and 2.
Cone-beam XMT with time-delay integrating
detectors.
Based on COTS infrastructure with in-house
software and detector hardware.
Cone Beam Tomography
Image:
Wikipedia (released into Public Domain)
Cone Beam Tomography
Image:
Wikipedia (released into Public Domain)
QMT at QMUL - TDI
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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.)
Applications of XMT
Video:
Dr G R Davis
Examining decayed or damaged scrolls.
Applications of XMT
Image:
F Ahmed
Analysis of biomaterial and artificial structures.
Applications of XMT
Video:
Dr G R Davis
Mineralization studies in hard tissue.
Sources of Artifacts
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Geometrical artifacts
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Centre-of-rotation errors.
Specimen motion errors.
Focus: grayscale artifacts.
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Beam Hardening
Scattering.
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
Observed LAC
Artifacts: Beam Hardening
• ‘Cupping’ artifact from beam hardening.
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
Artifacts: Scatter
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Instead of being absorbed, photons may be
deflected.
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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.
Artifacts: Scatter
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Correction: none at present at QMUL.
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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.
Conclusion: Research
Outcomes for QMUL
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Reconstruction developments fed into existing
MuCAT systems.
MuCAT 3 next-generation scanner.
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Improved spatial resolution.
Larger sample capacity.
In tender, for delivery during 2012.
Acknowledgements
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Supervisors:
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Dr Graham Davis (Institute of Dentistry)
Dr Andrea Cavallaro (School of Electronic
Engineering and Computer Science).
Post-doc:
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Dr David Mills.
Engineering and Physical Sciences Research
Council grant EP/G007845/1.