The Characterization of Uncertainties and

Artifacts in X-Ray Microtomography

Tony EvershedDental Biophysics Group, Institute of Dentistry

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.

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.

Cone Beam Tomography

Image: Wikipedia (released into Public Domain)

Cone Beam Tomography

Image: Wikipedia (released into Public Domain)

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.)

Applications of XMT

Examining decayed or damaged scrolls.

Video: Dr G R Davis

Applications of XMT

Analysis of biomaterial and artificial structures.

Image: F Ahmed

Applications of XMT

Mineralization studies in hard tissue.

Video:Dr G R Davis

Sources of Artifacts

• Geometrical artifacts

• Centre-of-rotation errors.

• Specimen motion errors.

• Focus: grayscale artifacts.

• 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

Artifacts: Beam Hardening

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• ‘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

• 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.

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.

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.

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.