| dc.description.abstract | X-ray microtomogaphy is a non-clinical, non-destructive, and quantitative technique for determining
three-dimensional mineral concentration distributions in variably radiolucent samples
with a spatial resolution on the micron scale. For reasons of practicality, particularly for longterm
studies, it is often not possible or desirable to utilise a monochromatic X-ray source and
so microtomography using a conventional impact-source X-ray generator to produce a polychromatic
photon beam is carried out instead. The use of photons of multiple energies causes the production
of projection artefacts arising from preferential absorption, which impair the greyscale
accuracy of the resulting reconstruction and the material concentration measurements that are
derived from the linear attenuation coefficients (LACs).
The purpose of the project described in this thesis is to identify weaknesses in the current method
of beam hardening correction and to develop and test a tomographic calibration and projection
processing method which may demonstrably improve the quality of current beam hardening correction
methods as used with the MuCAT microtomography equipment, which provides a worldclass
impact-source microtomography research and production facility at Barts and The London
School of Medicine and Dentistry.
An overview of the physical basis of X-ray computed tomography and X-ray microtomography
is given from first principles, and examples of quantitative applications of the techniques,
which generally depend on accurate reconstruction of linear attenuation coefficient values, are
discussed. The major sources of artefacts in X-ray microtomography are discussed, particularly those with a direct impact on reconstructed linear attenuation coefficient values. Beam hardening
is identified as an error source of particular interest, with secondary research on the effects of
any beam hardening correction method on the severity of Compton scatter artefacts, and a critical
review is carried out of historical attempts to reduce or mitigate these artefacts, particularly
the single-material parameter-optimisation approach in service at the beginning of the research
project.
A ‘carousel’ test piece comprising multiple attenuators of multiple materials along with attenuation
optimisation software based on varying multiple system parameters in order to extend
the functionality and usability of the existing correction model, and qualitative results have so
far been gathered suggesting the use of this system over the pre-existing attenuation wedge and
parameter-optimisation method.
A study of the effects of tuning the photon energy to which calibrations are made is carried out,
showing improved linear attenuation coefficient recovery at a higher energy than was previously
believed to be optimal, and a significant effect arising from X-ray generator target evaporation
leading to spatial changes and time-dependence of the target thickness parameter is measured,
suggesting that automated calibration as a standard part of the measurement process is required.
A stability experiment is carried out using this method in order to examine the possibility of
inconsistency resulting from ageing of the filament cathode, which is found not to significantly
impact the quality of results.
An immersion tank is developed in order to ensure beam hardening correction validity in the
case of dual-material specimens where a radiodensity-matching fluid can be provided and the
sample is suitable for immersion. Experimental comparison using a commercial beam hardening
calibration device as the specimen reveals significantly improved hydroxyapatite concentration
measurement recovery. An in-scatter experiment was carried out on the immersion tank, and it
was found that there was a significant scatter contribution when the tank was filled in the case
where the sample thickness is much less than the tank thickness. Proposals are presented for
further work to improve reconstruction quality through of scatter reduction techniques in impactsource
microtomographic systems, and to utilise the immersion tank for in situ chemical erosion
experiments.
The effects of the improvements to the beam hardening process are demonstrated using a biological
specimen to demonstrate qualitative changes in reconstruction, particularly in improved
dark levels surrounding the specimen. A second experiment is carried out in order to test the
reproducibility of results, which is found to be improved by approximately four times over the
same dataset corrected using the pre-existingbeam-hardening calibration method | en_US |
