Partially Coherent Lab Based X-ray Micro Computed Tomography
Date
2016
Authors
Li, Heyang (Thomas)
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Abstract
X-ray micro computed tomography (CT) is a useful tool for imaging
3-D internal structures. It has many applications in geophysics,
biology and materials science. Currently, micro-CT’s capability
are limited due to validity of assumptions used in modelling the
machines’ physical properties, such as penumbral blurring due
to non-point source, and X-ray refraction. Therefore many CT
research in algorithms and models are being carried out to
overcome these limitations.
This thesis presents methods to improve image resolution and
noise, and to enable material property estimation of the micro-CT
machine developed and in use at the ANU CTLab. This thesis is
divided into five chapters as
outlined below.
The broad background topics of X-ray modelling and CT
reconstruction are explored in Chapter 1, as required by later
chapters. It describes each X-ray CT component, including the
machines used at the ANU CTLab. The mathematical and statistical
tools, and electromagnetic physical models are provided and used
to characterise the scalar X-ray wave. This scalar wave equation
is used to derive the projection operator through matter and free
space, and basic reconstruction and phase retrieval algorithms.
It quantifies the four types of X-ray interaction with matter for
X-ray energy between 1 and 1000 keV, and presents common
assumptions used for the modelling of lab based X-ray micro-CT.
Chapter 2 is on X-ray source deblurring. The penumbral source
blurring for X-ray micro-CT systems are limiting its resolution.
This chapter starts with a geometrical framework to model the
penumbral source blurring. I have simulated the effect of source
blurring, assuming the geometry of the high-cone angle CT system,
used at the ANU CTLab. Also, I have developed the Multislice
Richardson-Lucy method that overcomes the computational
complexity of the conjugate gradient method, while produces less
artefacts compared to the standard Richardson-Lucy method. Its
performance is demonstrated for both simulated and real
experimental data.
X-ray refraction, phase contrast and phase retrieval (PR) are
investigated in Chapter 3. For weakly attenuating samples,
intensity variation due to phase contrast is a significant
fraction of the total signal. If phase contrast is incorrectly
modelled, the reconstruction would not correctly account the
phase contrast, therefore it would contribute to undesirable
artefacts in the reconstruction volume. Here I present a novel
Linear Iterative multi-energy PR algorithm. It enables material
property estimation for the near field submicron X-ray CT system
and reduces the noise and artefacts. This PR algorithm expands
the validity range in comparison to the single material and data
constrained modelling methods. I have also extended this novel PR
algorithm to assume a polychromatic incident spectrum for a
non-weakly absorbing object.
Chapter 4 outlines the space filling X-ray source trajectory and
reconstruction, on which I contributed in a minor capacity. This
space filling trajectory reconstruction have improved the
detector utilisation and reduced nonuniform resolution over the
state-of-the-art 3-D Katsevich’s helical reconstruction, this
patented work was done in collaboration with FEI Company.
Chapter 5 concludes my PhD research work and provides future
directions revealed by the present research.
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Keywords
micro computed tomography, penumbral blurring, non-point source, X-ray refraction, CT research, ANU CTLab, X-ray modelling, CT reconstruction, source deblurring, high-cone angle CT, phase contrast, phase retrieval, linear iterative multi-energy method, space filling trajectory
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