139 research outputs found
Polychromatic X-ray CT Image Reconstruction and Mass-Attenuation Spectrum Estimation
We develop a method for sparse image reconstruction from polychromatic
computed tomography (CT) measurements under the blind scenario where the
material of the inspected object and the incident-energy spectrum are unknown.
We obtain a parsimonious measurement-model parameterization by changing the
integral variable from photon energy to mass attenuation, which allows us to
combine the variations brought by the unknown incident spectrum and mass
attenuation into a single unknown mass-attenuation spectrum function; the
resulting measurement equation has the Laplace integral form. The
mass-attenuation spectrum is then expanded into first order B-spline basis
functions. We derive a block coordinate-descent algorithm for constrained
minimization of a penalized negative log-likelihood (NLL) cost function, where
penalty terms ensure nonnegativity of the spline coefficients and nonnegativity
and sparsity of the density map. The image sparsity is imposed using
total-variation (TV) and norms, applied to the density-map image and
its discrete wavelet transform (DWT) coefficients, respectively. This algorithm
alternates between Nesterov's proximal-gradient (NPG) and limited-memory
Broyden-Fletcher-Goldfarb-Shanno with box constraints (L-BFGS-B) steps for
updating the image and mass-attenuation spectrum parameters. To accelerate
convergence of the density-map NPG step, we apply a step-size selection scheme
that accounts for varying local Lipschitz constant of the NLL. We consider
lognormal and Poisson noise models and establish conditions for biconvexity of
the corresponding NLLs. We also prove the Kurdyka-{\L}ojasiewicz property of
the objective function, which is important for establishing local convergence
of the algorithm. Numerical experiments with simulated and real X-ray CT data
demonstrate the performance of the proposed scheme
Convex-Set–Constrained Sparse Signal Recovery: Theory and Applications
Convex-set constrained sparse signal reconstruction facilitates flexible measurement model and accurate recovery. The objective function that we wish to minimize is a sum of a convex differentiable data-fidelity (negative log-likelihood (NLL)) term and a convex regularization term. We apply sparse signal regularization where the signal belongs to a closed convex set within the closure of the domain of the NLL. Signal sparsity is imposed using the l1-norm penalty on the signal\u27s linear transform coefficients.
First, we present a projected Nesterov’s proximal-gradient (PNPG) approach that employs a projected Nesterov\u27s acceleration step with restart and a duality-based inner iteration to compute the proximal mapping. We propose an adaptive step-size selection scheme to obtain a good local majorizing function of the NLL and reduce the time spent backtracking. We present an integrated derivation of the momentum acceleration and proofs of O(k^(-2)) objective function convergence rate and convergence of the iterates, which account for adaptive step size, inexactness of the iterative proximal mapping, and the convex-set constraint. The tuning of PNPG is largely application independent. Tomographic and compressed-sensing reconstruction experiments with Poisson generalized linear and Gaussian linear measurement models demonstrate the performance of the proposed approach.
We then address the problem of upper-bounding the regularization constant for the convex-set--constrained sparse signal recovery problem behind the PNPG framework. This bound defines the maximum influence the regularization term has to the signal recovery. We formulate an optimization problem for finding these bounds when the regularization term can be globally minimized and develop an alternating direction method of multipliers (ADMM) type method for their computation. Simulation examples show that the derived and empirical bounds match.
Finally, we show application of the PNPG framework to X-ray computed tomography (CT) and outline a method for sparse image reconstruction from Poisson-distributed polychromatic X-ray CT measurements under the blind scenario where the material of the inspected object and the incident energy spectrum are unknown. To obtain a parsimonious mean measurement-model parameterization, we first rewrite the measurement equation by changing the integral variable from photon energy to mass attenuation, which allows us to combine the variations brought by the unknown incident spectrum and mass attenuation into a single unknown mass-attenuation spectrum function; the resulting measurement equation has the Laplace integral form. We apply a block coordinate-descent algorithm that alternates between an NPG image reconstruction step and a limited-memory BFGS with box constraints (L-BFGS-B) iteration for updating mass-attenuation spectrum parameters. Our NPG-BFGS algorithm is the first physical-model based image reconstruction method for simultaneous blind sparse image reconstruction and mass-attenuation spectrum estimation from polychromatic measurements. Real X-ray CT reconstruction examples demonstrate the performance of the proposed blind scheme
Blind polychromatic X-ray CT reconstruction from Poisson measurements
We develop a sparse image reconstruction method for Poisson distributed polychromatic X-ray computed tomography (CT) measurements under the blind scenario where the material of the inspected object and the incident energy spectrum are unknown. We employ our mass-attenuation spectrum parameterization of the noiseless measurements for single-material objects and express the mass-attenuation spectrum as a linear combination of B-spline basis functions of order one. A block coordinate descent algorithm is developed for constrained minimization of a penalized Poisson negative log-likelihood (NLL) cost function, where constraints and penalty terms ensure nonnegativity of the spline coefficients and nonnegativity and sparsity of the density-map image; the image sparsity is imposed using a convex total-variation (TV) norm penalty term. This algorithm alternates between a Nesterov’s proximal-gradient (NPG) step for estimating the density-map image and a limited-memory Broyden-Fletcher-Goldfarb-Shanno with box constraints (LBFGS- B) step for estimating the incident-spectrum parameters. We establish conditions for biconvexity of the penalized NLL objective function, which, if satisfied, ensures monotonicity of the NPG-BFGS iteration. We also show that the penalized NLL objective satisfies the Kurdyka-Łojasiewicz property, which is important for establishing local convergence of block-coordinate descent schemes in biconvex optimization problems. Simulation examples demonstrate the performance of the proposed scheme
Design and Model Verification of an Infrared Chromotomographic Imaging System
A prism chromotomographic hyperspectral imaging sensor is being developed to aid in the study of bomb phenomenology. Reliable chromotomographic reconstruction depends on accurate knowledge of the sensor specific point spread function over all wavelengths of interest. The purpose of this research is to generate the required point spread functions using wave optics techniques and a phase screen model of system aberrations. Phase screens are generated using the Richardson-Lucy algorithm for extracting point spread functions and Gerchberg-Saxton algorithm for phase retrieval. These phase screens are verified by comparing the modeled results of a blackbody source with measurements made using a chromotomographic sensor. The sensor itself is constructed as part of this research. Comparison between the measured and simulated results is based upon the noise statistics of the measured image. Four comparisons between measured and modeled data, each made at a different prism rotation angle, provide the basis for the conclusions of this research. Based on these results, the phase screen technique appears to be valid so long as constraints are placed on the field of view and spectral region over which the screens are applied
Markov random field image modelling
Includes bibliographical references.This work investigated some of the consequences of using a priori information in image processing using computer tomography (CT) as an example. Prior information is information about the solution that is known apart from measurement data. This information can be represented as a probability distribution. In order to define a probability density distribution in high dimensional problems like those found in image processing it becomes necessary to adopt some form of parametric model for the distribution. Markov random fields (MRFs) provide just such a vehicle for modelling the a priori distribution of labels found in images. In particular, this work investigated the suitability of MRF models for modelling a priori information about the distribution of attenuation coefficients found in CT scans
Partially Coherent Lab Based X-ray Micro Computed Tomography
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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