748 research outputs found
A Multi-Grid Iterative Method for Photoacoustic Tomography
Inspired by the recent advances on minimizing nonsmooth or bound-constrained
convex functions on models using varying degrees of fidelity, we propose a line
search multigrid (MG) method for full-wave iterative image reconstruction in
photoacoustic tomography (PAT) in heterogeneous media. To compute the search
direction at each iteration, we decide between the gradient at the target
level, or alternatively an approximate error correction at a coarser level,
relying on some predefined criteria. To incorporate absorption and dispersion,
we derive the analytical adjoint directly from the first-order acoustic wave
system. The effectiveness of the proposed method is tested on a total-variation
penalized Iterative Shrinkage Thresholding algorithm (ISTA) and its accelerated
variant (FISTA), which have been used in many studies of image reconstruction
in PAT. The results show the great potential of the proposed method in
improving speed of iterative image reconstruction
A finite element analysis of crack propagation problems with applications to seismology
Imperial Users onl
Refraction-corrected ray-based inversion for three-dimensional ultrasound tomography of the breast
Ultrasound Tomography has seen a revival of interest in the past decade,
especially for breast imaging, due to improvements in both ultrasound and
computing hardware. In particular, three-dimensional ultrasound tomography, a
fully tomographic method in which the medium to be imaged is surrounded by
ultrasound transducers, has become feasible. In this paper, a comprehensive
derivation and study of a robust framework for large-scale bent-ray ultrasound
tomography in 3D for a hemispherical detector array is presented. Two
ray-tracing approaches are derived and compared. More significantly, the
problem of linking the rays between emitters and receivers, which is
challenging in 3D due to the high number of degrees of freedom for the
trajectory of rays, is analysed both as a minimisation and as a root-finding
problem. The ray-linking problem is parameterised for a convex detection
surface and three robust, accurate, and efficient ray-linking algorithms are
formulated and demonstrated. To stabilise these methods, novel
adaptive-smoothing approaches are proposed that control the conditioning of the
update matrices to ensure accurate linking. The nonlinear UST problem of
estimating the sound speed was recast as a series of linearised subproblems,
each solved using the above algorithms and within a steepest descent scheme.
The whole imaging algorithm was demonstrated to be robust and accurate on
realistic data simulated using a full-wave acoustic model and an anatomical
breast phantom, and incorporating the errors due to time-of-flight picking that
would be present with measured data. This method can used to provide a
low-artefact, quantitatively accurate, 3D sound speed maps. In addition to
being useful in their own right, such 3D sound speed maps can be used to
initialise full-wave inversion methods, or as an input to photoacoustic
tomography reconstructions
State-dependent Kalman filters for robust engine control
Vehicle emissions variations impose significant challenges to the automotive industry. In these simulation studies, nonlinear estimation techniques based on state-dependent and extended Kalman filtering are developed for spark ignition engines to enhance robustness of the feedforward fuel controllers to changes in nominal system parameters and measurement errors. A model-based approach is used to derive the optimal filters. Numerical simulations indicate the superiority of estimation-based approaches to enhance robustness of in-cylinder air estimation which directly contributes to the precision of engine exhaust air-fuel ratio and, consequently the consistency of the tailpipe emissions. The results obtained are for an aggressive driving profile and are presented and discusse
Hessian-inversion-free ray-born inversion for quantitative ultrasound tomography
This study proposes a Hessian-inversion-free ray-born inversion approach for
biomedical ultrasound tomography. The proposed approach is a more efficient
version of the ray-born inversion approach proposed in [1]. Using these
approaches, the propagation of acoustic waves are modelled using a ray
approximation to heterogeneous Green's function. The inverse problem is solved
in the frequency domain by iteratively linearisation and minimisation of the
objective function from low to high frequencies. In [1], the linear subproblem
associated with each frequency interval is solved by an implicit and iterative
inversion of the Hessian matrix (inner iterations). Instead, this study applies
a preconditioning approach on each linear subproblem so that the Hessian matrix
becomes diagonalised, and can thus be inverted in a single step. Using the
proposed preconditioning approach, the computational cost of solving each
linear subproblem of the proposed ray-Born inversion approach becomes almost
the same as solving one linear subproblem associated with a radon-type
time-of-flight-based approach using bent rays. More importantly, the smoothness
assumptions made for diagonalising the Hessian matrix make the image
reconstruction more stable than the inversion approach in [1] to noise
On inclusion of source in the system of first-order linear acoustic wave equations
Simulating propagation of acoustic waves via solving a system of
three-coupled first-order linear differential equations using a k-space
pseudo-spectral method is popular for biomedical applications, firstly because
of availability of an open-source toolbox for implementation of this numerical
approach, and secondly because of its efficiency. The k-space pseudo-spectral
method is efficient, because it allows coarser computational grids and larger
time steps than finite difference and finite element methods for the same
accuracy. The goal of this study is to compare this numerical wave solver with
an analytical solution to the wave equation using the Green's function for
computing propagation of acoustic waves in homogeneous media. This comparison
is done in the frequency domain. Using the k-Wave solver, a match to the
Green's function is obtained after modifying the approach taken for including
mass source in the linearised equation of continuity (conservation of mass) in
the associated system of wave equations.Comment: List of changes: 1) In version 1, there was a mistake in displaying
figures 3(d), 3(e), 3(f), 6(d), 6(e) and 6(f). Those figures are corrected.
2) The word "acoustic" was added to the titl
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