493 research outputs found
Half-Precision Kronecker Product SVD Preconditioner for Structured Inverse Problems
In this paper we investigate the use of half-precision Kronecker product
singular value decomposition (SVD) approximations as preconditioners for
large-scale Tikhonov regularized least squares problems. Half precision reduces
storage requirements and has the potential to greatly speedup computations on
certain GPU architectures. We consider both standard PCG and flexible PCG
algorithms, and investigate, through numerical experiments on image deblurring
problems, the trade-offs between potentially faster convergence with the
additional cost per iteration when using this preconditioning approach.
Moreover, we also investigate the use of several regularization parameter
choice methods, including generalized cross validation and the discrepancy
principle
Bayesian inference for structured additive regression models for large-scale problems with applications to medical imaging
In der angewandten Statistik können Regressionsmodelle mit hochdimensionalen Koeffizienten auftreten, die sich nicht mit gewöhnlichen Computersystemen schätzen lassen. Dies betrifft unter anderem die Analyse digitaler Bilder unter Berücksichtigung räumlich-zeitlicher Abhängigkeiten, wie sie innerhalb der medizinisch-biologischen Forschung häufig vorkommen.
In der vorliegenden Arbeit wird ein Verfahren formuliert, das in der
Lage ist, Regressionsmodelle mit hochdimensionalen Koeffizienten und nicht-normalverteilten Zielgrößen unter moderaten Anforderungen an die benötigte Hardware zu schätzen. Hierzu wird zunächst im Rahmen strukturiert additiver Regressionsmodelle aufgezeigt, worin die Limitationen aktueller Inferenzansätze bei der Anwendung auf hochdimensionale Problemstellungen liegen, sowie Möglichkeiten diskutiert, diese zu umgehen. Darauf basierend wird ein Algorithmus formuliert, dessen Stärken und Schwächen anhand von Simulationsstudien analysiert werden. Darüber hinaus findet das Verfahren Anwendung in drei verschiedenen Bereichen der medizinisch-biologischen Bildgebung und zeigt dadurch, dass es ein vielversprechender Kandidat für die Beantwortung hochdimensionaler Fragestellungen ist.In applied statistics regression models with high-dimensional
coefficients can occur which cannot be estimated using ordinary computers. Amongst others, this applies to the analysis of digital images taking spatio-temporal dependencies into account as they commonly occur within bio-medical research.
In this thesis a procedure is formulated which allows to fit regression
models with high-dimensional coefficients and non-normal response
values requiring only moderate computational equipment. To this end, limitations of different inference strategies for structured additive regression models are demonstrated when applied to high-dimensional problems and possible solutions are discussed. Based thereon an algorithm is formulated whose strengths and weaknesses are subsequently analyzed using simulation studies. Furthermore, the procedure is applied to three different fields of bio-medical imaging from which can be concluded that the algorithm is a promising candidate for answering high-dimensional problems
A constructive arbitrary-degree Kronecker product decomposition of tensors
We propose the tensor Kronecker product singular value decomposition~(TKPSVD)
that decomposes a real -way tensor into a linear combination
of tensor Kronecker products with an arbitrary number of factors
. We generalize the matrix Kronecker product to
tensors such that each factor in the TKPSVD is a -way
tensor. The algorithm relies on reshaping and permuting the original tensor
into a -way tensor, after which a polyadic decomposition with orthogonal
rank-1 terms is computed. We prove that for many different structured tensors,
the Kronecker product factors
are guaranteed to inherit this structure. In addition, we introduce the new
notion of general symmetric tensors, which includes many different structures
such as symmetric, persymmetric, centrosymmetric, Toeplitz and Hankel tensors.Comment: Rewrote the paper completely and generalized everything to tensor
Conjugate-Gradient Preconditioning Methods for Shift-Variant PET Image Reconstruction
Gradient-based iterative methods often converge slowly for tomographic image reconstruction and image restoration problems, but can be accelerated by suitable preconditioners. Diagonal preconditioners offer some improvement in convergence rate, but do not incorporate the structure of the Hessian matrices in imaging problems. Circulant preconditioners can provide remarkable acceleration for inverse problems that are approximately shift-invariant, i.e., for those with approximately block-Toeplitz or block-circulant Hessians. However, in applications with nonuniform noise variance, such as arises from Poisson statistics in emission tomography and in quantum-limited optical imaging, the Hessian of the weighted least-squares objective function is quite shift-variant, and circulant preconditioners perform poorly. Additional shift-variance is caused by edge-preserving regularization methods based on nonquadratic penalty functions. This paper describes new preconditioners that approximate more accurately the Hessian matrices of shift-variant imaging problems. Compared to diagonal or circulant preconditioning, the new preconditioners lead to significantly faster convergence rates for the unconstrained conjugate-gradient (CG) iteration. We also propose a new efficient method for the line-search step required by CG methods. Applications to positron emission tomography (PET) illustrate the method.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85979/1/Fessler85.pd
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