16,405 research outputs found
Error analysis for filtered back projection reconstructions in Besov spaces
Filtered back projection (FBP) methods are the most widely used
reconstruction algorithms in computerized tomography (CT). The ill-posedness of
this inverse problem allows only an approximate reconstruction for given noisy
data. Studying the resulting reconstruction error has been a most active field
of research in the 1990s and has recently been revived in terms of optimal
filter design and estimating the FBP approximation errors in general Sobolev
spaces.
However, the choice of Sobolev spaces is suboptimal for characterizing
typical CT reconstructions. A widely used model are sums of characteristic
functions, which are better modelled in terms of Besov spaces
. In particular
with is a preferred
model in image analysis for describing natural images.
In case of noisy Radon data the total FBP reconstruction error
splits into an
approximation error and a data error, where serves as regularization
parameter. In this paper, we study the approximation error of FBP
reconstructions for target functions with positive and . We prove that the -norm
of the inherent FBP approximation error can be bounded above by
\begin{equation*} \|f - f_L\|_{\mathrm{L}^p(\mathbb{R}^2)} \leq c_{\alpha,q,W}
\, L^{-\alpha} \, |f|_{\mathrm{B}^{\alpha,p}_q(\mathbb{R}^2)} \end{equation*}
under suitable assumptions on the utilized low-pass filter's window function
. This then extends by classical methods to estimates for the total
reconstruction error.Comment: 32 pages, 8 figure
Sufficient Conditions for Feasibility and Optimality of Real-Time Optimization Schemes - II. Implementation Issues
The idea of iterative process optimization based on collected output
measurements, or "real-time optimization" (RTO), has gained much prominence in
recent decades, with many RTO algorithms being proposed, researched, and
developed. While the essential goal of these schemes is to drive the process to
its true optimal conditions without violating any safety-critical, or "hard",
constraints, no generalized, unified approach for guaranteeing this behavior
exists. In this two-part paper, we propose an implementable set of conditions
that can enforce these properties for any RTO algorithm. This second part
examines the practical side of the sufficient conditions for feasibility and
optimality (SCFO) proposed in the first and focuses on how they may be enforced
in real application, where much of the knowledge required for the conceptual
SCFO is unavailable. Methods for improving convergence speed are also
considered.Comment: 56 pages, 15 figure
Potential Output Estimations for Hungary: A Survey of Different Approaches
This paper is a comprehensive analysis of Hungary’s potential output. Since the concept of potential output is not unique, we present various interpretations of potential GDP, along with a large set of techniques for estimating it. Various estimates are presented and robustness analyses are performed. Finally, an illustrative scenario is outlined for the forthcoming few years.potential output, output gap, production function, business cycle, filtering.
TheanoLM - An Extensible Toolkit for Neural Network Language Modeling
We present a new tool for training neural network language models (NNLMs),
scoring sentences, and generating text. The tool has been written using Python
library Theano, which allows researcher to easily extend it and tune any aspect
of the training process. Regardless of the flexibility, Theano is able to
generate extremely fast native code that can utilize a GPU or multiple CPU
cores in order to parallelize the heavy numerical computations. The tool has
been evaluated in difficult Finnish and English conversational speech
recognition tasks, and significant improvement was obtained over our best
back-off n-gram models. The results that we obtained in the Finnish task were
compared to those from existing RNNLM and RWTHLM toolkits, and found to be as
good or better, while training times were an order of magnitude shorter
Numerical methods for coupled reconstruction and registration in digital breast tomosynthesis.
Digital Breast Tomosynthesis (DBT) provides an insight into the fine details of normal fibroglandular tissues and abnormal lesions by reconstructing a pseudo-3D image of the breast. In this respect, DBT overcomes a major limitation of conventional X-ray mam- mography by reducing the confounding effects caused by the superposition of breast tissue. In a breast cancer screening or diagnostic context, a radiologist is interested in detecting change, which might be indicative of malignant disease. To help automate this task image registration is required to establish spatial correspondence between time points. Typically, images, such as MRI or CT, are first reconstructed and then registered. This approach can be effective if reconstructing using a complete set of data. However, for ill-posed, limited-angle problems such as DBT, estimating the deformation is com- plicated by the significant artefacts associated with the reconstruction, leading to severe inaccuracies in the registration. This paper presents a mathematical framework, which couples the two tasks and jointly estimates both image intensities and the parameters of a transformation. Under this framework, we compare an iterative method and a simultaneous method, both of which tackle the problem of comparing DBT data by combining reconstruction of a pair of temporal volumes with their registration. We evaluate our methods using various computational digital phantoms, uncom- pressed breast MR images, and in-vivo DBT simulations. Firstly, we compare both iter- ative and simultaneous methods to the conventional, sequential method using an affine transformation model. We show that jointly estimating image intensities and parametric transformations gives superior results with respect to reconstruction fidelity and regis- tration accuracy. Also, we incorporate a non-rigid B-spline transformation model into our simultaneous method. The results demonstrate a visually plausible recovery of the deformation with preservation of the reconstruction fidelity
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