14,218 research outputs found
An efficient maximum entropy technique for 2-D isotropic random fields
Title from cover.Bibliography: p. 24-25.Supported in part by the National Science Foundation under Grant ECS-83-12921 and in part by the Army Research Office under Grant DAAG-29-84-K-0005.Ahmed H. Tewfik, Bernard C. Levy and Alan S. Willsky
An efficient maximum entropy technique for 2-D isotropic random fields
Bibliography: p. 36-40.Supported by the National Science Foundation under grant no. ECS-83-12921 Supported by the Army Research Office under grant no. DAAG-84-K-0005Ahmed H. Tewfik, Bernard C. Levy, Alan S. Willsky
Efficient Wiener filtering without preconditioning
We present a new approach to calculate the Wiener filter solution of general
data sets. It is trivial to implement, flexible, numerically absolutely stable,
and guaranteed to converge. Most importantly, it does not require an ingenious
choice of preconditioner to work well. The method is capable of taking into
account inhomogeneous noise distributions and arbitrary mask geometries. It
iteratively builds up the signal reconstruction by means of a messenger field,
introduced to mediate between the different preferred bases in which signal and
noise properties can be specified most conveniently. Using cosmic microwave
background (CMB) radiation data as a showcase, we demonstrate the capabilities
of our scheme by computing Wiener filtered WMAP7 temperature and polarization
maps at full resolution for the first time. We show how the algorithm can be
modified to synthesize fluctuation maps, which, combined with the Wiener filter
solution, result in unbiased constrained signal realizations, consistent with
the observations. The algorithm performs well even on simulated CMB maps with
Planck resolution and dynamic range.Comment: 5 pages, 2 figures. Submitted to Astronomy and Astrophysics. Replaced
to match published versio
Final report on estimation and statistical analysis of spatially distributed random processes
Includes bibliographical references.Final report;Supported by the NSF. ECS-8312921prepared by Alan S. Willsky, Bernard C. Levy, George C. Verghese
Detecting the orientation of magnetic fields in galaxy clusters
Clusters of galaxies, filled with hot magnetized plasma, are the largest
bound objects in existence and an important touchstone in understanding the
formation of structures in our Universe. In such clusters, thermal conduction
follows field lines, so magnetic fields strongly shape the cluster's thermal
history; that some have not since cooled and collapsed is a mystery. In a
seemingly unrelated puzzle, recent observations of Virgo cluster spiral
galaxies imply ridges of strong, coherent magnetic fields offset from their
centre. Here we demonstrate, using three-dimensional magnetohydrodynamical
simulations, that such ridges are easily explained by galaxies sweeping up
field lines as they orbit inside the cluster. This magnetic drape is then lit
up with cosmic rays from the galaxies' stars, generating coherent polarized
emission at the galaxies' leading edges. This immediately presents a technique
for probing local orientations and characteristic length scales of cluster
magnetic fields. The first application of this technique, mapping the field of
the Virgo cluster, gives a startling result: outside a central region, the
magnetic field is preferentially oriented radially as predicted by the
magnetothermal instability. Our results strongly suggest a mechanism for
maintaining some clusters in a 'non-cooling-core' state.Comment: 48 pages, 21 figures, revised version to match published article in
Nature Physics, high-resolution version available at
http://www.cita.utoronto.ca/~pfrommer/Publications/pfrommer-dursi.pd
Automatic Renal Segmentation in DCE-MRI using Convolutional Neural Networks
Kidney function evaluation using dynamic contrast-enhanced MRI (DCE-MRI)
images could help in diagnosis and treatment of kidney diseases of children.
Automatic segmentation of renal parenchyma is an important step in this
process. In this paper, we propose a time and memory efficient fully automated
segmentation method which achieves high segmentation accuracy with running time
in the order of seconds in both normal kidneys and kidneys with hydronephrosis.
The proposed method is based on a cascaded application of two 3D convolutional
neural networks that employs spatial and temporal information at the same time
in order to learn the tasks of localization and segmentation of kidneys,
respectively. Segmentation performance is evaluated on both normal and abnormal
kidneys with varying levels of hydronephrosis. We achieved a mean dice
coefficient of 91.4 and 83.6 for normal and abnormal kidneys of pediatric
patients, respectively
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