185 research outputs found
Optimal Data Collection For Informative Rankings Expose Well-Connected Graphs
Given a graph where vertices represent alternatives and arcs represent
pairwise comparison data, the statistical ranking problem is to find a
potential function, defined on the vertices, such that the gradient of the
potential function agrees with the pairwise comparisons. Our goal in this paper
is to develop a method for collecting data for which the least squares
estimator for the ranking problem has maximal Fisher information. Our approach,
based on experimental design, is to view data collection as a bi-level
optimization problem where the inner problem is the ranking problem and the
outer problem is to identify data which maximizes the informativeness of the
ranking. Under certain assumptions, the data collection problem decouples,
reducing to a problem of finding multigraphs with large algebraic connectivity.
This reduction of the data collection problem to graph-theoretic questions is
one of the primary contributions of this work. As an application, we study the
Yahoo! Movie user rating dataset and demonstrate that the addition of a small
number of well-chosen pairwise comparisons can significantly increase the
Fisher informativeness of the ranking. As another application, we study the
2011-12 NCAA football schedule and propose schedules with the same number of
games which are significantly more informative. Using spectral clustering
methods to identify highly-connected communities within the division, we argue
that the NCAA could improve its notoriously poor rankings by simply scheduling
more out-of-conference games.Comment: 31 pages, 10 figures, 3 table
Learned SVD: solving inverse problems via hybrid autoencoding
Our world is full of physics-driven data where effective mappings between
data manifolds are desired. There is an increasing demand for understanding
combined model-based and data-driven methods. We propose a nonlinear, learned
singular value decomposition (L-SVD), which combines autoencoders that
simultaneously learn and connect latent codes for desired signals and given
measurements. We provide a convergence analysis for a specifically structured
L-SVD that acts as a regularisation method. In a more general setting, we
investigate the topic of model reduction via data dimensionality reduction to
obtain a regularised inversion. We present a promising direction for solving
inverse problems in cases where the underlying physics are not fully understood
or have very complex behaviour. We show that the building blocks of learned
inversion maps can be obtained automatically, with improved performance upon
classical methods and better interpretability than black-box methods
A Partially Learned Algorithm for Joint Photoacoustic Reconstruction and Segmentation
In an inhomogeneously illuminated photoacoustic image, important information
like vascular geometry is not readily available when only the initial pressure
is reconstructed. To obtain the desired information, algorithms for image
segmentation are often applied as a post-processing step. In this work, we
propose to jointly acquire the photoacoustic reconstruction and segmentation,
by modifying a recently developed partially learned algorithm based on a
convolutional neural network. We investigate the stability of the algorithm
against changes in initial pressures and photoacoustic system settings. These
insights are used to develop an algorithm that is robust to input and system
settings. Our approach can easily be applied to other imaging modalities and
can be modified to perform other high-level tasks different from segmentation.
The method is validated on challenging synthetic and experimental photoacoustic
tomography data in limited angle and limited view scenarios. It is
computationally less expensive than classical iterative methods and enables
higher quality reconstructions and segmentations than state-of-the-art learned
and non-learned methods.Comment: "copyright 2019 IEEE. Personal use of this material is permitted.
Permission from IEEE must be obtained for all other uses, in any current or
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this work in other works.
Deep Learning-Based Carotid Artery Vessel Wall Segmentation in Black-Blood MRI Using Anatomical Priors
Carotid artery vessel wall thickness measurement is an essential step in the
monitoring of patients with atherosclerosis. This requires accurate
segmentation of the vessel wall, i.e., the region between an artery's lumen and
outer wall, in black-blood magnetic resonance (MR) images. Commonly used
convolutional neural networks (CNNs) for semantic segmentation are suboptimal
for this task as their use does not guarantee a contiguous ring-shaped
segmentation. Instead, in this work, we cast vessel wall segmentation as a
multi-task regression problem in a polar coordinate system. For each carotid
artery in each axial image slice, we aim to simultaneously find two
non-intersecting nested contours that together delineate the vessel wall. CNNs
applied to this problem enable an inductive bias that guarantees ring-shaped
vessel walls. Moreover, we identify a problem-specific training data
augmentation technique that substantially affects segmentation performance. We
apply our method to segmentation of the internal and external carotid artery
wall, and achieve top-ranking quantitative results in a public challenge, i.e.,
a median Dice similarity coefficient of 0.813 for the vessel wall and median
Hausdorff distances of 0.552 mm and 0.776 mm for lumen and outer wall,
respectively. Moreover, we show how the method improves over a conventional
semantic segmentation approach. These results show that it is feasible to
automatically obtain anatomically plausible segmentations of the carotid vessel
wall with high accuracy.Comment: SPIE Medical Imaging 202
4D imaging in tomography and optical nanoscopy
Diese Dissertation gehört zu den Gebieten mathematische Bildverarbeitung und inverse Probleme. Ein inverses Problem ist die Aufgabe, Modellparameter anhand von gemessenen Daten zu berechnen. Solche Probleme treten in zahlreichen Anwendungen in Wissenschaft und Technik auf, z.B. in medizinischer Bildgebung, Biophysik oder Astronomie. Wir betrachten Rekonstruktionsprobleme mit Poisson Rauschen in der Tomographie und optischen Nanoskopie. Bei letzterer gilt es Bilder ausgehend von verzerrten und verrauschten Messungen zu rekonstruieren, wohingegen in der Positronen-Emissions-Tomographie die Aufgabe in der Visualisierung physiologischer Prozesse eines Patienten besteht. Standardmethoden zur 3D Bildrekonstruktion berücksichtigen keine zeitabhängigen Informationen oder Dynamik, z.B. Herzschlag oder Atmung in der Tomographie oder Zellmigration in der Mikroskopie. Diese Dissertation behandelt Modelle, Analyse und effiziente Algorithmen für 3D und 4D zeitabhängige inverse Probleme. This thesis contributes to the field of mathematical image processing
and inverse problems. An inverse problem is a task, where the values of
some model parameters must be computed from observed data. Such problems
arise in a wide variety of applications in sciences and engineering,
such as medical imaging, biophysics or astronomy. We mainly consider
reconstruction problems with Poisson noise in tomography and optical
nanoscopy. In the latter case, the task is to reconstruct images from
blurred and noisy measurements, whereas in positron emission tomography
the task is to visualize physiological processes of a patient. In 3D
static image reconstruction standard methods do not incorporate
time-dependent information or dynamics, e.g. heart beat or breathing in
tomography or cell motion in microscopy. This thesis is a treatise on
models, analysis and efficient algorithms to solve 3D and 4D
time-dependent inverse problems
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