24,016 research outputs found
Accurate and reliable segmentation of the optic disc in digital fundus images
We describe a complete pipeline for the detection and accurate automatic segmentation of the optic disc in digital fundus images. This procedure provides separation of vascular information and accurate inpainting of vessel-removed images, symmetry-based optic disc localization, and fitting of incrementally complex contour models at increasing resolutions using information related to inpainted images and vessel masks. Validation experiments, performed on a large dataset of images of healthy and pathological eyes, annotated by experts and partially graded with a quality label, demonstrate the good performances of the proposed approach. The method is able to detect the optic disc and trace its contours better than the other systems presented in the literature and tested on the same data. The average error in the obtained contour masks is reasonably close to the interoperator errors and suitable for practical applications. The optic disc segmentation pipeline is currently integrated in a complete software suite for the semiautomatic quantification of retinal vessel properties from fundus camera images (VAMPIRE)
Image processing for plastic surgery planning
This thesis presents some image processing tools for plastic surgery planning. In particular,
it presents a novel method that combines local and global context in a probabilistic
relaxation framework to identify cephalometric landmarks used in Maxillofacial plastic
surgery. It also uses a method that utilises global and local symmetry to identify abnormalities
in CT frontal images of the human body. The proposed methodologies are
evaluated with the help of several clinical data supplied by collaborating plastic surgeons
Statistical analysis of coherent structures in transitional pipe flow
Numerical and experimental studies of transitional pipe flow have shown the
prevalence of coherent flow structures that are dominated by downstream
vortices. They attract special attention because they contribute predominantly
to the increase of the Reynolds stresses in turbulent flow. In the present
study we introduce a convenient detector for these coherent states, calculate
the fraction of time the structures appear in the flow, and present a Markov
model for the transition between the structures. The fraction of states that
show vortical structures exceeds 24% for a Reynolds number of about Re=2200,
and it decreases to about 20% for Re=2500. The Markov model for the transition
between these states is in good agreement with the observed fraction of states,
and in reasonable agreement with the prediction for their persistence. It
provides insight into dominant qualitative changes of the flow when increasing
the Reynolds number.Comment: 11 pages, 26 (sub)figure
Shear Alfven wave continuous spectrum within magnetic islands
The radial structure of the continuous spectrum of shear Alfven waves is
calculated in this paper within the separatrix of a magnetic island.
Geometrical effects due to the noncircularity of the flux surface's cross
section are retained to all orders. On the other hand, we keep only curvature
effects responsible for the beta-induced gap in the low-frequency part of the
continuous spectrum. Modes with different helicity from that of the magnetic
island are considered. The main result is that, inside a magnetic island, there
is a continuous spectrum very similar to that of tokamak plasmas, where a
generalized safety factor q can be defined and where a wide frequency gap is
formed, analogous to the ellipticity induced Alfven eigenmode gap in tokamaks.
The presence of this gap is due to the strong eccentricity of the island cross
section. The importance of the existence of such a gap is recognized in
potentially hosting magnetic-island induced Alfven eigenmodes (MiAE). Due to
the frequency dependence of the shear Alfven wave continuum on the
magnetic-island size, the possibility of utilizing MiAE frequency scalings as a
novel magnetic-island diagnostic is also discussed
SOFT: A synthetic synchrotron diagnostic for runaway electrons
Improved understanding of the dynamics of runaway electrons can be obtained
by measurement and interpretation of their synchrotron radiation emission.
Models for synchrotron radiation emitted by relativistic electrons are well
established, but the question of how various geometric effects -- such as
magnetic field inhomogeneity and camera placement -- influence the synchrotron
measurements and their interpretation remains open. In this paper we address
this issue by simulating synchrotron images and spectra using the new synthetic
synchrotron diagnostic tool SOFT (Synchrotron-detecting Orbit Following
Toolkit). We identify the key parameters influencing the synchrotron radiation
spot and present scans in those parameters. Using a runaway electron
distribution function obtained by Fokker-Planck simulations for parameters from
an Alcator C-Mod discharge, we demonstrate that the corresponding synchrotron
image is well-reproduced by SOFT simulations, and we explain how it can be
understood in terms of the parameter scans. Geometric effects are shown to
significantly influence the synchrotron spectrum, and we show that inherent
inconsistencies in a simple emission model (i.e. not modeling detection) can
lead to incorrect interpretation of the images.Comment: 24 pages, 12 figure
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