20,777 research outputs found
Asymptotic properties of eigenmatrices of a large sample covariance matrix
Let where is a matrix
with i.i.d. complex standardized entries having finite fourth moments. Let
in which
and where
is the Mar\v{c}enko--Pastur law with parameter ; which
converges to a positive constant as , and and are unit vectors in ,
having indices and , ranging in a compact subset
of a finite-dimensional Euclidean space. In this paper, we prove that the
sequence converges weakly to a
-dimensional Gaussian process. This result provides further evidence in
support of the conjecture that the distribution of the eigenmatrix of is
asymptotically close to that of a Haar-distributed unitary matrix.Comment: Published in at http://dx.doi.org/10.1214/10-AAP748 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Learning associations between clinical information and motion-based descriptors using a large scale MR-derived cardiac motion atlas
The availability of large scale databases containing imaging and non-imaging
data, such as the UK Biobank, represents an opportunity to improve our
understanding of healthy and diseased bodily function. Cardiac motion atlases
provide a space of reference in which the motion fields of a cohort of subjects
can be directly compared. In this work, a cardiac motion atlas is built from
cine MR data from the UK Biobank (~ 6000 subjects). Two automated quality
control strategies are proposed to reject subjects with insufficient image
quality. Based on the atlas, three dimensionality reduction algorithms are
evaluated to learn data-driven cardiac motion descriptors, and statistical
methods used to study the association between these descriptors and non-imaging
data. Results show a positive correlation between the atlas motion descriptors
and body fat percentage, basal metabolic rate, hypertension, smoking status and
alcohol intake frequency. The proposed method outperforms the ability to
identify changes in cardiac function due to these known cardiovascular risk
factors compared to ejection fraction, the most commonly used descriptor of
cardiac function. In conclusion, this work represents a framework for further
investigation of the factors influencing cardiac health.Comment: 2018 International Workshop on Statistical Atlases and Computational
Modeling of the Hear
Nonuniqueness in spin-density-functional theory on lattices
In electronic many-particle systems, the mapping between densities and spin
magnetizations, {n(r), m(r)}, and potentials and magnetic fields, {v(r), B(r)},
is known to be nonunique, which has fundamental and practical implications for
spin-density-functional theory (SDFT). This paper studies the nonuniqueness
(NU) in SDFT on arbitrary lattices. Two new, non-trivial cases are discovered,
here called local saturation and global noncollinear NU, and their properties
are discussed and illustrated. In the continuum limit, only some well-known
special cases of NU survive.Comment: 4 pages, 1 figur
A Population of Radio-loud Narrow Line Seyfert 1 Galaxies with Blazar-like Properties?
(abridged) We present a comprehensive study of a sample of 23 genuine
radio-loud NLS1 galaxies which have the radio-loudness parameters greater than
100. The radio sources of the sample are ubiquitously compact. A significant
fraction of these objects show interesting radio to X-ray properties that are
unusual to most of the previously known radio-loud NLS1 AGN, but are
reminiscent of blazars. These include flat radio spectra, large amplitude flux
and spectral variability, compact VLBI cores, very high brightness temperatures
derived from variability, enhanced optical emission in excess of the normal
ionising continuum, flat X-ray spectra, and blazar-like SEDs. We interpret them
as evidence for the postulated blazar nature of these very radio-loud NLS1 AGN,
which might possess at least moderately relativistic jets. Intrinsically, some
of the objects have relatively low radio power and would have been classified
as radio-intermediate AGN. The black hole masses are estimated to be within
10^{6-8}Msun, and the inferred Eddington ratios are around unity. The results
imply that radio-loud AGN may be powered by black holes with moderate masses
(10^{6-7}Msun) accreting at high rates. We find that a significant fraction of
the objects, despite having strong emission lines, resemble high-energy peaked
BL Lacs (HBL) in their SED. Given the peculiarities of blazar-like NLS1
galaxies, questions arise as to whether they are plain downsizing extensions of
normal radio-loud AGN, or whether they form a previously unrecognised
population.Comment: Comments: 29 pages, 16 figures, 4 tables, accepted for publication in
Ap
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Deep learning for cardiac image segmentation: A review
Deep learning has become the most widely used approach for cardiac image segmentation in recent years. In this paper, we provide a review of over 100 cardiac image segmentation papers using deep learning, which covers common imaging modalities including magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound (US) and major anatomical structures of interest (ventricles, atria and vessels). In addition, a summary of publicly available cardiac image datasets and code repositories are included to provide a base for encouraging reproducible research. Finally, we discuss the challenges and limitations with current deep learning-based approaches (scarcity of labels, model generalizability across different domains, interpretability) and suggest potential directions for future research
XMM-Newton View of PKS 2155-304: Characterizing the X-ray Variability Properties with EPIC-PN
Starting from XMM-Newton EPIC-PN data, we present the X-ray variability
characteristics of PKS 2155-304 using a simple analysis of the excess variance,
\xs, and of the fractional rms variability amplitude, fvar. The scatter in \xs\
and \fvar, calculated using 500 s long segments of the light curves, is smaller
than the scatter expected for red noise variability. This alone does not imply
that the underlying process responsible for the variability of the source is
stationary, since the real changes of the individual variance estimates are
possibly smaller than the large scatters expected for a red noise process. In
fact the averaged \xs and \fvar, reducing the fluctuations of the individual
variances, chang e with time, indicating non-stationary variability. Moreover,
both the averaged \sqxs (absolute rms variability amplitude) and \fvar show
linear correlation with source flux but in an opposite sense: \sqxs correlates
with flux, but \fvar anti-correlates with flux. These correlations suggest that
the variability process of the source is strongly non-stationary as random
scatters of variances should not yield any correlation. \fvar spectra were
constructed to compare variability amplitudes in different energy bands. We
found that the fractional rms variability amplitude of the source, when
significant variability is observed, increases logarithmically with the photon
energy, indicating significant spectral variability. The point-to-point
variability amplitude may also track this trend, suggesting that the slopes of
the power spectral density of the source are energy-independent. Using the
normalized excess variance the black hole mass of \pks was estimated to be
about . This is compared and contrasted with the
estimates derived from measurements of the host galaxies.Comment: Accepted for publication in The Astrophysical Journa
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Large-scale Quality Control of Cardiac Imaging in Population Studies: Application to UK Biobank
In large population studies such as the UK Biobank (UKBB), quality control of the acquired images by visual assessment is unfeasible. In this paper, we apply a recently developed fully-automated quality control pipeline for cardiac MR (CMR) images to the first 19,265 short-axis (SA) cine stacks from the UKBB. We present the results for the three estimated quality metrics (heart coverage, inter-slice motion and image contrast in the cardiac region) as well as their potential associations with factors including acquisition details and subject-related phenotypes. Up to 14.2% of the analysed SA stacks had sub-optimal coverage (i.e. missing basal and/or apical slices), however most of them were limited to the first year of acquisition. Up to 16% of the stacks were affected by noticeable inter-slice motion (i.e. average inter-slice misalignment greater than 3.4 mm). Inter-slice motion was positively correlated with weight and body surface area. Only 2.1% of the stacks had an average end-diastolic cardiac image contrast below 30% of the dynamic range. These findings will be highly valuable for both the scientists involved in UKBB CMR acquisition and for the ones who use the dataset for research purposes
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