815 research outputs found
A Mean-Field Theory for Coarsening Faceted Surfaces
A mean-field theory is developed for the scale-invariant length distributions
observed during the coarsening of one-dimensional faceted surfaces. This theory
closely follows the Lifshitz-Slyozov-Wagner theory of Ostwald ripening in
two-phase systems [1-3], but the mechanism of coarsening in faceted surfaces
requires the addition of convolution terms recalling the work of Smoluchowski
[4] and Schumann [5] on coalescence. The model is solved by the exponential
distribution, but agreement with experiment is limited by the assumption that
neighboring facet lengths are uncorrelated. However, the method concisely
describes the essential processes operating in the scaling state, illuminates a
clear path for future refinement, and offers a framework for the investigation
of faceted surfaces evolving under arbitrary dynamics.
[1] I. Lifshitz, V. Slezov, Soviet Physics JETP 38 (1959) 331-339.
[2] I. Lifshitz, V. Slyozov, J. Phys. Chem. Solids 19 (1961) 35-50.
[3] C. Wagner, Elektrochemie 65 (1961) 581-591.
[4] M. von Smoluchowski, Physikalische Zeitschrift 17 (1916) 557-571.
[5] T. Schumann, J. Roy. Met. Soc. 66 (1940) 195-207
Polarized Radio Sources: A Study of Luminosity, Redshift and Infrared Colors
The Dominion Radio Astrophysical Observatory Deep Field polarization study
has been matched with the Spitzer Wide-Area Infrared Extragalactic survey of
the European Large Area Infrared Space Observatory Survey North 1 field. We
have used VLA observations with a total intensity rms of 87 microJy beam^-1 to
match SWIRE counterparts to the radio sources. Infrared color analysis of our
radio sample shows that the majority of polarized sources are elliptical
galaxies with an embedded active galactic nucleus. Using available redshift
catalogs, we found 429 radio sources of which 69 are polarized with redshifts
in the range of 0.04 < z <3.2. We find no correlation between redshift and
percentage polarization for our sample. However, for polarized radio sources,
we find a weak correlation between increasing percentage polarization and
decreasing luminosity.Comment: 35 pages, 12 figures, accepted for publication in The Astrophysical
Journa
Spectator Rules: Shaping Making & Meaning in Contemporary Art
While considering works of art, how do we characterize roles played by spectatorship, whether in the making of (including by artists and viewers), or even in the making meaning of (including by scholars and viewers), such work? Speakers engage the question from a variety of positions: considering how curatorial practice not only steers but also gets steered by viewers; addressing the dynamism of spectator experience with installations that demand performative engagement; analyzing examples of work specifically invested in triggering an active spectatorship; and characterizing the roles of artists and audiences as inherently generative in the production of meaning
Fully automated 3D segmentation of dopamine transporter SPECT images using an estimation-based approach
Quantitative measures of uptake in caudate, putamen, and globus pallidus in
dopamine transporter (DaT) brain SPECT have potential as biomarkers for the
severity of Parkinson disease. Reliable quantification of uptake requires
accurate segmentation of these regions. However, segmentation is challenging in
DaT SPECT due to partial-volume effects, system noise, physiological
variability, and the small size of these regions. To address these challenges,
we propose an estimation-based approach to segmentation. This approach
estimates the posterior mean of the fractional volume occupied by caudate,
putamen, and globus pallidus within each voxel of a 3D SPECT image. The
estimate is obtained by minimizing a cost function based on the binary
cross-entropy loss between the true and estimated fractional volumes over a
population of SPECT images, where the distribution of the true fractional
volumes is obtained from magnetic resonance images from clinical populations.
The proposed method accounts for both the sources of partial-volume effects in
SPECT, namely the limited system resolution and tissue-fraction effects. The
method was implemented using an encoder-decoder network and evaluated using
realistic clinically guided SPECT simulation studies, where the ground-truth
fractional volumes were known. The method significantly outperformed all other
considered segmentation methods and yielded accurate segmentation with dice
similarity coefficients of ~ 0.80 for all regions. The method was relatively
insensitive to changes in voxel size. Further, the method was relatively robust
up to +/- 10 degrees of patient head tilt along transaxial, sagittal, and
coronal planes. Overall, the results demonstrate the efficacy of the proposed
method to yield accurate fully automated segmentation of caudate, putamen, and
globus pallidus in 3D DaT-SPECT images
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