47,202 research outputs found
Increasing the Analytical Accessibility of Multishell and Diffusion Spectrum Imaging Data Using Generalized Q-Sampling Conversion
Many diffusion MRI researchers, including the Human Connectome Project (HCP),
acquire data using multishell (e.g., WU-Minn consortium) and diffusion spectrum
imaging (DSI) schemes (e.g., USC-Harvard consortium). However, these data sets
are not readily accessible to high angular resolution diffusion imaging (HARDI)
analysis methods that are popular in connectomics analysis. Here we introduce a
scheme conversion approach that transforms multishell and DSI data into their
corresponding HARDI representations, thereby empowering HARDI-based analytical
methods to make use of data acquired using non-HARDI approaches. This method
was evaluated on both phantom and in-vivo human data sets by acquiring
multishell, DSI, and HARDI data simultaneously, and comparing the converted
HARDI, from non-HARDI methods, with the original HARDI data. Analysis on the
phantom shows that the converted HARDI from DSI and multishell data strongly
predicts the original HARDI (correlation coefficient > 0.9). Our in-vivo study
shows that the converted HARDI can be reconstructed by constrained spherical
deconvolution, and the fiber orientation distributions are consistent with
those from the original HARDI. We further illustrate that our scheme conversion
method can be applied to HCP data, and the converted HARDI do not appear to
sacrifice angular resolution. Thus this novel approach can benefit all
HARDI-based analysis approaches, allowing greater analytical accessibility to
non-HARDI data, including data from the HCP
Constraining Dark Energy by Combining Cluster Counts and Shear-Shear Correlations in a Weak Lensing Survey
We study the potential of a large future weak-lensing survey to constrain
dark energy properties by using both the number counts of detected galaxy
clusters (sensitive primarily to density fluctuations on small scales) and
tomographic shear-shear correlations (restricted to large scales). We use the
Fisher matrix formalism, assume a flat universe and parameterize the equation
of state of dark energy by w(a)=w_0+w_a(1-a), to forecast the expected
statistical errors from either observable, and from their combination. We show
that the covariance between these two observables is small, and argue that
therefore they can be regarded as independent constraints. We find that when
the number counts and the shear-shear correlations (on angular scales l < 1000)
are combined, an LSST (Large Synoptic Survey Telescope)-like survey can yield
statistical errors on (Omega_DE, w_0, w_a) as tight as (0.003, 0.03, 0.1).
These values are a factor of 2-25 better than using either observable alone.
The results are also about a factor of two better than those from combining
number counts of galaxy clusters and their power spectrum.Comment: 17 pages, 5 figures, 10 tables, submitted to PR
ALTKAL: An optimum linear filter for GEOS-3 altimeter data
ALTKAL is a computer program designed to smooth sea surface height data obtained from the GEOS 3 altimeter, and to produce minimum variance estimates of sea surface height and sea surface slopes, along with their standard derivations. The program operates by processing the data through a Kalman filter in both the forward and backward directions, and optimally combining the results. The sea surface height signal is considered to have a geoid signal, modeled by a third order Gauss-Markov process, corrupted by additive white noise. The governing parameters for the signal and noise processes are the signal correlation length and the signal-to-noise ratio. Mathematical derivations of the filtering and smoothing algorithms are presented. The smoother characteristics are illustrated by giving the frequency response, the data weighting sequence and the transfer function of a realistic steady-state smoother example. Based on nominal estimates for geoidal undulation amplitude and correlation length, standard deviations for the estimated sea surface height and slope are 12 cm and 3 arc seconds, respectively
Topological magnetoplasmon
Classical wave fields are real-valued, ensuring the wave states at opposite
frequencies and momenta to be inherently identical. Such a particle-hole
symmetry can open up new possibilities for topological phenomena in classical
systems. Here we show that the historically studied two-dimensional (2D)
magnetoplasmon, which bears gapped bulk states and gapless one-way edge states
near zero frequency, is topologically analogous to the 2D topological p+\Ii p
superconductor with chiral Majorana edge states and zero modes. We further
predict a new type of one-way edge magnetoplasmon at the interface of opposite
magnetic domains, and demonstrate the existence of zero-frequency modes bounded
at the peripheries of a hollow disk. These findings can be readily verified in
experiment, and can greatly enrich the topological phases in bosonic and
classical systems.Comment: 12 pages, 6 figures, 1 supporting materia
Bifurcation Boundary Conditions for Switching DC-DC Converters Under Constant On-Time Control
Sampled-data analysis and harmonic balance analysis are applied to analyze
switching DC-DC converters under constant on-time control. Design-oriented
boundary conditions for the period-doubling bifurcation and the saddle-node
bifurcation are derived. The required ramp slope to avoid the bifurcations and
the assigned pole locations associated with the ramp are also derived. The
derived boundary conditions are more general and accurate than those recently
obtained. Those recently obtained boundary conditions become special cases
under the general modeling approach presented in this paper. Different analyses
give different perspectives on the system dynamics and complement each other.
Under the sampled-data analysis, the boundary conditions are expressed in terms
of signal slopes and the ramp slope. Under the harmonic balance analysis, the
boundary conditions are expressed in terms of signal harmonics. The derived
boundary conditions are useful for a designer to design a converter to avoid
the occurrence of the period-doubling bifurcation and the saddle-node
bifurcation.Comment: Submitted to International Journal of Circuit Theory and Applications
on August 10, 2011; Manuscript ID: CTA-11-016
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