128,261 research outputs found
Does higher sampling rate (multiband + SENSE) improve group statistics - An example from social neuroscience block design at 3T
Multiband (MB) or Simultaneous multi-slice (SMS) acquisition schemes allow the acquisition of MRI signals from more than one spatial coordinate at a time. Commercial availability has brought this technique within the reach of many neuroscientists and psychologists. Most early evaluation of the performance of MB acquisition employed resting state fMRI or the most basic tasks. In this study, we tested whether the advantages of using MB acquisition schemes generalize to group analyses using a cognitive task more representative of typical cognitive neuroscience applications. Twenty-three subjects were scanned on a Philips 3 T scanner using five sequences, up to eight-fold acceleration with MB-factors 1 to 4, SENSE factors up to 2 and corresponding TRs of 2.45s down to 0.63s, while they viewed (i) movie blocks showing complex actions with hand object interactions and (ii) control movie blocks without hand object interaction. Data were processed using a widely used analysis pipeline implemented in SPM12 including the unified segmentation and canonical HRF modelling. Using random effects group-level, voxel-wise analysis we found that all sequences were able to detect the basic action observation network known to be recruited by our task. The highest t-values were found for sequences with MB4 acceleration. For the MB1 sequence, a 50% bigger voxel volume was needed to reach comparable t-statistics. The group-level t-values for resting state networks (RSNs) were also highest for MB4 sequences. Here the MB1 sequence with larger voxel size did not perform comparable to the MB4 sequence. Altogether, we can thus recommend the use of MB4 (and SENSE 1.5 or 2) on a Philips scanner when aiming to perform group-level analyses using cognitive block design fMRI tasks and voxel sizes in the range of cortical thickness (e.g. 2.7 mm isotropic). While results will not be dramatically changed by the use of multiband, our results suggest that MB will bring a moderate but significant benefit
Nonparametric inference of quantile curves for nonstationary time series
The paper considers nonparametric specification tests of quantile curves for
a general class of nonstationary processes. Using Bahadur representation and
Gaussian approximation results for nonstationary time series, simultaneous
confidence bands and integrated squared difference tests are proposed to test
various parametric forms of the quantile curves with asymptotically correct
type I error rates. A wild bootstrap procedure is implemented to alleviate the
problem of slow convergence of the asymptotic results. In particular, our
results can be used to test the trends of extremes of climate variables, an
important problem in understanding climate change. Our methodology is applied
to the analysis of the maximum speed of tropical cyclone winds. It was found
that an inhomogeneous upward trend for cyclone wind speeds is pronounced at
high quantile values. However, there is no trend in the mean lifetime-maximum
wind speed. This example shows the effectiveness of the quantile regression
technique.Comment: Published in at http://dx.doi.org/10.1214/09-AOS769 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Regularized Newton Methods for X-ray Phase Contrast and General Imaging Problems
Like many other advanced imaging methods, x-ray phase contrast imaging and
tomography require mathematical inversion of the observed data to obtain
real-space information. While an accurate forward model describing the
generally nonlinear image formation from a given object to the observations is
often available, explicit inversion formulas are typically not known. Moreover,
the measured data might be insufficient for stable image reconstruction, in
which case it has to be complemented by suitable a priori information. In this
work, regularized Newton methods are presented as a general framework for the
solution of such ill-posed nonlinear imaging problems. For a proof of
principle, the approach is applied to x-ray phase contrast imaging in the
near-field propagation regime. Simultaneous recovery of the phase- and
amplitude from a single near-field diffraction pattern without homogeneity
constraints is demonstrated for the first time. The presented methods further
permit all-at-once phase contrast tomography, i.e. simultaneous phase retrieval
and tomographic inversion. We demonstrate the potential of this approach by
three-dimensional imaging of a colloidal crystal at 95 nm isotropic resolution.Comment: (C)2016 Optical Society of America. One print or electronic copy may
be made for personal use only. Systematic reproduction and distribution,
duplication of any material in this paper for a fee or for commercial
purposes, or modifications of the content of this paper are prohibite
Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe
The Standard Model of particle physics assumes that the so-called fundamental
constants are universal and unchanging. Absorption lines arising in molecular
clouds along quasar sightlines offer a precise test for variations in the
proton-to-electron mass ratio, mu, over cosmological time and distance scales.
The inversion transitions of ammonia are particularly sensitive to mu compared
to molecular rotational transitions. Comparing the available ammonia spectra
observed towards the quasar B0218+357 with new, high-quality rotational
spectra, we present the first detailed measurement of mu with this technique,
limiting relative deviations from the laboratory value to |dmu/mu| <
1.8x10^{-6} (95% confidence level) at approximately half the Universe's current
age - the strongest astrophysical constraint to date. Higher-quality ammonia
observations will reduce both the statistical and systematic uncertainties in
these measurements.Comment: Science, 20th June 2008. 22 pages, 5 figures (12 EPS files), 2
tables, including Supporting Online Material; v2: Corrected reference for
laboratory mu-variation bound
The Spectroscopic Age of 47 Tuc
High signal-to-noise integrated spectra of the metal-rich globular cluster 47
Tuc, spanning the H-gamma(HR) and Fe4668 line indices, have been obtained. The
combination of these indices has been suggested (Jones & Worthey 1995, ApJ,
446, L31) as the best available mechanism for cleanly separating the
age-metallicity degeneracy which hampers the dating of distant, unresolved,
elliptical galaxies. For the first time, we apply this technique to a nearby
spheroidal system, 47 Tuc, for which independent ages, based upon more
established methods, exist. Such an independent test of the technique's
suitability has not been attempted before, but is an essential one before its
application to more distant, unresolved, stellar populations can be considered
valid. Because of its weak series of Balmer lines, relative to model spectra,
our results imply a spectroscopic ``age'' for 47 Tuc well in excess of 20 Gyr,
at odds with the colour-magnitude diagram age of 14+/-1 Gyr. The derived metal
abundance, however, is consistent with the known value. Emission ``fill-in'' of
the H-gamma line as the source of the discrepancy cannot be entirely excluded
by existing data, although the observational constraints are restrictive.Comment: 17 pages, 4 figures, LaTeX, accepted for publication in The
Astronomical Journal, also available at
http://casa.colorado.edu/~bgibson/publications.htm
A new measurement method for separating airborne and structureborne noise radiated by aircraft type panels
The theoretical basis for and experimental validation of a measurement method for separating airborne and structure borne noise radiated by aircraft type panels are presented. An extension of the two microphone, cross spectral, acoustic intensity method combined with existing theory of sound radiation of thin shell structures of various designs, is restricted to the frequency range below the coincidence frequency of the structure. Consequently, the method lends itself to low frequency noise problems such as propeller harmonics. Both an aluminum sheet and two built up aircraft panel designs (two aluminum panels with frames and stringers) with and without added damping were measured. Results indicate that the method is quick, reliable, inexpensive, and can be applied to thin shell structures of various designs
Production of Ultrafine, High-purity Ceramic Powders Using the US Bureau of Mines Developed Turbomill
Turbomilling, an innovative grinding technology developed by the U.S. Bureau of Mines in the early 1960's for delaminating filler-grade kaolinitic clays, has been expanded into the areas of particle size reduction, material mixing, and process reaction kinetics. The turbomill, originally called an attrition grinder, has been used for particle size reduction of many minerals, including natural and synthetic mica, pyrophyllite, talc, and marble. In recent years, an all-polymer version of the turbomill has been used to produce ultrafine, high-purity, advanced ceramic powders such as SiC, Si3N4, TiB2, and ZrO2. In addition to particle size reduction, the turbomill has been used to produce intimate mixtures of high surface area powders and whiskers. Raw materials, TiN, AlN, and Al2O3, used to produce a titanium nitride/aluminum oxynitride (TiN/AlON) composite, were mixed in the turbomill, resulting in strength increases over samples prepared by dry ball milling. Using the turbomill as a leach vessel, it was found that 90.4 pct of the copper was extracted from the chalcopyrite during a 4-hour leach test in ferric sulfate versus conventional processing which involves either roasting of the ore for Cu recovery or leaching of the ore for several days
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