928 resultados para Post-and-core technique
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Mode of access: Internet.
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Shaw & Shoemaker
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Mode of access: Internet.
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Thesis (Ph.D.)--University of Washington, 2016-04
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Due to complex field/tissue interactions, high-field magnetic resonance (MR) images suffer significant image distortions that result in compromised diagnostic quality. A new method that attempts to remove these distortions is proposed in this paper and is based on the use of transceiver-phased arrays. The proposed system uses, in the examples presented herein, a shielded four-element transceive-phased array head coil and involves performing two separate scans of the same slice with each scan using different excitations during transmission. By optimizing the amplitudes and phases for each scan, antipodal signal profiles can be obtained, and by combining both the images together, the image distortion can be reduced several fold. A combined hybrid method of moments (MoM)/finite element method (FEM) and finite-difference time-domain (FDTD) technique is proposed and used to elucidate the concept of the new method and to accurately evaluate the electromagnetic field (EMF) in a human head model. In addition, the proposed method is used in conjunction with the generalized auto-calibrating partially parallel acquisitions (GRAPPA) reconstruction technique to enable rapid imaging of the two scans. Simulation results reported herein for 11-T (470-MHz) brain imaging applications show that the new method with GRAPPA reconstruction theoretically results in improved image quality and that the proposed combined hybrid MoM/FEM and FDTD technique is. suitable for high-field magnetic resonance imaging (MRI) numerical analysis.
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A simple technique for direct real-time assessment of a fiber laser cavity-mode condition during operation is demonstrated. Mode stabilization and optimization with this cavity-mode monitoring and conditioning feedback scheme shows significant improvements to the output performance.
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Single crystal platelets of AlN were successfully grown on 6H-SiC(0001) by a novel technique designed to suppress SiC decomposition, promote two-dimensional growth, and eliminate cracking in the AlN. X-ray diffractometry and synchrotron white beam X-ray topography demonstrate that the final AlN single crystal is of high structural quality.
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A simple technique for direct real-time assessment of a fiber laser cavity-mode condition during operation is demonstrated. Mode stabilization and optimization with this cavity-mode monitoring and conditioning feedback scheme shows significant improvements to the output performance.
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Planktic foraminifera have been used as recorders of the neodymium (Nd) isotopic composition of seawater, although there is still controversy over the precise provenance of the Nd signal. We present an extensive, multispecific plankton tow Nd/Ca data set from several geographic locations (SE Atlantic, NE Atlantic, Norwegian Sea, and western Mediterranean), together with core top samples from the Mediterranean region. The range of Nd/Ca ratios in plankton-towed foraminifera, cleaned only of organic material, from all regions (0.01-0.7 µmol/mol), is similar to previously published analyses of sedimentary foraminifera cleaned using both oxidative and reductive steps, with distribution coefficients (Kd) ranging between 4 and 302. For the Mediterranean, where core top and plankton tow data are both available, the range for plankton tows (0.05-0.7 µmol/mol) is essentially identical to that for the core tops (0.1-0.5 µmol/mol). Readsorption of Nd during cleaning is ruled out by the fact that the plankton tow samples underwent only an oxidative cleaning process. We find a relationship between manganese (Mn) and Nd in plankton tow samples that is mirrored by a similar correlation in core top samples. This relationship suggests that Fe-Mn coatings are of negligible importance to the Nd budgets of foraminifera as the Nd/Mn ratio it implies is over an order of magnitude greater than that seen in other Fe-Mn oxide phases. Rather, since both plankton tows and core tops present a similar behavior, the Nd/Mn relationship must originate in the upper water column. The data are consistent with the acquisition of Nd and Mn from the water column by binding to organic material and the fact that intratest organic material is shielded from both aggressive cleaning and diagenetic processes. Collectively, the results help to explain two abiding puzzles about Nd in sedimentary planktic foraminifera: their high REE contents and the fact that they record a surface water Nd isotopic signal, regardless of the cleaning procedure used.
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In the reconstruction of sea surface temperature (SST) from sedimentary archives, secondary sources, lateral transport and selective preservation are considered to be mainly negligible in terms of influencing the primary signal. This is also true for the archaeal glycerol dialkyl glycerol tetraethers (GDGTs) that form the basis for the TEX86 SST proxy. Our samples represent four years variability on a transect off Cape Blanc (NW Africa). We studied the subsurface production, vertical and lateral transport of intact polar lipids and core GDGTs in the water column at high vertical resolution on the basis of suspended particulate matter (SPM) samples from the photic zone, the subsurface oxygen minimum zone (OMZ), nepheloid layers (NL) and the water column between these. Furthermore we compared the water column SPM GDGT composition with that in underlying surface sediments. This is the first study that reports TEX86 values from the precursor intact polar lipids (IPLs) associated with specific head groups (IPL -specific TEX86). We show a clear deviation from the sea surface GDGT composition in the OMZ between 300 and 600 m. Since neither lateral transport nor selective degradation provides a satisfactory explanation for the observed TEX-derived temperature profiles with a bias towards higher temperatures for both core- and IPL -specific TEX86 values, we suggest that subsurface in situ production of archaea with a distinct relationship between lipid biosynthesis and temperature is the responsible mechanism. However, in the NW-African upwelling system the GDGT contribution of the OMZ to the surface sediments does not seem to affect the sedimentary TEX86 as it shows no bias and still reflects the signal of the surface waters between 0 and 60 m.