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Fast, variable system delay correction for spiral MRI

Description

Magnetic Resonance Imaging using spiral trajectories has many advantages in speed, efficiency in data-acquistion and robustness to motion and flow related artifacts. The increase in sampling speed, however, requires high performance of the gradient system. Hardware inaccuracies from system delays

Magnetic Resonance Imaging using spiral trajectories has many advantages in speed, efficiency in data-acquistion and robustness to motion and flow related artifacts. The increase in sampling speed, however, requires high performance of the gradient system. Hardware inaccuracies from system delays and eddy currents can cause spatial and temporal distortions in the encoding gradient waveforms. This causes sampling discrepancies between the actual and the ideal k-space trajectory. Reconstruction assuming an ideal trajectory can result in shading and blurring artifacts in spiral images. Current methods to estimate such hardware errors require many modifications to the pulse sequence, phantom measurements or specialized hardware. This work presents a new method to estimate time-varying system delays for spiral-based trajectories. It requires a minor modification of a conventional stack-of-spirals sequence and analyzes data collected on three orthogonal cylinders. The method is fast, robust to off-resonance effects, requires no phantom measurements or specialized hardware and estimate variable system delays for the three gradient channels over the data-sampling period. The initial results are presented for acquired phantom and in-vivo data, which show a substantial reduction in the artifacts and improvement in the image quality.

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2013

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Advancing access to biodiversity data using the SALIX method and digital field guides

Description

The Arizona State University Herbarium began in 1896 when Professor Fredrick Irish collected the first recorded Arizona specimen for what was then called the Tempe Normal School - a Parkinsonia microphylla. Since then, the collection has grown to approximately 400,000

The Arizona State University Herbarium began in 1896 when Professor Fredrick Irish collected the first recorded Arizona specimen for what was then called the Tempe Normal School - a Parkinsonia microphylla. Since then, the collection has grown to approximately 400,000 specimens of vascular plants and lichens. The most recent project includes the digitization - both the imaging and databasing - of approximately 55,000 vascular plant specimens from Latin America. To accomplish this efficiently, possibilities in non-traditional methods, including both new and existing technologies, were explored. SALIX (semi-automatic label information extraction) was developed as the central tool to handle automatic parsing, along with BarcodeRenamer (BCR) to automate image file renaming by barcode. These two developments, combined with existing technologies, make up the SALIX Method. The SALIX Method provides a way to digitize herbarium specimens more efficiently than the traditional approach of entering data solely through keystroking. Using digital imaging, optical character recognition, and automatic parsing, I found that the SALIX Method processes data at an average rate that is 30% faster than typing. Data entry speed is dependent on user proficiency, label quality, and to a lesser degree, label length. This method is used to capture full specimen records, including close-up images where applicable. Access to biodiversity data is limited by the time and resources required to digitize, but I have found that it is possible to do so at a rate that is faster than typing. Finally, I experiment with the use of digital field guides in advancing access to biodiversity data, to stimulate public engagement in natural history collections.

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2012

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Stabilization and imaging of cohesionless soil specimens

Description

This dissertation describes development of a procedure for obtaining high quality, optical grade sand coupons from frozen sand specimens of Ottawa 20/30 sand for image processing and analysis to quantify soil structure along with a methodology for quantifying the microstructure

This dissertation describes development of a procedure for obtaining high quality, optical grade sand coupons from frozen sand specimens of Ottawa 20/30 sand for image processing and analysis to quantify soil structure along with a methodology for quantifying the microstructure from the images. A technique for thawing and stabilizing frozen core samples was developed using optical grade Buehler® Epo-Tek® epoxy resin, a modified triaxial cell, a vacuum/reservoir chamber, a desiccator, and a moisture gauge. The uniform epoxy resin impregnation required proper drying of the soil specimen, application of appropriate confining pressure and vacuum levels, and epoxy mixing, de-airing and curing. The resulting stabilized sand specimen was sectioned into 10 mm thick coupons that were planed, ground, and polished with progressively finer diamond abrasive grit levels using the modified Allied HTP Inc. polishing method so that the soil structure could be accurately quantified using images obtained with the use of an optical microscopy technique. Illumination via Bright Field Microscopy was used to capture the images for subsequent image processing and sand microstructure analysis. The quality of resulting images and the validity of the subsequent image morphology analysis hinged largely on employment of a polishing and grinding technique that resulted in a flat, scratch free, reflective coupon surface characterized by minimal microstructure relief and good contrast between the sand particles and the surrounding epoxy resin. Subsequent image processing involved conversion of the color images first to gray scale images and then to binary images with the use of contrast and image adjustments, removal of noise and image artifacts, image filtering, and image segmentation. Mathematical morphology algorithms were used on the resulting binary images to further enhance image quality. The binary images were then used to calculate soil structure parameters that included particle roundness and sphericity, particle orientation variability represented by rose diagrams, statistics on the local void ratio variability as a function of the sample size, and the local void ratio distribution histograms using Oda's method and Voronoi tessellation method, including the skewness, kurtosis, and entropy of a gamma cumulative probability distribution fit to the local void ratio distribution.

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Date Created
2011

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Optical and crystal structure characterizations of nanowires for infrared applications

Description

Semiconductor nanowires (NWs) are one dimensional materials and have size quantization effect when the diameter is sufficiently small. They can serve as optical wave guides along the length direction and contain optically active gain at the same time. Due to

Semiconductor nanowires (NWs) are one dimensional materials and have size quantization effect when the diameter is sufficiently small. They can serve as optical wave guides along the length direction and contain optically active gain at the same time. Due to these unique properties, NWs are now very promising and extensively studied for nanoscale optoelectronic applications. A systematic and comprehensive optical and microstructural study of several important infrared semiconductor NWs is presented in this thesis, which includes InAs, PbS, InGaAs, erbium chloride silicate and erbium silicate. Micro-photoluminescence (PL) and transmission electron microscope (TEM) were utilized in conjunction to characterize the optical and microstructure of these wires. The focus of this thesis is on optical study of semiconductor NWs in the mid-infrared wavelengths. First, differently structured InAs NWs grown using various methods were characterized and compared. Three main PL peaks which are below, near and above InAs bandgap, respectively, were observed. The octadecylthiol self-assembled monolayer was employed to passivate the surface of InAs NWs to eliminate or reduce the effects of the surface states. The band-edge emission from wurtzite-structured NWs was completely recovered after passivatoin. The passivated NWs showed very good stability in air and under heat. In the second part, mid-infrared optical study was conducted on PbS wires of subwavelength diameter and lasing was demonstrated under optical pumping. The PbS wires were grown on Si substrate using chemical vapor deposition and have a rock-salt cubic structure. Single-mode lasing at the wavelength of ~3000-4000 nm was obtained from single as-grown PbS wire up to the temperature of 115 K. PL characterization was also utilized to demonstrate the highest crystallinity of the vertical arrays of InP and InGaAs/InP composition-graded heterostructure NWs made by a top-down fabrication method. TEM-related measurements were performed to study the crystal structures and elemental compositions of the Er-compound core-shell NWs. The core-shell NWs consist of an orthorhombic-structured erbium chloride silicate shell and a cubic-structured silicon core. These NWs provide unique Si-compatible materials with emission at 1530 nm for optical communications and solid state lasers.

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Date Created
2011

Study of a night sky radiator cooling system utilizing direct fluid radiation emission and varying cover materials

Description

As the demand for power increases in populated areas, so will the demand for water. Current power plant technology relies heavily on the Rankine cycle in coal, nuclear and solar thermal power systems which ultimately use condensers to cool the

As the demand for power increases in populated areas, so will the demand for water. Current power plant technology relies heavily on the Rankine cycle in coal, nuclear and solar thermal power systems which ultimately use condensers to cool the steam in the system. In dry climates, the amount of water to cool off the condenser can be extremely large. Current wet cooling technologies such as cooling towers lose water from evaporation. One alternative to prevent this would be to implement a radiative cooling system. More specifically, a system that utilizes the volumetric radiation emission from water to the night sky could be implemented. This thesis analyzes the validity of a radiative cooling system that uses direct radiant emission to cool water. A brief study on potential infrared transparent cover materials such as polyethylene (PE) and polyvinyl carbonate (PVC) was performed. Also, two different experiments to determine the cooling power from radiation were developed and run. The results showed a minimum cooling power of 33.7 W/m2 for a vacuum insulated glass system and 37.57 W/m2 for a tray system with a maximum of 98.61 Wm-2 at a point when conduction and convection heat fluxes were considered to be zero. The results also showed that PE proved to be the best cover material. The minimum numerical results compared well with other studies performed in the field using similar techniques and materials. The results show that a radiative cooling system for a power plant could be feasible given that the cover material selection is narrowed down, an ample amount of land is available and an economic analysis is performed proving it to be cost competitive with conventional systems.

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Date Created
2011

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Strain-balanced InAs-InAsSb type-II superlattices on GaSb substrates for infrared photodetector applications

Description

Infrared photodetectors, used in applications for sensing and imaging, such as military target recognition, chemical/gas detection, and night vision enhancement, are predominantly comprised of an expensive II-VI material, HgCdTe. III-V type-II superlattices (SLs) have been studied as viable alternatives for

Infrared photodetectors, used in applications for sensing and imaging, such as military target recognition, chemical/gas detection, and night vision enhancement, are predominantly comprised of an expensive II-VI material, HgCdTe. III-V type-II superlattices (SLs) have been studied as viable alternatives for HgCdTe due to the SL advantages over HgCdTe: greater control of the alloy composition, resulting in more uniform materials and cutoff wavelengths across the wafer; stronger bonds and structural stability; less expensive substrates, i.e., GaSb; mature III-V growth and processing technologies; lower band-to-band tunneling due to larger electron effective masses; and reduced Auger recombination enabling operation at higher temperatures and longer wavelengths. However, the dark current of InAs/Ga1-xInxSb SL detectors is higher than that of HgCdTe detectors and limited by Shockley-Read-Hall (SRH) recombination rather than Auger recombination. This dissertation work focuses on InAs/InAs1-xSbx SLs, another promising alternative for infrared laser and detector applications due to possible lower SRH recombination and the absence of gallium, which simplifies the SL interfaces and growth processes. InAs/InAs1-xSbx SLs strain-balanced to GaSb substrates were designed for the mid- and long-wavelength infrared (MWIR and LWIR) spectral ranges and were grown using MOCVD and MBE by various groups. Detailed characterization using high-resolution x-ray diffraction, atomic force microscopy, photoluminescence (PL), and photoconductance revealed the excellent structural and optical properties of the MBE materials. Two key material parameters were studied in detail: the valence band offset (VBO) and minority carrier lifetime. The VBO between InAs and InAs1-xSbx strained on GaSb with x = 0.28 - 0.41 was best described by Qv = ÄEv/ÄEg = 1.75 ± 0.03. Time-resolved PL experiments on a LWIR SL revealed a lifetime of 412 ns at 77 K, one order of magnitude greater than that of InAs/Ga1-xInxSb LWIR SLs due to less SRH recombination. MWIR SLs also had 100's of ns lifetimes that were dominated by radiative recombination due to shorter periods and larger wave function overlaps. These results allow InAs/InAs1-xSbx SLs to be designed for LWIR photodetectors with minority carrier lifetimes approaching those of HgCdTe, lower dark currents, and higher operating temperatures.

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Date Created
2012

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Characterization of HgCdTe and HgCdSe materials for third generation infrared detectors

Description

HgCdTe is the dominant material currently in use for infrared (IR) focal-plane-array (FPA) technology. In this dissertation, transmission electron microscopy (TEM) was used for the characterization of epitaxial HgCdTe epilayers and HgCdTe-based devices. The microstructure of CdTe surface passivation layers

HgCdTe is the dominant material currently in use for infrared (IR) focal-plane-array (FPA) technology. In this dissertation, transmission electron microscopy (TEM) was used for the characterization of epitaxial HgCdTe epilayers and HgCdTe-based devices. The microstructure of CdTe surface passivation layers deposited either by hot-wall epitaxy (HWE) or molecular beam epitaxy (MBE) on HgCdTe heterostructures was evaluated. The as-deposited CdTe passivation layers were polycrystalline and columnar. The CdTe grains were larger and more irregular when deposited by HWE, whereas those deposited by MBE were generally well-textured with mostly vertical grain boundaries. Observations and measurements using several TEM techniques showed that the CdTe/HgCdTe interface became considerably more abrupt after annealing, and the crystallinity of the CdTe layer was also improved. The microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures grown by MBE was investigated. Many inclined {111}-type stacking faults were present throughout the thin ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Lattice parameter measurement and elemental profiles indicated that some local intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd, Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively. Initial observations of CdTe(211)B/GaAs(211) heterostructures indicated much reduced defect densities near the vicinity of the substrate and within the CdTe epilayers. HgCdTe epilayers grown on CdTe(211)B/GaAs(211) composite substrate were generally of high quality, despite the presence of precipitates at the HgCdTe/CdTe interface. The microstructure of HgCdSe thin films grown by MBE on ZnTe/Si(112) and GaSb(112) substrates were investigated. The quality of the HgCdSe growth was dependent on the growth temperature and materials flux, independent of the substrate. The materials grown at 100°C were generally of high quality, while those grown at 140°C had {111}-type stacking defects and high dislocation densities. For epitaxial growth of HgCdSe on GaSb substrates, better preparation of the GaSb buffer layer will be essential in order to ensure that high-quality HgCdSe can be grown.

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Date Created
2011

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Valuation Tools of Conglomerates and Their Application in the Case of Fosun International

Description

There has been much debate in the world of academia over the valuation of conglomerates. This thesis proposes the use of the EVA theory in explaining fluctuations in conglomerates’ valuation, and we believe that ROIC, WACC, and ROIC-WACC are three

There has been much debate in the world of academia over the valuation of conglomerates. This thesis proposes the use of the EVA theory in explaining fluctuations in conglomerates’ valuation, and we believe that ROIC, WACC, and ROIC-WACC are three indicators that to a certain extent explain these valuation fluctuations. Through analysis of a sample containing 23 conglomerates, this thesis finds that ROIC, WACC, and ROIC-WACC exhibit positive correlation with valuation fluctuations. In the case study on Fosun, this thesis finds that ROIC-WACC is highly correlated with Fosun’s valuation fluctuations and next with ROIC. Thus this thesis conjectures that for investment companies for which investment capital is derived largely from insurance float, such as Fosun, ROIC-WACC is a better valuation tool.

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Date Created
2015

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Assessing martian bedrock mineralogy through "windows" in the dust using near- and thermal infrared remote sensing

Description

Much of Mars' surface is mantled by bright dust, which masks the spectral features used to interpret the mineralogy of the underlying bedrock. Despite the wealth of near-infrared (NIR) and thermal infrared data returned from orbiting spacecraft in recent decades,

Much of Mars' surface is mantled by bright dust, which masks the spectral features used to interpret the mineralogy of the underlying bedrock. Despite the wealth of near-infrared (NIR) and thermal infrared data returned from orbiting spacecraft in recent decades, the detailed bedrock composition of approximately half of the martian surface remains relatively unknown due to dust cover. To address this issue, and to help gain a better understanding of the bedrock mineralogy in dusty regions, data from the Thermal Emission Spectrometer (TES) Dust Cover Index (DCI) and Mars Reconnaissance Orbiter (MRO) Mars Color Imager (MARCI) were used to identify 63 small localized areas within the classical bright dusty regions of Arabia Terra, Elysium Planitia, and Tharsis as potential "windows" through the dust; that is, areas where the dust cover is thin enough to permit infrared remote sensing of the underlying bedrock. The bedrock mineralogy of each candidate "window" was inferred using processed spectra from the Mars Express (MEx) Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité (OMEGA) NIR spectrometer and, where possible, TES. 12 areas of interest returned spectra that are consistent with mineral species expected to be present at the regional scale, such as high- and low-calcium pyroxene, olivine, and iron-bearing glass. Distribution maps were created using previously defined index parameters for each species present within an area. High-quality TES spectra, if present within an area of interest, were deconvolved to estimate modal mineralogy and support NIR results. OMEGA data from Arabia Terra and Elysium Planitia are largely similar and indicate the presence of high-calcium pyroxene with significant contributions of glass and olivine, while TES data suggest an intermediate between the established southern highlands and Syrtis Major compositions. Limited data from Tharsis indicate low-calcium pyroxene mixed with lesser amounts of high-calcium pyroxene and perhaps glass. TES data from southern Tharsis correlate well with the previously inferred compositions of the Aonium and Mare Sirenum highlands immediately to the south.

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Agent

Created

Date Created
2014

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A Low Cost, High Dynamic Range, Versatile Digital Readout Integrated Circuit Unit Cell Prototype for Infrared Imaging Applications

Description

Readout Integrated Circuits(ROICs) are important components of infrared(IR) imag

ing systems. Performance of ROICs affect the quality of images obtained from IR

imaging systems. Contemporary infrared imaging applications demand ROICs that

can support large dynamic range, high frame rate, high output data rate,

Readout Integrated Circuits(ROICs) are important components of infrared(IR) imag

ing systems. Performance of ROICs affect the quality of images obtained from IR

imaging systems. Contemporary infrared imaging applications demand ROICs that

can support large dynamic range, high frame rate, high output data rate, at low

cost, size and power. Some of these applications are military surveillance, remote

sensing in space and earth science missions and medical diagnosis. This work focuses

on developing a ROIC unit cell prototype for National Aeronautics and Space Ad

ministration(NASA), Jet Propulsion Laboratory’s(JPL’s) space applications. These

space applications also demand high sensitivity, longer integration times(large well

capacity), wide operating temperature range, wide input current range and immunity

to radiation events such as Single Event Latchup(SEL).

This work proposes a digital ROIC(DROIC) unit cell prototype of 30ux30u size,

to be used mainly with NASA JPL’s High Operating Temperature Barrier Infrared

Detectors(HOT BIRDs). Current state of the art DROICs achieve a dynamic range

of 16 bits using advanced 65-90nm CMOS processes which adds a lot of cost overhead.

The DROIC pixel proposed in this work uses a low cost 180nm CMOS process and

supports a dynamic range of 20 bits operating at a low frame rate of 100 frames per

second(fps), and a dynamic range of 12 bits operating at a high frame rate of 5kfps.

The total electron well capacity of this DROIC pixel is 1.27 billion electrons, enabling

integration times as long as 10ms, to achieve better dynamic range. The DROIC unit

cell uses an in-pixel 12-bit coarse ADC and an external 8-bit DAC based fine ADC.

The proposed DROIC uses layout techniques that make it immune to radiation up to

300krad(Si) of total ionizing dose(TID) and single event latch-up(SEL). It also has a

wide input current range from 10pA to 1uA and supports detectors operating from

Short-wave infrared (SWIR) to longwave infrared (LWIR) regions.

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Agent

Created

Date Created
2019