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RF Pulse Design for Parallel Excitation in Magnetic Resonance ImagingLiu, Yinan 2012 May 1900 (has links)
Parallel excitation is an emerging technique to improve or accelerate multi-dimensional spatially selective excitations in magnetic resonance imaging (MRI) using multi-channel transmit arrays. The technique has potential in many applications, such as accelerating imaging speed, mitigating field inhomogeneity in high-field MRI, and alleviating the susceptibility artifact in functional MRI (fMRI). In these applications, controlling radiofrequency (RF) power deposition (quantified by Specific Absorption Rate, or SAR) under safe limit is a critical issue, particularly in high-field MRI. This \dissertation will start with a review of multidimensional spatially selective excitation in MRI and current parallel excitation techniques. Then it will present two new RF pulse design methods to achieve reduced local/global SAR for parallel excitation while preserving the time duration and excitation pattern quality. Simulations incorporating human-model based tissue density and dielectric property were performed. Results have show that the proposed methods can achieve significant SAR reductions without enlonging the pulse duration at high-fields.
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Improved fluorescence-enhanced optical imaging and tomography by enhanced excitation light rejectionHwang, Kil Dong 15 May 2009 (has links)
Fluorescence enhanced optical imaging and tomography studies involve the
detection of weak fluorescent signals emanating from nano- to picomolar concentrations
of exogenous or endogenously produced fluorophore concurrent with the rejection of an
overwhelmingly large component of backscattered excitation light. The elimination of
the back-reflected excitation light of the collected signal remains a major and often
unrecognized challenge for further reducing the noise floor and increasing sensitivity of
small animal fluorescence imaging.
In this dissertation, we adapted collimating and gradient index (GRIN) lenses in
an existing frequency-domain system to improve excitation light rejection and enhance
planar and tomographic imaging. To achieve this goal, we developed planar and
tomographic imaging systems based upon ray tracing calculations for improved rejection
of excitation light. The “out-of-band (S (λx))” to “in-band (S (λm) - S (λx))” signal ratio
assessing excitation leakage was acquired with and without collimating optics. The
addition of collimating optics resulted in a 51 to 75% reduction in the transmission ratio of (S (λx))/ (S (λm) - S (λx)) for the phantom studies and an increase of target to
background ratio (TBR) from 11% to 31% in animal studies. Additionally, we presented
results demonstrating the improvement of model match between experiments and
forward simulation models by adaptation of GRIN lens optics to a breast phantom study.
In particular, 128 GRIN lenses on the fiber bundle face were employed to align the
collected excitation and emission light normal to the filter surface in an existing
frequency-domain system. As a result of GRIN lens collimation, we reduced the
transmission ratio between 10 and 86 % and improved the model match for tomographic
reconstruction of one (1 cm3) and two (0.1 cm3) targets in a 1087 cm3 of breast phantom.
Ultimately, this work improves the sensitivity of NIR fluorescence imaging by
enhancing the rejection of excitation light and shows that the current sensitivity
challenges for translating fluorescence-enhanced optical imaging into the clinic can be
overcome.
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Parametric Excitation of a DWSCLakhotia, Chandan 2010 May 1900 (has links)
Parametric excitation of the DWSC (Deep Water Stable Craneship) is studied in this thesis. It occurs for a system without any external forcing, when one of the coefficients in the equation of motion (EOM) modeling the system varies with time. Parametric instability might be triggered for certain values of the parameters describing the time-varying coefficient. The DWSC, basically a stepped classic spar with a catamaran as its deck, because of certain unique features, may be susceptible to parametric excitation. This thesis examines the phenomenon of parametric excitation with respect to roll motion in head seas, using time-domain simulation and stability analysis. It examines the DWSC's susceptibility to parametric instability using the same methods of analysis and the effect of damping (especially viscous drag) on parametric excitation and instability. The thesis uses Mathieu's equation as the basis for stability analysis and time-simulates the coupled heave-sway-roll EOM.
Time-domain simulation is done for two reasons: firstly for determining the variation in roll stiffness because of a regular wave (the variation in roll stiffness is an input to the stability analysis) and secondly for simulating the coupled heave-sway-roll EOM. Both time-domain analysis and stability analysis are done for sea states of interest and for examining interesting phenomena like roll resonance (due to body-wave interaction) and parametric instability.
Results highlight: 1) a "cancellation frequency" in the heave wave exciting force; 2) the effect of viscous drag on coupled heave-sway-roll motions; 3) time-simulations validating the stability analysis; 4) the trend of stability with increasing sea states, wave periods and amplitudes; 5) characteristics of parametric instability; 6) the methodology used to predict or detect parametric instability and 7) the effect of viscous drag on parametric instability.
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Adaptive excitation control in power systemsChiu, Pei-Chen 16 August 2006 (has links)
This thesis presents an adaptive excitation controller of power systems. The control
law is derived by using model reference adaptive control (MRAC) or adaptive pole
placement control (APPC) and an equilibrium tracking mechanism is implemented to
obtain equilibrium. By our approaches, system damping improvement is achieved to
increase loadability as well as strengthen stability properties.
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Mean lives and relative intensities in the Balmer series of hydrogenCurnan, William Allen, 1945- January 1970 (has links)
No description available.
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Excitation by ion impact on surfacesMetz, Werner Adam 12 1900 (has links)
No description available.
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Small angle scattering of metastable hydrogen formed by collisional excitation of neutral hydrogenSauers, Isidor 08 1900 (has links)
No description available.
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Signature splitting and inversion in the 186-194 Au Nuclei predicted by the total routhian surface (TRS) and cranked shell model (CSM) calculations.Shirinda, Obed. January 2007 (has links)
<p>The nearly oblate deformed Au nuclei show rotational bands built on multi quasiparticle excitations [Bou89, Bou92, Gue03, Gue01, Ven92]. Several of these bands are built on rotationally aligned high-j proton and neutron excitations. In many cases bands consisting of two or three signature partner E2 sequences are observed. For some fo these bands signature inversion is found and this feature gives a great challenge to the theoretical models. In this study the researcher performed TRS and CSM calculations for all high-j rotational bands in the p186-194s Au nuclei aiming to predict the signature splitting and inversion phemomena, alignments, gains in alignments, gains in alignment and band crossing frequencies observed.</p>
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Study of octupole collectivity in '1'4'8NdWhite, Christopher Andrew January 1990 (has links)
No description available.
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Roton-roton scattering in liquid helium HE IIForbes, Andrew Christopher January 1991 (has links)
No description available.
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