An object-oriented designed finite-difference time-domain simulator for electromagnetic analysis and design in MRI - application to high field analyses

This paper presents a finite-difference time-domain (FDTD) simulator for electromagnetic analysis and design applications in MRI. It is intended to be a complete FDTD model of an MRI system including all RF and low-frequency field generating units and electrical models of the patient. The pro-ram has been constructed in an object-oriented framework. The design procedure is detailed and the numerical solver has been verified against analytical solutions for simple cases and also applied to various field calculation problems. In particular, the simulator is demonstrated for inverse RF coil design, optimized source profile generation, and parallel imaging in high-frequency situations. The examples show new developments enabled by the simulator and demonstrate that the proposed FDTD framework can be used to analyze large-scale computational electromagnetic problems in modern MRI engineering. (C) 2004 Elsevier Inc. All rights reserved.

Drying process of microcrystalline cellulose studied by attenuated total reflection IR spectroscopy with two-dimensional correlation spectroscopy and principal component analysis

Molecular interactions between microcrystalline cellulose (MCC) and water were investigated by attenuated total reflection infrared (ATR/IR) spectroscopy. Moisture-content-dependent IR spectra during a drying process of wet MCC were measured. In order to distinguish overlapping O–H stretching bands arising from both cellulose and water, principal component analysis (PCA) and, generalized two-dimensional correlation spectroscopy (2DCOS) and second derivative analysis were applied to the obtained spectra. Four typical drying stages were clearly separated by PCA, and spectral variations in each stage were analyzed by 2DCOS. In the drying time range of 0–41 min, a decrease in the broad band around 3390 cm−1 was observed, indicating that bulk water was evaporated. In the drying time range of 49–195 min, decreases in the bands at 3412, 3344 and 3286 cm−1 assigned to the O6H6cdots, three dots, centeredO3′ interchain hydrogen bonds (H-bonds), the O3H3cdots, three dots, centeredO5 intrachain H-bonds and the H-bonds in Iβ phase in MCC, respectively, were observed. The result of the second derivative analysis suggests that water molecules mainly interact with the O6H6cdots, three dots, centeredO3′ interchain H-bonds. Thus, the H-bonding network in MCC is stabilized by H-bonds between OH groups constructing O6H6cdots, three dots, centeredO3′ interchain H-bonds and water, and the removal of the water molecules induces changes in the H-bonding network in MCC.

Biochemical sensor based on a novel all-fibre cavity ring down spectroscopy technique incorporating a tilted fibre Bragg grating

A novel all-fibre cavity ring down spectroscopy technique is proposed where a tilt fibre Bragg grating (TFBG) or long-period grating (LPG) in the cavity provides sensitivity to surrounding medium. Such configuration with an LPG as the representative was theoretically analyzed. Two spectral bands were identified employable for sensing of surrounding refractive index for a weak LPG while only one band existed for a strong LPG. A TFBG, with enhanced sensitivity compared to usual LPGs, was used in a ring down cavity of 1 m constructed with 2 fibre Bragg gratings as the reflectors and the decay time changed from 220 to 450 ns when the TFBG was immersed into water from air.

Numerically optimized implementations, mitigation and simulations of polarisation mode dispersion in real-time transmission systems

The aim of this thesis is to present numerical investigations of the polarisation mode dispersion (PMD) effect. Outstanding issues on the side of the numerical implementations of PMD are resolved and the proposed methods are further optimized for computational efficiency and physical accuracy. Methods for the mitigation of the PMD effect are taken into account and simulations of transmission system with added PMD are presented. The basic outline of the work focusing on PMD can be divided as follows. At first the widely-used coarse-step method for simulating the PMD phenomenon as well as a method derived from the Manakov-PMD equation are implemented and investigated separately through the distribution of a state of polarisation on the Poincaré sphere, and the evolution of the dispersion of a signal. Next these two methods are statistically examined and compared to well-known analytical models of the probability distribution function (PDF) and the autocorrelation function (ACF) of the PMD phenomenon. Important optimisations are achieved, for each of the aforementioned implementations in the computational level. In addition the ACF of the coarse-step method is considered separately, based on the result which indicates that the numerically produced ACF, exaggerates the value of the correlation between different frequencies. Moreover the mitigation of the PMD phenomenon is considered, in the form of numerically implementing Low-PMD spun fibres. Finally, all the above are combined in simulations that demonstrate the impact of the PMD on the quality factor (Q=factor) of different transmission systems. For this a numerical solver based on the coupled nonlinear Schrödinger equation is created which is otherwise tested against the most important transmission impairments in the early chapters of this thesis.

Design and development of a time of flight fast scattering spectrometer : a quantitative surface analysis technique and a new approach towards the experimental investigation of the surface particle interactions

A new surface analysis technique has been developed which has a number of benefits compared to conventional Low Energy Ion Scattering Spectrometry (LEISS). A major potential advantage arising from the absence of charge exchange complications is the possibility of quantification. The instrumentation that has been developed also offers the possibility of unique studies concerning the interaction between low energy ions and atoms and solid surfaces. From these studies it may also be possible, in principle, to generate sensitivity factors to quantify LEISS data. The instrumentation, which is referred to as a Time-of-Flight Fast Atom Scattering Spectrometer has been developed to investigate these conjecture in practice. The development, involved a number of modifications to an existing instrument, and allowed samples to be bombarded with a monoenergetic pulsed beam of either atoms or ions, and provided the capability to analyse the spectra of scattered atoms and ions separately. Further to this a system was designed and constructed to allow incident, exit and azimuthal angles of the particle beam to be varied independently. The key development was that of a pulsed, and mass filtered atom source; which was developed by a cyclic process of design, modelling and experimentation. Although it was possible to demonstrate the unique capabilities of the instrument, problems relating to surface contamination prevented the measurement of the neutralisation probabilities. However, these problems appear to be technical rather than scientific in nature, and could be readily resolved given the appropriate resources. Experimental spectra obtained from a number of samples demonstrate some fundamental differences between the scattered ion and neutral spectra. For practical non-ordered surfaces the ToF spectra are more complex than their LEISS counterparts. This is particularly true for helium scattering where it appears, in the absence of detailed computer simulation, that quantitative analysis is limited to ordered surfaces. Despite this limitation the ToFFASS instrument opens the way for quantitative analysis of the 'true' surface region to a wider range of surface materials.

NMR spectroscopy in the presence of ultrasound and other perturbations

The work described in this thesis has been concerned with exploring the potential uses of ultrasound in Nuclear Magnetic Resonance (NMR) spectroscopy, The NMR spectra of liquids provide detailed structural information that may be deduced from the chemical shifts and spin-spin coupling, that are evident in the narrow resonances, arising from some of the nuclear broadening interactions being reduced to zero. In the solid state, all of the nuclear broadening interactions are present and broad lines in the NMR spectrum are observed. Current techniques employed to reduce the line widths in solids are based on coherent averaging techniques such as MAS NMR1,2 which can remove first order interactions. Recently DOR3 and DAS4 have become available to remove higher order interactions. SINNMR (Sonically Induced Narrowing of the NMR spectra of solids) has been reported by Homer et al5 and developed by Homer and Howard6 to reduce the line widths of solids. The basis of their work is the proposal that a colloidal suspension of solid particles can be made to move like large molecules by using ultrasonic agitation. The advantage of the technique is that the particles move incoherently removing all of the nuclear interactions responsible for broad lines. This thesis describes work on the extension of SINNMR by showing that the line width of 27AI and 11B for the glass Na20/B203/AI203 can be reduced by placing solid particles in a colloidal suspension. Further line width reduction is possible by applying ultrasound, at 2 MHz, of sufficient intensity. It is proposed that a cavitation field is responsible for imparting sufficient rotational motion to the solid particles to partially average the nuclear interactions responsible for broad lines. Rapid stirring of the colloidal suspension generates turbulent flow, however, the motion is insufficient to narrow the line widths for 27AI in the glass. Investigations of sonochemical reactions for in situ rate measurements by NMR have been made. 8y using the Weissler reaction7, it has been shown that ultrasonic cavitation is possible up to 10MHz. Preliminary studies have been carried out into the rate of ultrasonic polymerisation of methylmethacrylate by NMR. Long range order in liquid crystals can imposed when they are aligned in the presence a magnetic field. The degree of alignment can be monitored by NMR using, for example a deuterated solute added to the liquid crystal8. Ultrasonic streaming can then be employed to deflect the directors of the liquid crystal from their equilibrium position, resulting in a change In the NMR spectrum. The angle of deflection has been found for the thermotropic liquid crystal (I35) to be ca, 35° and for the lyotropic (ZLI-1167) to be ca, 20°, Mechanical stirring can used to re- orientate the liquid crystal but was found to give a smaller deflection, In a separate study, that did not use ultrasound, it has been found that the signal to noise ratio of 13C NMR signals can be enhanced by rapidly stirring a Iiquid. Accelerating the diffusion of nuclei out of the coil region enables M0 to be re-established more rapidly than the normal relaxation process. This allows the pulse repetition rate to be reduced without saturating the spin system. The influence of varying the relaxation delay, acquisition time and inter-pulse delay have been studied and parameters optimised. By studying cholesterol the technique was found to be most effective for nuclei with long relaxation times, such as quaternary carbon sites. Key Worde: NMR, Ulf.rasciund, 1,.lqi.fi!:l cryllltalt!h SCll1C1otlemlstryl I!r1hano~d algnflllf

Run-time resolution of uncertainty

Requirements awareness should help optimize requirements satisfaction when factors that were uncertain at design time are resolved at runtime. We use the notion of claims to model assumptions that cannot be verified with confidence at design time. By monitoring claims at runtime, their veracity can be tested. If falsified, the effect of claim negation can be propagated to the system's goal model and an alternative means of goal realization selected automatically, allowing the dynamic adaptation of the system to the prevailing environmental context. © 2011 IEEE.

Matrix-assisted diffusion-ordered spectroscopy:application of surfactant solutions to the resolution of isomer spectra

The component spectra of a mixture of isomers with nearly identical diffusion coefficients cannot normally be distinguished in a standard diffusion-ordered spectroscopy (DOSY) experiment but can often be easily resolved using matrix-assisted DOSY, in which diffusion behaviour is manipulated by the addition of a co-solute such as a surfactant. Relatively little is currently known about the conditions required for such a separation, for example, how the choice between normal and reverse micelles affects separation or how the isomer structures themselves affect the resolution. The aim of this study was to explore the application of sodium dodecyl sulfate (SDS) normal micelles in aqueous solution and sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) aggregates in chloroform, at a range of concentrations, to the diffusion resolution of some simple model sets of isomers such as monomethoxyphenols and short chain alcohols. It is shown that SDS micelles offer better resolution where these isomers differ in the position of a hydroxyl group, whereas AOT aggregates are more effective for isomers differing in the position of a methyl group. For both the normal SDS micelles and the less well-defined AOT aggregates, differences in the resolution of the isomers can in part be rationalised in terms of differing degrees of hydrophobicity, amphiphilicity and steric effects. Copyright © 2012 John Wiley & Sons, Ltd.

Matrix-assisted diffusion-ordered spectroscopy:mixture resolution by NMR using SDSmicelles

Diffusion-ordered spectroscopy (DOSY) is a powerful technique for mixture analysis, but in its basic form it cannot separate the component spectra for species with very similar diffusion coefficients. It has been recently demonstrated that the component spectra of a mixture of isomers with nearly identical diffusion coefficients (the three dihydroxybenzenes) can be resolved using matrix-assisted DOSY (MAD), in which diffusion is perturbed by the addition of a co-solute such as a surfactant [R. Evans, S. Haiber, M. Nilsson, G. A. Morris, Anal. Chem. 2009, 81, 4548-4550]. However, little is known about the conditions required for such a separation, for example, the concentrations and concentration ratios of surfactant and solutes. The aim of this study was to explore the concentration range over whichmatrix-assisted DOSY using the surfactant SDS can achieve diffusion resolution of a simple model set of isomers, the monomethoxyphenols. The results show that the separation is remarkably robust with respect to both the concentrations and the concentration ratios of surfactant and solutes, supporting the idea that MAD may become a valuable tool formixture analysis. © 2010 John Wiley & Sons, Ltd.

Biochemical sensor based on a novel all-fibre cavity ring down spectroscopy technique incorporating a tilted fibre Bragg grating

A novel all-fibre cavity ring down spectroscopy technique is demonstrated where a tilted fibre Bragg grating in the cavity provides sensitivity to surrounding refractive index. A decay time of 450ns was attained when sensing water.