Thesis (M.S.)--University of Wisconsin--Madison, 1963. / Typescript. eContent provider-neutral record in process. Description based on print version record.

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Elasticity of α-helices and coiled coils have often been described by a linear response to local bending with bending stiffness (Kb) and persistence length (Lp) describing their flexibility. However, we observed that the non-bonded forces along the length of these structures are not screened at physiological conditions and introduce a buckling instability. For α-helical systems of same composition, but different lengths, this is identified by a drop in Kb for longer helices and the length where this drop is triggered is referred to as the critical buckling length. When shorter than their critical buckling length they behave linearly, and Kb calculated using normal mode analysis in this regime is about (3.0−3.4)×10-28 Nm2 for α-helices with varying compositions, and about (1.9 − 2.1) × 10−27 Nm2 for coiled coils with leucine zipper periodicity. Beyond the critical buckling length, normal mode solutions turn imaginary, leading to an eventual disappearance of bending modes. Investigations with one dimensional (1-D) linear chains of beads (a simplistic representation of bio-filaments) show that non-bonded forces have a reciprocal relation with the critical buckling length (no buckling instability existed in the absence of non-bonded forces). Critical buckling length is 115.3 ± 2.9 °A for α-helices and 695.1 ± 44.8 Å for coiled coils with leucine zipper periodicity, which is much smaller than their Lp (~ 800 Å for α-helices and ~ 3000 Å for coiled coils).

Magnetic Resonance Spectroscopy is known as a valuable diagnostic tool for physicians as well as a research tool for biochemists. In addition to hydrogen (which is the most abundant atom with nuclear magnetic resonance capability), other species (such as 31P or 13C) are used as well, to obtain certain information such as metabolite concentrations in neural or muscular tissues. However, this requires nuclear magnetic resonance (NMR) transmitter/receivers (coils) capable of operating at multiple frequencies, while maintaining a good performance at each frequency. The objective of this work is to discuss various design approaches used for second-nuclei RF (radio frequency) coils, and to analyze the performance of a particular design, which includes using inductor-capacitor (LC) trap circuits on a 31P coil. The method can be easily applied to other nuclei. The main advantage of this trapping method is the enabling design of second-nuclei coils that are insertable into standard proton coils, maintaining a near-optimum performance for both nuclei. This capability is particularly applicable as MRI field strengths increase and the use of specialized proton coils becomes more prevalent. A thorough performance analysis shows the benefit of this method over other designs, which usually impose a significant signal-to-noise (SNR) sacrifice on one of the nuclei. A methodology based on a modular coil configuration was implemented, which allowed for optimization of LC trap decoupling as well as performance analysis. The 31P coil was used in conjunction with various standard 1H coil configurations (surface/volume/array), using the trap design to overcome the coupling problem (degraded SNR performance) mentioned above. An analytical model was developed and guidelines on trap design were provided to help optimize sensitivity. The performance was analyzed with respect to the untrapped case, using RF bench measurements as well as data obtained from the NMR scanner. Insertability of this coil design was then verified by using it with general-purpose proton coils available. Phantoms were built to mimic the phosphorus content normally found in biologic tissues in order to verify applicability of this coil for in vivo studies. The contribution of this work lies in the quantification of general design parameters to enable “insertable” second-nuclei coils, in terms of the effects on SNR and resonance frequency of a given proton coil.

Phased array surface coils offer high SNR over a large field of view. Phased array volume coils have high SNR at the surface and centre of the volume. Most array coil designs typically employ a combination of geometrical and additional techniques, such as isolating preamplifiers for element-to-element decoupling. The development of array coils for small animal MRI is of increasing interest. However isolation preamplifiers are expensive and not ubiquitous at the field strengths typically employed for small animal work (4.7T, 9.4T, etc). In addition, isolating preamps complicates the designs of coils for transmit SENSE since they do not decouple during transmitting. Therefore, this thesis reexamines a "tried and true" method for decoupling coil elements. In this work five different coils for mouse imaging at 200MHz are presented: a 16 leg trombone design quadrature birdcage coil and four geometrically decoupled volume phased array coils. The first mouse array coil is a two saddle quadrature coil with a circularly polarized field. The second coil is a four channel transmit/receive volume array coil that is decoupled purely geometrically, without the need for other forms of decoupling. The third array coil is a modified 'open' configuration to facilitate the loading of animals. The fourth coil presented is a 'tunable' decoupling coil, where the geometric decoupling between elements is 'tunable', in order to compensate for different loading conditions of the coil. Tunable decoupling between elements was achieved using two mechanisms, a decoupling paddle for isolation of top to bottom elements, with a variable overlap mechanism for decoupling diagonal elements. Bench measurements demonstrate good decoupling (better than -20dB) of the coil elements and 'tunability' of both mechanisms. Phantom images from all coils are presented.

A target field method approach to the design of RF phase gradient fields, intended for TRASE MRI, produced a superposition of axial currents C_m*sin(m*phi) for m=1,2,3..., and a solenoidal current C_0*z (m=0), where C_m are constants. Omission of terms m>2 produced a phase gradient field with a linear phase and uniform magnitude within a target ROI of 2.5 cm diameter. A set of three RF coils (uniform birdcage, gradient mode birdcage, and 4-loop Maxwell) was found to be sufficient to generate both positive and negative x and y phase gradients. In addition, the phase gradient amplitude can be controlled by simply adjusting the power split to the three RF component coils. Bench measurements of an experimentally constructed 1.8 deg/mm transverse phase gradient showed excellent agreement with predicted results. A linear phase and magnitude within ± 4% of the median value was achieved within the ROI. / October 2015

No description available.

Thesis (M.S.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on January 11, 2008) Includes bibliographical references.

Thesis (M.S.)--Virginia Polytechnic Institute, 1952. / Vita. Includes bibliographical references (leaves 83-85). Also available via the Internet.

Thesis (M.S.)--University of Missouri-Columbia, 2008. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 13, 2009) Includes bibliographical references.