Design of Radio-Frequency Arrays for Ultra-High Field MRI

Magnetic Resonance Imaging (MRI) is an indispensable, non-invasive diagnostic tool for the assessment of disease and function. As an investigational device, MRI has found routine use in both basic science research and medicine for both human and non-human subjects. Due to the potential increase in spatial resolution, signal-to-noise ratio (SNR), and the ability to exploit novel tissue contrasts, the main magnetic field strength of human MRI scanners has steadily increased since inception. Beginning in the early 1980’s, 0.15 T human MRI scanners have steadily risen in main magnetic field strength with ultra-high field (UHF) 8 T MRI systems deemed to be insignificant risk by the FDA (as of 2016). However, at UHF the electromagnetic fields describing the collective behaviour of spin dynamics in human tissue assume ‘wave-like’ behaviour due to an increase in the processional frequency of nuclei at UHF. At these frequencies, the electromagnetic interactions transition from purely near-field interactions to a mixture of near- and far-field mechanisms. Due to this, the transmission field at UHF can produce areas of localized power deposition – leading to tissue heating – as well as tissue-independent contrast in the reconstructed images. Correcting for these difficulties is typically achieved via multi-channel radio-frequency (RF) arrays. This technology allows multiple transmitting elements to synthesize a more uniform field that can selectively minimize areas of local power deposition and remove transmission field weighting from the final reconstructed image. This thesis provides several advancements in the design and construction of these arrays. First, in Chapter 2 a general framework for modeling the electromagnetic interactions occurring inside an RF array is adopted from multiply-coupled waveguide filters and applied to a subset of decoupling problems encountered when constructing RF arrays. It is demonstrated that using classic filter synthesis, RF arrays of arbitrary size and geometry can be decoupled via coupling matrix synthesis. Secondly, in Chapters 3 and 4 this framework is extended for designing distributed filters for simple decoupling of RF arrays and removing the iterative tuning portion of utilizing decoupling circuits when constructing RF arrays. Lastly, in Chapter 5 the coupling matrix synthesis framework is applied to the construction of a conformal transmit/receive RF array that is shape optimized to minimize power deposition in the human head during any routine MRI examination

Synthesis of multi-layer frequency selective surfaces of quasi-optical systems

This thesis investigate design techniques for multilayer Frequency Selective Surfaces (FSS) and its applications in quasi-optical (QO) systems. Design challenges that involve higher order filter and practical implementation of multilayer FSS at higher frequencies are reviewed. Multilayer FSS structures are commonly realized by cascading two or more FSS panel to achieve higher order responses, which usually rely on dielectric substrates to support the FSS arrays. It is noted that existing design approaches involved elaborate manufacturing processes as well as the requirement of custom dielectric thickness for the implementation of multilayer FSS. These design issues poses practical problems in the realization of multilayer FSS of higher order and its demonstration at higher frequencies. Furthermore, realization of higher order multilayer FSS with custom dielectric thicknesses are not feasible with low cost Printed Circuit Board (PCB) technology. As a result of this investigation, a novel design and synthesis technique is developed to address the aforementioned design issues. Equivalent circuit modelling and full wave electromagnetic simulation are employed for this purpose. The developed design technique enable practical realization of QO filter to have all transmission lines of predefined fix length. As a result, the proposed technique is able to resolve the limited availability of custom dielectric thicknesses, thus enable demonstration of multilayer FSS of higher order at higher frequencies. Particularly, the proposed design methodology allow rectification by design to adapt to any small variations in the dielectric thicknesses. Subsequently, based on this technique, a novel QO reflector design is developed to demonstrate proof of concept for time delay multiplexing that are employed in a radar system. The implementation of time delay between two polarization multiplexed beams initially requires true time delay structures that are difficult to integrate due to their electrically large structure. In order to address this problem, the designed QO reflector is able to perform same functionalities, i.e. a significant group delay difference for the two orthogonal linear polarization. Specifically, the designed QO reflector has the capability to de-multiplex an incoming wave into two linear polarized waves, whereby one of the reflected wave is time delayed while the other wave is unaffected. A synthesis method for QO reflector design with time delay multiplexing has been presented. Based on the design procedures reported in this thesis, prototypes for both QO filter and QO reflector of fourth order has been developed to operate at 15 GHz with 5% and 3.5% bandwidth respectively. The performances of the developed prototypes are verified with free-space measurement setup. The measured insertion loss of the QO filter is observed to be in the range of 0.5 dB – 2.83 dB, while the measured return loss of the QO reflector is the range of 1.5 dB – 2.3 dB. In order to demonstrate the effect of the group delay from the QO reflector, frequency domain analysis is performed by post-processing the measured data to obtain the required time domain signals. Overall the experimental measurement results corroborate well with both full-wave and circuit simulation

The Fabrication and Characterization of a Superconducting Birdcage Resonator

In this thesis we presents the fabrication and characterization of a superconducting Birdcage style radiofrequency (RF) coil, a commonly used coil design in nuclear magnetic resonance (NMR) spectrometers. The Birdcage was fabricated from a series of superconducting Niobium thin film resonators, deposited onto sapphire wafer by sputtering deposition. The resonators were assembled into the 3 dimensional birdcage coil, and housed in a custom machined RF package. The fabrication, assembly tools, and procedures are presented. A numerical model, based on lumped-element eigenanalysis, was used to describe the resonance structure of the coil. We show that misalignment in the assembly can lead to an increase in the resonant frequency of the transverse mode when compared to the analytical description. A set of low temperature measurements were performed to characterize the quality factor, magnetic field, and power dependence. A modest improvement in the $Q$, over non-superconducting Birdcage coil, is shown. Simulations, in HFSS, of the field and current distribution, as well as changes in the quality factor from experimental measurements of the coil are presented. The coil was perturbed with an aluminum ball bearing and aluminum sheet in order to demonstrate the existence circularly polarized magnetic field within the Birdcage coil

RF Coil Design, Imaging Methods and Measurement of Ventilation with 19F C3F8 MRI

This thesis attempts to address the challenge of low signal in fluorinated gas ventilation imaging and optimize imaging methods considering the particular MR parameters of C3F8 by the following approaches: (i) Exploration of coil designs capable of imaging both proton (1H – 63.8 MHz at 1.5T) and fluorine (19F – 60.1 MHz at 1.5T) nuclei involved: 1. The novel use of microelectromechanical systems to switch a single transceive vest coil between the two nuclei was compared to hard-wired or PIN diode switching. 2. The design of an 8 element transceive array with an additional 6 receive only coils for 19F imaging. MEMs was utilized for broadband transmit-receive switching. 3. The amalgamation of a ladder resonator coil with a 6-element transceive array to reduce SAR and improve transmit homogeneity when compared to standard vest coil designs. (ii) Development of imaging methods involved: 1. The optimization and comparison of steady-state free precession and spoiled gradient 19F imaging with C3F8 at 1.5T and 3T. Simulation of the optimal SNR was verified through comprehensive phantom and in-vivo imaging experiments. 2. The investigation of compressed sensing via incoherent sparse k-space sampling to maximize the resolution in 19F ventilation imaging under the constraint of low SNR. Retrospective simulation with hyperpolarized gas images were corroborated by prospective 19F imaging of a 3D printed lung phantom and in-vivo measurements of the lungs. (iii) In-vivo ventilation metrics obtained by 19F ventilation imaging were explored by: 1. The in-vivo mapping of T1 at 1.5T and 3T and mapping of FV and T2* at 3 T. The apparent diffusion coefficient (1.5T) and the evaluation of ventilated volume (1.5T and 3T) was also compared to imaging performed with 129Xe (1.5T). 2. The optimization of imaging for the evaluation of percent ventilated volume with 19F at 3T with a commercial birdcage coil

Controlling phonons and photons at the wavelength-scale: silicon photonics meets silicon phononics

Radio-frequency communication systems have long used bulk- and surface-acoustic-wave devices supporting ultrasonic mechanical waves to manipulate and sense signals. These devices have greatly improved our ability to process microwaves by interfacing them to orders-of-magnitude slower and lower loss mechanical fields. In parallel, long-distance communications have been dominated by low-loss infrared optical photons. As electrical signal processing and transmission approaches physical limits imposed by energy dissipation, optical links are now being actively considered for mobile and cloud technologies. Thus there is a strong driver for wavelength-scale mechanical wave or "phononic" circuitry fabricated by scalable semiconductor processes. With the advent of these circuits, new micro- and nanostructures that combine electrical, optical and mechanical elements have emerged. In these devices, such as optomechanical waveguides and resonators, optical photons and gigahertz phonons are ideally matched to one another as both have wavelengths on the order of micrometers. The development of phononic circuits has thus emerged as a vibrant field of research pursued for optical signal processing and sensing applications as well as emerging quantum technologies. In this review, we discuss the key physics and figures of merit underpinning this field. We also summarize the state of the art in nanoscale electro- and optomechanical systems with a focus on scalable platforms such as silicon. Finally, we give perspectives on what these new systems may bring and what challenges they face in the coming years. In particular, we believe hybrid electro- and optomechanical devices incorporating highly coherent and compact mechanical elements on a chip have significant untapped potential for electro-optic modulation, quantum microwave-to-optical photon conversion, sensing and microwave signal processing.Comment: 26 pages, 5 figure

FEM based analysis of highpass birdcage resonators for B1 field mapping

3D full wave finite element method (FEM) based electromagnetic (EM) analysis is a technique to map EM fields generated by electrical devices. To better understand and apply this technique to magnetic resonance imaging (MRI) radio frequency (RF) birdcage resonators, a vast number of 3D full wave EM simulations are required for validation and optimization of the B1 field generated by them since they have to be tuned to a particular Larmor frequency. In the past RF birdcage resonators were constructed without doing any 3D full wave EM analysis and more emphasis was laid on tuning and matching the electrical circuits used to make these resonators. However modeling birdcage resonators in a 3D computer aided engineering (CAE) simulation environment is important to observe the resonance behavior and the B1 field distribution inside the birdcage resonator volume before its construction thus saving valuable resources. In this work we have attempted to map B1 field distribution inside the full and half birdcage resonators tuned to Larmor frequency for proton nuclei at 3 Tesla with the help of FEM. FEM essentially converts the problem of solving Maxwell’s partial differential equations into solving a large system of linear equations. In this work we make use of the ANSYS high frequency structure simulator (HFSS) which is an FEM based frequency domain solver. The results of the full birdcage resonator are further compared with experimental outcomes. The phantoms used for experiments and simulation are both symmetric and non-symmetric ones. It can be concluded that HFSS or similar FEM based EM simulator may be used to predict the B1 field inside loaded RF resonators to obtain information of the B1 field behavior. It is observed that B1 field distribution inside the birdcage resonator varies with different types of phantoms used to mimic small animals for MRI. B1 field maps and resonance results from simulation and experiment are presented. Finally this thesis concludes with areas of improvement and a road map for future work

Superconducting Nonlinear Kinetic Inductance Devices

We describe a novel class of devices based on the nonlinearity of the kinetic inductance of a superconducting thin film. By placing a current-dependent inductance in a microwave resonator, small currents can be measured through their effect on the resonator’s frequency. By using a high-resistivity material for the film and nanowires as kinetic inductors, we can achieve a large coefficient of nonlinearity to improve device sensitivity. We demonstrate a current sensitivity of 8 pA/&#8730;Hz, making this device useful for transition-edge sensor (TES) readout and other cutting-edge applications. An advantage of these devices is their natural ability to be multiplexed in the frequency domain, enabling large detector arrays for TES-based instruments. A traveling-wave version of the device, consisting of a thin-film microwave transmission line, is also sensitive to small currents as they change the phase length of the line due to their effect on its inductance. We demonstrate a current sensitivity of 5 pA/&#8730;Hz for this version of the device, making it also suitable for TES readout as well as other current-detection applications. It has the advantage of multi-gigahertz bandwidth and greater dynamic range, offering a different approach to the resonator version of the device. Finally, we also demonstrate a transmission-line resonator version of the device that combines some of the advantages of the nanowire resonator and the traveling-wave device. This version of the device has high dynamic range but can also be easily multiplexed in the frequency domain. A lumped-element resonator similar to the first device can be placed in a loop configuration to make it sensitive to magnetic fields. We demonstrate an example of such a device whose sensitivity could ultimately reach levels similar to those of state-of-the-art DC SQUIDs, making it potentially useful for many magnetometry applications given its ease of multiplexing. Finally, a similar microwave resonator is shown to exhibit parametric gain of up to 29 dB in the presence of a strong pump tone. The noise performance of this parametric amplifier approaches the quantum limit, making it useful for applications in quantum information and metrology.</p