Electrical Properties Mapping and Coil Characterization at High Magnetic Fields

Electrical properties (EP), namely conductivity and permittivity, can provide endogenous (having an internal origin) contrast for tissue characterization. EP of the biological tissue are strongly related to essential determinants of the physiological state of tissue such as ionic concentration and mobility, water content and cell structure. As a result, non-invasive measurement of EP has been increasingly used in neuroimaging and other areas. Magnetic resonance electrical property tomography (EPT) is a recently introduced technique that can provide maps of EP from conventional MRI data by measuring the distortions induced on the radio frequency (RF) field (B1+). Although its feasibility has been shown at clinical field strengths (1-7T), the application of EPT to routine medical protocols is limited, partly due to reconstruction inaccuracies and variations. In this dissertation, the first application of EPT at 21.1 T (900 MHz), the highest magnetic field available for MRI, is presented with a focus on pre-clinical research. The ultra-high pre-clinical field provides improved signal-to-noise ratios and higher interaction between sample EP and the applied RF field that can enhance EPT accuracy and precision. Helmholtz-based EPT was implemented in its full-form, which demands the complex B1+ field, and a simplified form requiring either just the B1+ field phase for conductivity or the B1+ field magnitude for permittivity. Experiments were conducted at 21.1 T using birdcage and saddle coils operated in linear or quadrature transceive mode, respectively. Feasibility and accuracy of EPT approaches at this field were evaluated using a phantom, ex and in vivo Sprague-Dawley rats under the conditions of naïve and ischemic stroke via transient middle cerebral artery occlusion. Different conductivity reconstruction approaches applied to the phantom displayed average errors of 23-86% to target values. Permittivity reconstructions showed higher agreement and an average 5-8% error to the target depending on the reconstruction approach. The full-form technique generated from the linear birdcage provided the best accuracy for the EP of the phantom. Phase-based conductivity and magnitude-based permittivity mapping provided reasonable estimates but also demonstrated the limitations of Helmholtz-based EPT at 21.1 T. Conductivity and permittivity of ex and in vivo rodent brains also were measured. With the aim to demonstrate the applicability of EPT for ischemic stroke studies, EP of the in vivo rat brain with and without ischemia were measured. The findings demonstrate significantly elevated conductivity and permittivity in the ischemic stroke lesion compared to the contralateral non-pathological side correlated with the increased sodium content and the influx of water intracellularly following ischemia. Permittivity reconstruction was improved significantly over lower fields, suggesting a novel metric for in vivo brain studies. The last section of this dissertation aims to address the implementation of high temperature superconducting (HTS) coils for nuclear magnetic resonance (NMR) spectroscopy with a particular focus on the transmit coil’s characteristics for 13C NMR. NMR is widely used to study the molecular structure and dynamics of molecules in solution, and 13C NMR is critical for structural elucidation in organic chemistry. However, the low sensitivity of NMR has meant that relatively large amounts of the sample or alternate techniques are needed to improve sensitivity. Replacing the normal-metal pickup coils with thin-film HTS resonators has been shown to increase the sensitivity of NMR and reduce the amount of sample required. It also would be convenient and beneficial to use HTS resonators to excite as well as to detect the NMR signal. However, producing a sufficiently strong and rapidly switched excitation field is more challenging with thin-film HTS resonators than with the normal metal coils that they would replace. While double-sided HTS resonators can significantly increase the achievable RF field, the high Q factor of the HTS resonator limits the pulse bandwidth and the minimum dead time following a pulse before reception can begin. This study explored several important aspects of the use of HTS resonators as NMR excitation coils. The presented analysis showed non-linearity in the coil’s response and current compression when high power levels are applied to the coil. Additionally, time domain representation of the excitation pulses generated by the coil showed long ring-up and ring-down times as well as distorted pulse shapes at different power levels. The Fourier transformation of these pulses displayed the limited bandwidth of the coil, which can be problematic for exciting and receiving the whole 13C spectrum. To mitigate the elongated pulse shapes, a shorted stub was added to the transmission path. The result demonstrated improved pulse shapes and reduced phase transients. A similar technique is expected to be applied to the HTS NMR probe to increase the applicability of HTS resonators for transmission.