Radiofrequency solutions in clinical high field magnetic resonance

Abstract

Magnetic resonance imaging (MRI) and spectroscopy (MRS) benefit from the sensitivity gain at high field (≥7T). However, high field brings also certain challenges associated with growing frequency and spectral dispersion. Frequency growth results in degraded performance of large volume radiofrequency (RF) probe design, successfully used at lower field strengths. As a consequence, a body RF coil is absent in current commercial human MR scanners with the main magnetic field strength above 3 Tesla. The first part of this thesis was devoted to exploring several fundamental concepts for alternative, waveguide based high frequency RF probe, i.e. waveguide MRI. To facilitate the increased spectral dispersion at high field, broad band RF strategies were designed for accurate and efficient MRS. This was applied in the second part of the thesis aiming for efficient detection and quantification of weak metabolite signals in human brain at 7T by means of spectral editing MRS. The aspects of large volume RF probe for high fields such as transmit field (B1+) efficiency, RF shimming and safety were addressed in the thesis. A coaxial waveguide of proper length increases delivered power to distant locations in the human body, which results in the escalated B1+ amplitude there. The coaxial waveguide also prevents premature power losses in human tissue and, thus, increases RF safety of waveguide MRI. High permittivity dielectric lining of the scanner bore is another way to boost B1+ efficiency of waveguide MRI. The dielectric lining increases both radial and longitudinal power flows towards the patient. RF shimming based on waveguide modes is also feasible. A multimode, coaxial waveguide was created to facilitate RF shimming in human head or body torso. A method to evaluate the number of waveguide modes in B1+ pattern was proposed. We have shown that RF shimming with the multimode, coaxial waveguide is efficient in three principle directions and demands low number (eight) of transmit channels compared to dedicated surface transmit arrays. In addition, the multimode, coaxial waveguide can be used not only for proton but also for fluorine MRI without any modifications. To improve sensitivity but preserve the same receive path for both nuclei, shared receive coils tuned to the middle frequency are feasible at 7T to detect both proton and fluorine signals with only negligible SNR loss. The design of dedicated, waveguide based RF probe was also created for 9.4T head MRI. The RF probe was based on the coaxial waveguide cavity, which effectively couples RF field to the head through the slot antennas and open end. A spectral editing MRS technique was developed using adiabatic frequency swept RF pulses to obtain high bandwidth at relatively low B1+ fields, while using narrow band pulses for improved selectivity in editing. The technique was successfully tested for detection of GABA signal in human brain within clinically acceptable scan times and voxel volumes. To quantify this signal, a method was introduced to measure T2 relaxation time within spectral editing MRS sequence and was successfully applied to measure GABA T2 time in number of healthy volunteers.