Development of novel radiofrequency coils for hyperpolarized magnetic resonance imaging
Keywords: Spectroscopy
X-nuclei
Radiofrequency coil
Signal to noise
Electromagnetic simulation
Flexible PCB
Radiofrequency scattering
Multinuclear MRI

Magnetic Resonance Imaging (MRI) is the gold-standard diagnostic tool for cardiovascular conditions. However, this imaging technique is currently insensitive with a limited signal-to-noise ratio (SNR), leading to long acquisition times, poor spatial resolution and negatively impacts on the diagnostic quality of scans. These issues can be alleviated through improvements in hardware. The SNR is strongly determined by the performance of Radio-Frequency (RF) coils, devices that transmit and receive the electromagnetic (EM) fields which interact with the patient to generate the MR signal.
X-nuclear MRI, investigating non-proton ‘X-’ nuclei, provides unique metabolic information in vivo. Compared to conventional proton MRI scans, X-nuclei scans are performed at a different range of RF frequencies, therefore requiring special RF coils. However, due to limited wideband power amplification at these frequencies and relatively low natural abundance of these X-nuclei in the body, this metabolic imaging heavily relies on efficient RF coils with strong local EM fields to achieve sufficient SNR. Conventionally, X-nuclei RF coils utilise a ‘surface coil’ design, comprising of one to several individual conducting loops positioned on the surface of the body. Using EM simulations, we show that for a given region of interest (ROI), surface coil designs inherently trade-off field homogeneity against field strength. In this work, methods were developed to compare and optimise coil design.
The main achievement of this project is the development of a novel class of RF coils, utilizing a flexible multi-layer design that offers significantly improved transmit and receive sensitivity over that achieved by conventional surface elements. EM simulations of parameterized four- and six-layer multi-layer designs identified promising 3D conductor geometries that generated both a stronger and more uniform field profile than conventional circular conducting elements. Preclinical and clinical multi-layer coils were constructed for carbon (13C) MRI at 7 T and 3 T respectively and compared to a range of conventional loop coils; improvements in transmit and receive field were consistent with simulation results. Subsequently, a two-fold increase in SNR was observed using a human multi-layer coil for cardiac phosphorus (31P) spectroscopy on five human participants.
Commonly, cardiac RF receive coils comprise of several single elements partially overlapped in an array. These array elements are geometrically decoupled to allow good field coverage with the high sensitivity of small individual elements. This project proceeds to investigate the optimisation and decoupling ability of multi-layer elements to form multi-layer receive arrays, whilst preserving the increased sensitivity of the multi-layer design. This design offers a direct replacement to conventional loop elements in existing coil configurations, allowing increased SNR to be obtained at minimal additional cost beyond design and manufacturing complexity.