| dc.description.abstract | Multinuclear imaging is a topic of research with great potential within Magnetic Resonance Imaging (MRI). Since the first Nuclear Magnetic Resonance experiments, it was known that many different nuclei could be analyzed with this technique, for example, hydrogen (1H), carbon (13C), and sodium (23Na). However, clinical use of MRI today has focused on the hydrogen nucleus because of its superior magnetic properties. The availability of hydrogen MRI systems at high (3T) and ultra-high (7T) magnetic fields has increased because of a desire for images with higher resolution. Increased field strengths increase the Signal to Noise Ratio (SNR) of the scans, which can be exploited to increase the spatial resolution of the final images. Higher fields also increase the SNR of the nuclei with less favorable magnetic properties than hydrogen. An intriguing nucleus with suitable MRI properties in the context of human physiology is 23Na, despite its intrinsically low SNR caused by its lower gyromagnetic ratio, low concentrations in vivo, and shorter relaxation time. 23Na plays a significant role in several parts of the human body, for example, the musculoskeletal system and the brain. The physiological information of a sodium MRI (23Na-MRI) scan combined with the anatomical information from a 1H-MRI scan has the potential to answer questions about human physiology in general and abnormal conditions. This project aimed to develop sodium Radio Frequency (RF) surface coils for 23Na-MRI at 3T with an increased SNR in a particular Region of Interest (ROI) in the brain (visual cortex area) compared to a birdcage head coil and general-purpose surface coils.
Three designs were included: a simple transceiver single-loop surface coil, a simple enhancing secondary resonator to enhance the field of a birdcage head coil during reception, and a transceiver quadrature surface coil consisting of two loops similar to the single-loop. All coils had geometric shapes fitting to the ROI. The coils' performances were all analyzed through laboratory measurements and phantom MRI scans with a radial 3D cones (CN) Ultra-short Echo Time (UTE) sequence specialized for imaging 23Na with its short relaxation times.
The two transceiver coils successfully increased SNR in the defined ROI. The quadrature coil had the superior SNR because of its circularly polarized magnetic field combined with the specialized shape. This coil provided a more inhomogeneous field than the single-loop coil, but inhomogeneity corrections were successfully applied using the Double-Angle Method (DAM). The enhancing coil successfully enhanced the birdcage coil's field in a slightly different region than expected. Static field calculations were used to explain this, requiring further confirmation. The three in-house developed coils may be suitable for in vivo 23Na-MRI scans. | |
