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Abstract:
The motivation to perform magnetic resonance imaging (MRI) at ultra-high field strength (UHF) (B0 ≥ 7 Tesla) is primarily driven by the increased sensitivity compared to low field MRI. This is especially true for nuclei which exhibit intrinsically a low signal-to-noise ratio (SNR) either due to their physical properties or their small in vivo concentrations. The aim of this thesis was to establish the measurement techniques required for sodium magnetic resonance imaging at 9.4 Tesla and to overcome some of the limitations faced at lower field strengths. For this purpose, the hardware as well as the software used for the acquisition of the MR signal were designed and adapted to each other with great care in order to harness the full potential offered by UHF MRI. In the first part of this thesis, a novel coil setup consisting of a single-tuned sodium birdcage coil and a proton patch antenna was used to acquire high-resolution quantitative sodium images of several healthy volunteers. This setup provided a satisfactory sensitivity at the sodium frequency and offered at the same time the possibility to acquire the proton signal for anatomical localization and B0 shimming. Correction methods for inhomogeneities of the B0 and radio-frequency (RF) transmit field (B1) were implemented and partial volume effects were mitigated by the reduced voxel size, which enabled a more accurate quantification of the sodium concentration in the human brain. However, the spatial resolution was insufficient to completely avoid quantifications errors at tissue boundaries, although the achieved sensitivity was considerably higher compared to previous studies. The second part of the thesis focused on further increasing the sensitivity of the coil setup at the sodium frequency without sacrificing the proton imaging capability. The final coil design was made up of an assembly of three coils arranged in layers. The innermost layer consisted of a multi-channel receiver array to boost the sensitivity for sodium imaging. The middle layer comprised the sodium transmit array and the outer layer was formed by a dipole array to enable proton imaging. It could be shown that the proposed coil setup possessed all the required features needed for efficient multi-nuclear MRI at UHF and enabled the acquisition of sodium images having a quality not previously achieved. In the last part of the thesis, the high sensitivity provided by the multi-channel coil array and the strong static magnetic field was used to perform sodium triple quantum filtered (TQF) imaging, which is known to be an SNR-critical application. The latter allows differentiating between intra- and extracellular sodium, which might be valuable information for disease diagnosis and monitoring. Apart from the low SNR, the high power deposition rates associated with this type of imaging technique are challenging, especially at UHF. To overcome this problem, at least partially, a modulation of the flip angles of the TQ preparation module was proposed and shown to improve the sensitivity by about 20%.
