• fMRI
• High-field
• High-resolution
• EPI

The great interest in ultra-high field functional magnetic resonance imaging (fMRI) comes from the observation that the functional contrast increases supra-linearly with field strength, allowing imaging to be performed at unprecedented spatial resolutions. However, simply reducing partial volume effects is not sufficient to precisely localize task-induced activation due to the indirect mechanisms that relate brain function and the changes in the measured signal. Also, the increased magnetic field strength increases the effect of problematic image artefacts which arise from local B0 inhomogeneities.
In the first experimental chapter, a Gradient-Echo Echo Planar Imaging (GRE-EPI) and Spin-Echo EPI (SE-EPI) were evaluated at 7 T at sub-millimetre resolutions in comparison to larger voxel sizes. While similar activation foci were obtained in all acquisitions, SE-EPI acquired with sub-millimetre voxels showed a significant decrease in temporal signal-to-noise ratio which hampered the extent of the activation clusters.
At these resolutions, T2* -weighted data collected with GRE-EPI provided higher functional contrast and sensitivity. The selection of active voxels within an appropriate gray-matter mask enabled satisfactory specificity to the true site of activation. This work was published in the proceedings of the 24th ISMRM meeting and led to a first-author paper in the Magnetic Resonance Imaging journal.
In a second study, Tailored Radio-Frequency (TRF) pulses optimized to compensate for through-plane field inhomogeneities in GRE-EPI at 7 T were tested with an fMRI experiment targeting the ventral occipito-temporal cortex. The TRF showed BOLD signal recovery in signal dropout areas, highlighting its potential use at 7 T. This work was published in the proceedings of the 23rd ISMRM meeting and is currently under
review in the Magnetic Resonance Materials in Physics, Biology and Medicine journal.
In a third study, a Multi-Echo EPI sequence was modified to acquire a field map at each fMRI volume by switching the phase encoding polarity of even echoes. Tests on healthy subjects revealed localized physiology-related motion effects captured by the dynamic field maps. In addition, the dynamically distortion corrected fMRI showed sufficient unwarping fidelity and no observable loss of functional sensitivity. This work was published in the proceedings of the 25th ISMRM meeting.
The work presented in this thesis has demonstrated improvements in localization of functional activity at 7 T.