| Over the recent years, researchers have been increasingly pushing towards using ultra high magnetic field (7 Tesla and higher) for magnetic resonance imaging in human in order to benefit from substantial increases in signal to noise ratio and contrast. However, at ultra high field, severe transmit B1 ( B+1 ) inhomogeneity occurs, limiting applications of most conventional MR techniques. Multidimensional spatially selective RF pulses have been proposed as a method to mitigate B+1 inhomogeneity. However, those RF pulses are typically very long and are impractical at high field. Parallel transmission, an emerging technique, makes it possible to design sufficiently short selective RF pulses for use in actual experiments. In this thesis, we demonstrate the first successful implementation of parallel transmission at an ultra high field of 9.4 Tesla (T) in the human brain with an eight-channel transmit system, using accelerated (x4) RF pulses designed to create arbitrarily shaped excitation profiles. To achieve satisfactory excitation accuracy, k-space errors due to gradient system imperfection had to be accurately calibrated and integrated in RF pulse calculation. In order to limit RF power deposition in tissues, an inherent concern for patient safety at very high field, we introduced and demonstrated a new 2D RF pulse design method that effectively reduces specific absorption rate (SAR) while preserving excitation pattern fidelity. SAR reduction efficiency was demonstrated with numerical simulations while excitation pattern fidelity was experimentally verified at 9.4T. Additional preliminary work relevant to B+1 manipulation at high field were also conducted through the course of this thesis, including the implementation of spoke trajectory based transmit excitation with 16 channels at 9.4T, a fast 2D B 1 mapping technique and in-depth simulation of SAR in the human brain with multi transmission. |
