Research In Parallel Transmission In High Field MRI

Magnetic resonance imaging (MRI), with its excellent soft tissue contrast, non-invasive character, is a promising medical imaging technique used in radiology to visualize internal structures and functions of the body in detail. However, the process of MRI formation introduces various artifacts that may corrupt a truly quantitative evaluation. One of major artifacts stems from the intra-slice intensity non-uniformity due to severe radio-frequency (RF) inhomogeneity in high field or ultra high field MRI, since the RF field propagating through the object has shorter wavelengths and greater attenuation, which induces unwanted intensity variations of the signal. This problem will be addressed throughout this thesis, by researching in transmit pulse sequence design, coil array design, and K-space trajectory measurement.(1) Transmit pulse sequence designBased on recently-developed parallel transmission techniques, an optimized3D tailored RF (TRF) pulse, designed with a novel3D adaptive trajectory, is proposed to improve and accelerate volume selective excitation. The feasibility of this method is confirmed by simulations of ultra-high field cases.There are three key factors concerning a practical use of a transmit pulse sequence: excitation uniformity, pulse duration, and RF energy relating to clinical safety issues. The method optimally defines the k-space trajectory to improve all of the three key factors.In detail, the method dedicatedly defines and limits the traversing area of the k-space trajectory, to find an appropriate way to subsample the k-space, so as to shorten the pulse duration. The dedicatedly defined traversing area is supposed to be mostly responsible for the excitation target, so that a promising uniform excitation can be achieved in the target region. Moreover, because the method allows designer to introduce various types of3D k-space trajectory, one can optimally choose one type to help reduce the peak RF pulse amplitude and the entire energy for the excitation, so as to improve RF safety margins in clinical applications.(2) Coil array designA brand new type of coil array geometry is proposed for3D parallel excitation. Such geometry is expected to be used to explore excitation acceleration in an extra dimension, compared to the conventional transmit coil array, so that to mitigate degradation in excitation accuracy at high acceleration factors. Similar type of coil array geometry specialized for parallel imaging is used as reference for those for parallel excitation. The performance of the proposed coil array geometry is demonstrated via simulation based comparison, showing its ability to adapt the3D parallel transmit pulse sequence, to further accelerate excitation pulse length, and to improve excitation accuracy. An evaluation method is also proposed to assess the parallel excitation performance of coil array geometries.(3) K-space trajectory measurementActual K-space trajectory measurement techniques was studied and their performance are tested on a permanent magnet scanner, in accounting for the deviations from the theoretical k-space trajectory and achieving accurate excitation of target pattern. The feasibility of the techniques is confirmed by comparisons of image reconstruction results based on the expected and the measured trajectories.