Highly Parallel Magnetic Resonance Imaging with a Fourth Gradient Channel for Compensation of RF Phase Patterns
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A fourth gradient channel was implemented to provide slice dependent RF coil phase compensation for arrays in dual-sided or "sandwich" configurations. The use of highly parallel arrays for single echo acquisition magnetic resonance imaging allows both highly accelerated imaging and capture of dynamic and single shot events otherwise inaccessible to MRI. When using RF coils with dimensions on the order of the voxel size, the array coil element phase patterns adversely affect image acquisition, requiring correction. This has previously been accomplished using a pulse of the gradient coil, imparting a linear phase gradient across the sample opposite of that due to the RF coil elements. However, the phase gradient due to the coil elements reverses on opposite sides of the coils, preventing gradient-based phase compensation with sandwich arrays. To utilize such arrays, which extend the imaging field of view of this technique, a fourth gradient channel and coil were implemented to simultaneously provide phase compensation of opposite magnitude to the lower and upper regions of a sample, imparting opposite phase gradients to compensate for the opposite RF coil phase patterns of the arrays. The fourth gradient coil was designed using a target field approach and constructed using printed circuit boards. This coil was integrated with an RF excitation coil, dual-sided receive array, and sample loading platform to form a single imaging probe capable of both ultra-fast and high resolution magnetic resonance imaging. By employing the gradient coil, this probe was shown to simultaneously provide improved phase compensation throughout a sample, enabling simultaneous SEA imaging using arrays placed below and above a sample. The fourth gradient coil also improves the acquisition efficiency of highly accelerated imaging using both arrays for receive. The same imaging probe was shown to facilitate accelerated MR microscopy over the field of view of the entire array with no changes to the hardware configuration. The spatio-temporal imaging capabilities of this system were explored with magnetic resonance elastography.
