Basic and Tailored RF Shimming in a Multi-transmit Whole Body MR System

Wave propagation effects diminish the quality of MR images at main fields of 3T or above. Parallel RF transmission has the potential of compensating for these effects through RF shimming. RF shimming can be performed in two different ways. The basic way of RF shimming is to adjust the global amplitude and phase of the currents in each transmit element, aiming at a constant B1 in the region of interest. For 3D volume imaging, 3D RF shimming is facilitated using multiple frequencies for the different transmit elements. On the other hand, “tailored” RF shimming can be performed via multi-dimensional RF pulses designed to achieve a spatially constant excitation pattern. Using an MR system equipped with parallel RF channels, these multi-dimensional RF pulses can be accelerated via Transmit SENSE. The potential of both, basic and tailored RF shimming, can be enhanced significantly, if only a constant B1 amplitude is demanded, and an arbitrary spatial distribution of the resulting B1 phase is allowed. This is the case, if only magnitude images are of interest. However, this approach introduces a non-linear problem, requiring corresponding numerical techniques. Optimal results for basic RF shimming are obtained with transmit arrays owning preferably homogeneous sensitivity distributions of the individual elements. On the other hand, for tailored shimming, the transmit elements require orthogonal sensitivity distributions, which are easier to achieve with inhomogeneous sensitivity distributions. Thus, the transmit coil array cannot be optimized for both, basic and tailored RF shimming simultaneously, and a suitable compromise has to be found. This study compares the different RF shimming approaches using a whole-body, 8-channel Tx/Rx system at 3T. It shows, that basic RF shimming is able to remove B1 inhomogeneities to a high degree, both in phantoms and in vivo. Tailored RF shimming is able to achieve even higher levels of B1 homogeneity, which, however, might not be necessary for the majority of clinical applications.