MR Physics for Physicists / B1 Shimming and Parallel Transmission

METHODS: Currently, among other means, modifications of the applied RF pulses are investigated to cope with the mentioned B1 inhomogeneities. First, using a coil array for RF transmission, standard slice selective pulses can be transmitted with different weights in the different array elements (see, e.g., [2-4]). These weights, i.e., amplitude and phase of each element, are designed to yield a flip angle variation as small as possible (“basic” B1 shimming). On the other hand, a twoor three-dimensional RF pulse can be applied, whose target pattern corresponds to the reciprocal of the B1 inhomogeneity to be compensated. Thus, the resulting flip angle should be constant across the field of excitation (“tailored” B1 shimming). Using not a single RF transmit coil, but again a transmit coil array, the required multidimensional RF pulses can be based on short trajectories, which are “sparse” in the excitation k-space. The resulting gaps in the excitation k-space can be filled by utilizing the different sensitivity profiles of the elements of the transmit coil array used (“Transmit SENSE”, see, e.g., [5-9]). These sensitivity profiles have to be measured in a preparation step prior to scanning (so-called “B1-mapping”, see, e.g., [10-13]), both for basic as well as for tailored RF shimming. A much debated topic for B1 shimming and parallel transmission is the specific energy absorption rate (SAR) in the framework of RF patient safety (see, e.g., [14-17]). On one hand, the new degrees of freedom introduced by parallel transmission remove the proportionality of RF power and SAR as found for single channel systems. RF safety concepts have to be adapted accordingly, which is a highly non-trivial challenge for RF pulse design as well as for online RF/SAR monitoring. On the other hand, parallel transmission also offers the possibility to reduce SAR, and thus, to fully exploit the capabilities of the MR system used.