Improved B1-Mapping for Multi RF Transmit Systems

Fig. 1 shows the B1-maps for the individual coil elements. The maps clearly reflect the regular arrangement of the transmit coil array. The single coil mode results in B1-maps suffering from strong noise in the regions of low B1. In contrast, hardly any noise can be noticed in the maps derived from the inverted coil mode. In Fig. 2, selected profiles of the B1-maps are shown as a graph for both modes. In the high B1 regions, both curves are in good agreement, indicating a reasonable linearity of the transmit system. However, for flip angles smaller than 10°, the maps derived from the single coil mode are corrupted by noise. This is a result of the lower SNR of the source data and the adverse noise propagation of the AFI technique for small flip angles. In contrast, the maps based on the inverted coil mode are smooth for the entire B1 range. Discussion As a result of the linearity of the transmit system, the coil basis functions used for multi-transmit applications (e.g. RF shimming) may be freely chosen. This may be utilized for adapting the RF field to a favourable operational range of the B1-mapping technique. This potentially results in improved B1-maps, as demonstrated in this work for the example of the AFI approach. In practice, basis functions with constructive superposition of the coil element sensitivities will be favourable. If reasonable estimates for the phases of the coil elements are not available, signal cancellation may occur and spoil the quality of the maps. In this case, the phases of the coil elements can still be estimated and proper B1-maps can be acquired in a second iteration.