Optimal linear combinations of array elements for B1 mapping.

Accuracy of B(1) mapping for array coils can be improved by mapping the fields produced by driving linear combinations of the array elements, chosen to produce a more uniform distribution of B(1) amplitude. Quality of the resulting single element B(1) maps is influenced by the transformation used both via the uniformity of the resulting linear combination fields and by the degree to which these linear combinations differ from one another. In this work we investigate the effect of using different transformations on the quality of B(1) maps by simulating the B(1) mapping process for two different techniques, using real data from a 3T 8-channel body transmit system. Different transformations are generated using a single complex parameter. It is demonstrated that the optimal transformation within this framework is different for different imaging targets (pelvis and brain of healthy volunteers, and water and oil phantoms). For the same target (pelvis) the optimum condition, however, is similar for a number of subjects, suggesting that optimal configurations to be used for calibrating coils in specific anatomical contexts can be determined in advance. Potential gains may be translated into significant reductions in scan time for equivalent signal-to-noise ratio coil maps. (c) 2009 Wiley-Liss, Inc.