Eigenmode analysis of transmit coil array for tailored B1 mapping.

In vivo radiofrequency (RF) field B(1) mapping represents an essential prerequisite for parallel transmit applications. However, the large dynamic range of the transmit fields of the individual coil elements challenges the accuracy of MR-based B(1) mapping techniques. In the present work, the B(1) mapping error and its impact on the RF performance are studied based on a coil eigenmode analysis. Furthermore, the linear properties of the transmit chain are exploited to virtually adjust the weighting of the different coil eigenmodes in the B(1) mapping procedure, resulting in considerably reduced mapping errors. In addition, the weighting of the eigenmodes is tailored to potential target applications, e.g., specific absorption rate (SAR) reduced RF shimming or multidimensional RF pulses, resulting in improved RF performance. The basic theoretic principles of the concept are elaborated and validated by corresponding simulations. Furthermore, results on B(1) mapping and RF shimming experiments, performed on phantoms and in vivo using a 3-T scanner equipped with an eight-channel transmit/receive body coil, are presented to prove the feasibility of the approach.