Precise and robust B1+ characterization of transmit coil arrays

Introduction: A common approach to B1+ characterization of TX-arrays involves transmitting with different combinations of array elements and decoding the combined field maps to obtain single channel field maps [1,2]. This leads to an improved SNR behavior compared to the straightforward approach of B1+ characterization where only one array element with limited spatial TX-sensitivity in the FOV is successively used for transmission. This combined transmission approach can in principle be used with any common B1+ mapping technique. TX-element combination can be employed in a more sophisticated manner when using a B1+ mapping method based on a saturation pulse with different flip angles and on imaging the remaining longitudinal magnetization with a gradient echo sequence. In such a method B1+ values are determined by fitting a function to the measured data [3]. When using TX-arrays it is possible, as proposed by Fautz et al. [4], to perform the saturation with a single transmit element while the excitation pulse can be performed with a B1+-shimmed TX-element combination ensuring a sufficient B1+ amplitude and therefore a sufficient SNR in the whole FOV. With this technique the decoding process can be omitted which prevents errors occurring during the fitting process to be propagated to the B1+ maps of all elements. The present study proposes improvements for the common combined transmission approach for TX-array B1+ mapping as well as for the saturation-based technique. Furthermore, the performance of the improved techniques is experimentally compared to the classical single-element B1+ mapping approach.