B1+ Interferometry for the Calibration of RF Transmitter Arrays

Multiple-channel RF transmission holds great promise for MRI, especially for human applications at high fields. For calibration it requires mapping the effective RF magnetic fields, B-1(+), of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B-1(+) and tight SAR constraints. In the present work, this problem is revisited and solved by a novel mapping approach relying on an interference principle. The B-1(+) fields of individual transmitter elements are measured indirectly by observing their interference with a SAR-efficient baseline RF field. In this fashion even small RF fields can be observed in the B-1(+)-sensitive large-flip-angle regime. Based on a set of such experiments B-1(+) maps of the individual transmitter channels are obtained by solving a linear inverse problem. Confounding relaxation and off-resonance effects are addressed by an extended signal model and nonlinear fitting. Using the novel approach, 2D mapping of an 8-channel transmitter array was accomplished in less than a minute. For validation it is demonstrated that mapping results do not vary with T, or parameters of the mapping sequence. In RF shimming experiments it is shown that the measured B-1(+) maps accurately reflect the linearity of RF superposition. Magn Reson Med 61:1480-1488, 2009. (C) 2009 Wiley-Liss, Inc.