A modified multi-echo AFI for simultaneous B1+ magnitude and phase mapping

To simultaneously acquire the B1+ magnitude and B1+ phase, a modified multi-echo actual flip-angle imaging (AFI) sequence is proposed. A multi-echo gradient echo sequence was integrated into every even TR of AFI to measure both magnitude and phase of B1+. In addition, to increase the signal-to-noise ratio of the B1+ phase, a double-angle multi-echo AFI sequence, in which the flip-angle of the RF pulses is α at the odd TR and 2α at the even TR is proposed. Images were simulated to evaluate the performance of this method under various imaging and physical parameters. The performance was compared to the spin echo based B1+ mapping method in phantom and in vivo studies.In the simulation, the estimation error decreased as TR1/T1 decreased and TR2/TR1 increased. For double-angle AFI, flip-angle ranges that could estimate B1+ magnitude and phase better than the original AFI were identified. Using the proposed method, B1+ phase estimation was similar to spin echo phase. In the phantom study, correlation coefficient between the estimated B1+ phases using the spin echo and the proposed method was 0.9998. The results show that the B1+ magnitude and B1+ phase can be simultaneously acquired and accurately estimated using the proposed double-angle AFI method.