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.