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.