A modified multi-echo AFI for simultaneous B-1(+) magnitude and phase mapping

作者：Choi Narae; Lee Joonsung; Kim Min Oh; Shin Jaewook; Kim Dong Hyun^*

来源：Magnetic Resonance Imaging, 2014, 32(4): 314-320.

DOI：10.1016/j.mri.2013.12.005

• B-1 mapping
• B-1 phase
• Actual flip-angle imaging
• Double-angle AFI (DA AFI)
• Multi-echo AFI
• B-1 complex mapping

To simultaneously acquire the B-1(+) magnitude and B-1(+) 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 B-1(+). In addition, to increase the signal-to-noise ratio of the B-1(+) phase, a double-angle multi-echo AFI sequence, in which the flip-angle of the RF pulses is alpha at the odd TR and 2 alpha 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 B-1(+) mapping method in phantom and in vivo studies.
In the simulation, the estimation error decreased as TR1/T-1 decreased and TR2/TR1 increased. For double-angle AFI, flip-angle ranges that could estimate B-1(+) magnitude and phase better than the original AFI were identified. Using the proposed method, B-1(+) phase estimation was similar to spin echo phase. In the phantom study, correlation coefficient between the estimated B-1(+) phases using the spin echo and the proposed method was 0.9998. The results show that the B-1(+) magnitude and B-1(+) phase can be simultaneously acquired and accurately estimated using the proposed double-angle AFI method.