Phase-sensitive B₁ mapping: Effects of relaxation and RF spoiling

Purpose: To develop a phase-based B₁ mapping technique accounting for the effects of imperfect RF spoiling and magnetization relaxation. Theory and Methods: The technique is based on a multi-gradient-echo sequence with 2 successive orthogonal radiofrequency (RF) excitation pulses followed by the train of gradient echoes measurements. We have derived a theoretical expression relating the MR signal phase produced by the 2 successive RF pulses to the B₁ field and B₀-related frequency shift. The expression takes into account effects of imperfections of RF spoiling and T₁ and T₂ ^* relaxations. Results: Our computer simulations and experiments revealed that imperfections of RF spoiling cause significant errors in B₁ mapping if not accounted for. By accounting for these effects along with effects of magnetization relaxation and frequency shift, we demonstrated the high accuracy of our approach. The technique has been tested on spherical phantoms and a healthy volunteer. Conclusion: In this paper, we have proposed, implemented, and demonstrated the accuracy of a new phase-based technique for fast and robust B₁ mapping based on the measured MR signal phase, frequency, and relaxation. Because imperfect RF spoiling effects are accounted for, this technique can be applied with short TRs and therefore substantially reduces the scan time. Magn Reson Med 80:101–111, 2018.