Phase-sensitive B mapping: Effects of relaxation and RF spoiling.

PURPOSE

To develop a phase-based B 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 B field and B -related frequency shift. The expression takes into account effects of imperfections of RF spoiling and T and T2* relaxations.

RESULTS

Our computer simulations and experiments revealed that imperfections of RF spoiling cause significant errors in B 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 B 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. © 2017 International Society for Magnetic Resonance in Medicine.