Robust Refocusing Pulses of Limited Power

Abstract A series of optimized 180° refocusing pulses are derived using simulated annealing. These pulses are insensitive to simultaneous RF and static magnetic field inhomogeneity. They are superior to previously published pulses with similar characteristics, including a series derived by a different numerical optimization approach. The effect of limited integrated rotation angles (which are manifested as power and pulse length) on the phase dispersion and magnitude performance of the refocusing was investigated. Both phase uniformity and magnitude performance improve with increasing integrated rotation angle. Performance gain from optimization is substantial only at an integrated rotation angle of 2.5π or higher. At a particular integrated rotation angle, there is tradeoff between the phase uniformity and magnitude performance of refocusing. The tradeoff becomes more significant at smaller integrated rotation angle. Because of the need to limit pulse power forin vivoMR applications, the findings highlight the importance of optimizing pulse design for individual applications.