Designing RF Refocusing Pulses for Parallel Transmit Spin Echo Sequences

INTRODUCTION Parallel transmission of RF pulses [1, 2] has shown great potential in B1 inhomogeneity correction at high fields [3]. Most of the existing applications of parallel transmission, however, have been restricted to only gradient echo due to lack of systematic design of parallel transmit refocusing pulses. The difficulty of designing such pulses lies in that 1) the large flip angle nature of refocusing pulses requires addressing the Bloch equation nonlinearity, 2) the refocusing pulses need to “pancake” flip a spin ensemble (rather than a single spin) where each spin can have different initial phase in the transverse plane, and 3) parallel transmission in general does not favor a spatially constant nutation axis due to the complex nature of B1+ maps. In this paper, we propose an optimal control [4, 5] based method for designing parallel transmit refocusing pulses that addresses all the above difficulties. Bloch simulation results show that refocusing pulses designed by the proposed method can produce homogeneous refocusing and are expected to enable spin echo applications of parallel transmission.