Sequential Optimal Spoke Selection for Spoke Trajectory Based RF Pulses Design in Parallel Excitation

INTRODUCTION: The spoke trajectory [1,2] is a preferred choice of k-space trajectory in designing 3D RF pulses for applications requiring thin slice selection and tailored in-plane excitation. It typically consists of several spokes of samples, each spoke extensively covering the kz (throughplane) axis at a distinct (kx,ky) (in-plane axes) location. Ideally, a large number of spokes should be used to ensure sufficient coverage of the kx-ky plane. In practice, however, only a small number of spokes can be used due to the pulse duration limitation. The locations of these spokes are therefore critical to the performance of the resulting RF pulses and should in principle be optimized for a given excitation pattern and B1 field map, especially in parallel transmission. In this paper, we propose a sequential selection [3,4] based algorithm to find the optimal sampling locations that generates minimum excitation error. Both Bloch simulation and experimental results show that the proposed method produces significantly smaller excitation error and/or shorter pulses than several conventional designs. PROPOSED METHOD: Suppose there are L transmit coils and N spokes whose locations KN={(kn,x kn,y)}, n=1,...,N are given, Sinc/SLR subpulses are used in the 3D design and the weights are determined by minimizing the following cost function: