Average SAR Constrained Local RF Shimming

Introduction Achieving given homogeneity and SAR constraints in RF shimming process is an important problem in high field MRI. There are different studies in the literature which dealt with the optimization of these parameters by using non-convex optimization techniques. In a previous work whole body SAR constrained RF shimming problem was solved in order to achieve whole slice transmit homogeneity [1]. For the cases in which a smaller target is imaged it might be sufficient to obtain a homogenous transmit profile in the vicinity of the target organ/tissue. In a previous work local shimming problem was solved for prostate imaging with local SAR constraints [2]. Although both local and whole body SAR are important limitations in high field MRI we focused on the relation between whole body SAR and transmit field homogeneity for local targets In this work an optimization method called Particle Swarm Optimization (PSO)[3] is used to solve the RF shimming problem mentioned above. The method is verified by phantom simulations of a 16 channel transmit coil array at 7 T. Theory Solving an RF shimming problem for a given coil geometry requires non-linear optimization algorithms. The initial point of the nonlinear algorithms should be chosen in a rational manner in order to come up with useful solutions. Let α denote the set of complex