Sparse spokes slice selective design for B1 inhomogeneity correction at 7T

Introduction: At high magnetic field strength, B1 inhomogeneity causes undesired effects of non-uniformity in SNR and contrast. For slice selective excitation, 3D RF pulse design proposed in [1,2,3], can be used to mitigate this B1 inhomogeneity effect. These designs are based on playing kz sinc “spokes” of different amplitude at a number of (kx,ky) locations around the k-space origin. The sinc spokes in kz create slice selective excitation in z, while the amplitude weighted placement of these spokes in kxky provides modulation in excitation in xy plane. This modulation is designed so that it cancels the effect of the B1 inhomogeneity. Such designs require the use of relatively few kz-spokes to correct for the mild B1 inhomogeneity observed in the brain at 3T. At 7T however, B1 inhomogeneity can be very large and many spokes are required for the correction, causing long RF pulse to be used. With the goal being a reduction of the number of kz spokes, in this work we propose a novel method to optimally select (kx,ky) locations for the placement of the spokes.