Mitigate B1+ inhomogeneity by nonlinear gradients and RF shimming

High-field MRI has the challenge of inhomogeneous B1+ and consequently an inhomogeneous flip angle distribution. This causes spatially dependent contrast and makes clinical diagnosis difficult. Under the small flip angle approximation and using nonlinear spatial encoding magnetic fields (SEMs), we propose a method to remap the B1+ map into a lower dimension coordinate system. Combining with RF shimming method, a simple pulse sequence design using nonlinear SEMs can achieve a homogenous flip angle distribution efficiently. Using simulations, we demonstrate that combining RF shimming and spatially selective RF excitation using generalized SEMs (SAGS) using linear and quadratic SEMs in a multi-spoke k-space trajectory can mitigate the B1+ inhomogeneity at 7T efficiently without using parallel RF transmission.