Mitigate B₁⁺ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields

Purpose High-field magnetic resonance imaging (MRI) has the challenge of inhomogeneous B₁⁺, and consequently inhomogeneous flip angle distribution, which causes spatially dependent contrast and makes clinical diagnosis difficult. Method We propose a two-step pulse design procedure in which (1) a combination of linear and nonlinear spatial encoding magnetic fields (SEMs) is used to remap the B₁⁺ map in order to reduce the dimensionality of the problem, (2) the locations, amplitudes, and phases of spoke pulses are estimated in one dimension. The advantage of this B ₁⁺ remapping is that when the isointensity contours of a linear combination of SEMs are similar to the isointensity contours of B ₁⁺, a simple pulse sequence design using time-varying SEMs can achieve a homogenous flip-angle distribution efficiently. Results We demonstrate that spatially selective radiofrequency (RF) excitation with generalized SEMs (SAGS) using both linear and quadratic SEMs in a multi-spoke k-space trajectory can mitigate the B₁⁺ inhomogeneity at 7T efficiently. Numerical simulations based on experimental data suggest that, compared with other methods, SAGS provide a formulation allowing multiple-pulse design, a similar average flip-angle distribution with less RF power, and/or a more homogeneous flip-angle distribution. Conclusion Without using multiple RF coils for parallel transmission, SAGS can be used to mitigate the B ₁⁺ inhomogeneity in high-field MRI experiments.