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

ABSTRACT

PURPOSE: High-field magnetic resonance imaging (MRI) has the challenge of inhomogeneous B(1)(+), 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 B(1)(+) 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 B(1)(+) 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.