Mitigation of B1+ inhomogeneity using spatially selective excitation with jointly designed quadratic spatial encoding magnetic fields and RF shimming

Abstract

Purpose The inhomogeneity of flip angle distribution is one major challenge impeding the application of high field magnetic resonance imaging (MRI). Here we report a method combining SpAtially selective excitation using Generalized SEMs (SAGS) with RF shimming to achieve homogeneous excitation. This method can be an alternative approach to address the challenge of B1+ inhomogeneity using nonlinear gradients. Method We proposed a two-step algorithm, which first jointly optimizes the combination of nonlinear spatial encoding magnetic fields (SEMs) and the combination of multiple RF transmitter coils, and then optimizes the locations, RF amplitudes, and phases of the spokes. Results Our results show that jointly-designed SAGS and RF shimming can provide a more homogeneous flip angle distribution than using SAGS or RF shimming alone. Compared to RF shimming alone, our approach can reduce the relative standard deviation of flip angle by 56% and 52% using phantom and human head data, respectively. Conclusion The jointly-designed SAGS and RF shimming method can be used to achieve homogeneous flip angle distributions when fully parallel RF transmission is not available.

Simultaneous use of linear and nonlinear gradients for B1+ inhomogeneity correction

“…As an extension of this work, our excitation k ‐space formulation defines independent variables for each N‐SEM channel, and a Fourier transform relationship is still re‐established in the increased dimension without requiring the parameterization of the target distribution. Later, this work was extended to parallel transmission with a similar design principle for a spatial phase profile . Although the number of spokes is greater than 2, the approach is also suboptimal since it uses the same combination of N‐SEMs and L‐SEMs for all spokes; in other words, the phase profiles created between the spokes are only linearly scaled versions of each other.…”

“…Although the number of spokes is greater than 2, the approach is also suboptimal since it uses the same combination of N‐SEMs and L‐SEMs for all spokes; in other words, the phase profiles created between the spokes are only linearly scaled versions of each other. Previous methods might be extremely effective for correcting even highly variant B 1 + inhomogeneities in space if there is a high number of independent N‐SEM channels to implement the required phase profile at the expense of hardware complexity. However, the approaches are still suboptimal because the required phase profile might not be created by the available hardware; therefore, an optimization that considers the available field profiles at the first stage can still be useful.…”

Stochastic‐offset‐enhanced restricted slice excitation and 180° refocusing designs with spatially non‐linear ΔB0 shim array fields

“…[14][15][16] RF pulse designs taking non-linear gradient fields into design considerations have also been studied with quadratic coordinates transformations under the small-tip-angle regime. [17][18][19] To address the nonlinearity of the Bloch equations for large-tip-angle excitations, the Shinnar-Le Roux (SLR) algorithm proposed a mapping of RF pulses to pairs of polynomials that represent Cayley-Klein parameters with the hard pulse assumption. 20,21 Later, Setsompop et al 22 proposed a two-stage design with an initial (small-tip) approximation refined through iterative Bloch optimization.…”

“…Parallel transmission coils are also used to mitigate the long RF duration issues with the help of independently driven transmit coils 14–16 . RF pulse designs taking non‐linear gradient fields into design considerations have also been studied with quadratic coordinates transformations under the small‐tip‐angle regime 17–19 …”