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

Abstract

PurposeDeveloping a general framework with a novel stochastic offset strategy for the design of optimized RF pulses and time‐varying spatially non‐linear ΔB0 shim array fields for restricted slice excitation and refocusing with refined magnetization profiles within the intervals of the fixed voxels.MethodsOur framework uses the decomposition property of the Bloch equations to enable joint design of RF‐pulses and shim array fields for restricted slice excitation and refocusing with auto‐differentiation optimization. Bloch simulations are performed independently on orthogonal basis vectors, Mx, My, and Mz, which enables designs for arbitrary initial magnetizations. Requirements for refocusing pulse designs are derived from the extended phase graph formalism obviating time‐consuming sub‐voxel isochromatic simulations to model the effects of crusher gradients. To refine resultant slice‐profiles because of voxelwise optimization functions, we propose an algorithm that stochastically offsets spatial points at which loss is computed during optimization.ResultsWe first applied our proposed design framework to standard slice‐selective excitation and refocusing pulses in the absence of non‐linear ΔB0 shim array fields and compared them against pulses designed with Shinnar‐Le Roux algorithm. Next, we demonstrated our technique in a simulated setup of fetal brain imaging in pregnancy for restricted‐slice excitation and refocusing of the fetal brain.ConclusionsOur proposed framework for optimizing RF pulse and time‐varying spatially non‐linear ΔB0 shim array fields achieve high fidelity restricted‐slice excitation and refocusing for fetal MRI, which could enable zoomed fast‐spin‐echo‐MRI and other applications.

Multiphoton parallel transmission (MP‐pTx): Pulse design methods and numerical validation

“…While the gradient coils alone could provide these fields, their spatial pattern and slew rate limit the mitigation performance (Figure S12). Previously reported approaches have used static, 16,17 and dynamic shim fields [18][19][20][21][22] to perturb the local B z field and therefore alter the local off-resonance. In contrast, MP-pTx directly incorporates the shim array fields in the multiphoton excitation.…”

“…6,7,[13][14][15] A more recent excitation scheme supplements the gradient waveforms characterizing excitation k-space with nonlinear encoding fields from a multichannel ΔB 0 shim array. [16][17][18][19][20][21][22] While the shim array was originally introduced to address B 0 field variations from air-tissue interfaces, [23][24][25][26][27] the additional degrees-of-freedom can also be used during excitation for flip angle correction by spatially modulating the transverse magnetization during RF irradiation.…”