Joint Design of RF and Gradient Waveforms via Auto-differentiation for 3D Tailored Excitation in MRI

Purpose: To demonstrate, through numerical simulations, novel designs of spatially selective radiofrequency (RF) excitations of the fetal brain by both a restricted 2D slice and 3D inner-volume selection. These designs exploit a singlechannel RF pulse, conventional gradient fields, and the spatially non-linear ΔB 0 fields of a multi-coil shim array, using an auto-differentiation optimization algorithm. Methods:The design algorithm jointly optimizes the RF pulse and the timevarying ΔB 0 fields, which is produced by a 64-channel multi-coil ΔB 0 body array to augment the RF and the linear gradient fields, using an auto-differentiation approach. Two design targets were specified, one a 4-mm thick slice with a limited in-slice extent in one dimension ("restricted slice"), and the other a 3D inner-volume selection encompassing the fetal brain ("inner volume"). The RF duration was limited to 2 ms for the restricted slice excitation and 6 ms for the inner-volume excitation.Results: Excitation profiles were achieved for both the restricted slice excitation task (one-minus-minimum magnitude, 8%) within the region of interest (ROI) and (maximum-minus-zero magnitude, 8%) in the suppressed regions and the fetal brain volume excitation task (13% and 9%, respectively). Conclusions:The proposed joint design of RF and time-varying, spatially nonlinear ΔB 0 fields achieves the target excitation profiles with short RF pulse durations and demonstrates the potential to enhance fetal MRI with multi-channel body shim arrays.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Optimization For Joint Design Of Rf Pulse and Time-varying S...mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Selective RF excitation designs enabled by time‐varying spatially non‐linear ΔB₀ fields with applications in fetal MRI

Zhang

Arango

Stockmann

et al. 2021

Selective excitation localized by the Bloch–Siegert shift and a B1+ gradient

Martin

Srinivas

Vaughn

et al. 2022

Purpose To perform B1+$$ {B}_1^{+} $$‐selective excitation using the Bloch–Siegert shift for spatial localization. Theory and Methods A B1+$$ {B}_1^{+} $$‐selective excitation is produced by an radiofrequency (RF) pulse consisting of two summed component pulses: an off‐resonant pulse that induces a B1+$$ {B}_1^{+} $$‐dependent Bloch–Siegert frequency shift and a frequency‐selective excitation pulse. The passband of the pulse can be tailored by adjusting the frequency content of the frequency‐selective pulse, as in conventional B0$$ {B}_0 $$ gradient‐localized excitation. Fine magnetization profile control is achieved by using the Shinnar–Le Roux algorithm to design the frequency‐selective excitation pulse. Simulations analyzed the pulses' robustness to off‐resonance, their suitability for multi‐echo spin echo pulse sequences, and how their performance compares to that of rotating‐frame selective excitation pulses. The pulses were evaluated experimentally on a 47.5 mT MRI scanner using an RF gradient transmit coil. Multiphoton resonances produced by the pulses were characterized and their distribution across B1+$$ {B}_1^{+} $$ predicted. Results With correction for varying B1+$$ {B}_1^{+} $$ across the desired profile, the proposed pulses produced selective excitation with the specified profile characteristics. The pulses were robust against off‐resonance and RF amplifier distortion, and suitable for multi‐echo pulse sequences. Experimental profiles closely matched simulated patterns. Conclusion The Bloch–Siegert shift can be used to perform B0$$ {B}_0 $$‐gradient‐free selective excitation, enabling the excitation of slices or slabs in RF gradient‐encoded MRI.

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

Zhang,

Arango,

Arefeen

et al. 2023

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.