Stochastic-offset-enhanced restricted slice excitation and 180° refocusing designs with spatially non-linear ΔB₀ shim...

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Pulse Sequence Design
• Biomedical Engineering

Background:

• Optimizing radiofrequency (RF) pulses and magnetic field gradient shim arrays is crucial for precise slice selection in MRI.
• Existing methods face challenges in achieving refined magnetization profiles for restricted excitation and refocusing.

Purpose of the Study:

• To develop a general framework for designing optimized RF pulses and time-varying, spatially non-linear ΔB0 shim array fields.
• To achieve restricted slice excitation and refocusing with refined magnetization profiles using a novel stochastic offset strategy.

Main Methods:

• Utilized the decomposition property of Bloch equations for joint RF pulse and shim field design.
• Employed auto-differentiation optimization and performed Bloch simulations on orthogonal basis vectors (Mx, My, Mz).
• Derived refocusing pulse design requirements from extended phase graph formalism and introduced a stochastic offset algorithm for slice-profile refinement.

Main Results:

• The framework was successfully applied to standard slice-selective excitation and refocusing pulses, showing comparable or improved performance against the Shinnar-Le Roux algorithm.
• Demonstrated efficacy in a simulated fetal brain MRI scenario, achieving restricted-slice excitation and refocusing.

Conclusions:

• The proposed framework enables high-fidelity restricted-slice excitation and refocusing for fetal MRI.
• This advancement could facilitate applications like zoomed fast-spin-echo MRI and other specialized imaging techniques.