VERSE-guided parallel RF excitations using dynamic field correction.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Pulse Design
• Pulse Sequence Engineering

Background:

• Variable Rate Selective Excitation (VERSE) limits peak RF power and specific absorption rate (SAR) by reshaping RF and gradient waveforms.
• VERSE accuracy relies heavily on hardware performance; deviations in gradient fields cause excitation errors and prevent convergence to target RF power.
• Iterative VERSE-guided (reVERSE) pulse design assumes a constant k-space trajectory, which is violated by hardware imperfections.

Purpose of the Study:

• To theoretically and experimentally investigate the impact of gradient system imperfections on iteratively VERSEd parallel RF excitations.
• To propose and validate methods for improving excitation accuracy and maintaining RF power below thresholds in the presence of gradient field deviations.
• To enhance the robustness of VERSE-based RF pulse design for accelerated and spatially selective applications.

Main Methods:

• Theoretical analysis and experimental validation of gradient system imperfections' effects on VERSE.
• Integration of gradient field monitoring or Gradient Impulse Response Function (GIRF) estimation into the iterative RF pulse design.
• Utilizing a third-order dynamic field camera with NMR field sensors and GIRFs to measure/estimate actual gradient waveforms.
• Counteracting deviating and variable k-space trajectories at each iteration of the VERSE-guided design.

Main Results:

• Demonstrated that gradient system imperfections significantly compromise excitation accuracy in iteratively VERSEd parallel RF pulses.
• Showcased the effectiveness of integrating real-time gradient field information (GIRF) into the iterative design process.
• Achieved highly improved experimental performance for accelerated, multi-channel, spatially selective RF pulses at both 3T and 7T.
• Validated the proposed approach in mitigating excitation errors caused by hardware imperfections.

Conclusions:

• Gradient field monitoring and GIRF estimation are crucial for accurate VERSE-based RF pulse design, especially in iterative schemes.
• The proposed method effectively compensates for hardware-induced k-space trajectory deviations, leading to superior RF pulse performance.
• This work advances the reliability and accuracy of parallel RF pulse design, enabling more robust accelerated MRI at high field strengths.