Time optimal control-based RF pulse design under gradient imperfections.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Optimal Control Theory
• Pulse Sequence Design

Background:

• Gradient system imperfections, such as limited bandwidth and eddy currents, can degrade radio frequency (RF) pulse performance in MRI.
• Accurate RF pulse design requires accounting for these hardware limitations and system disturbances.

Purpose of the Study:

• To develop an optimal control framework for designing radio frequency (RF) and slice selective gradient pulses that compensate for gradient system imperfections.
• To ensure the practical applicability and robustness of optimized RF waveforms under realistic hardware constraints.

Main Methods:

• Formulated the joint design of minimum-time RF and slice selective gradient shapes as an optimal control problem.
• Integrated a measured gradient impulse response function (GIRF) to model system disturbances like limited bandwidth and eddy currents.
• Included hardware limitations (maximal amplitudes, slew rates) as hard constraints for practical waveform design.

Main Results:

• Generated optimized RF and pre-distorted slice selective gradient shapes capable of fully compensating modeled gradient system imperfections.
• Achieved results nearly matching those of an idealized gradient system, demonstrating effective correction.
• Validated numerical Bloch simulations with phantom and in-vivo experiments on 3T scanners.

Conclusions:

• The presented optimal control approach successfully corrects gradient system imperfections in RF pulse design.
• This method enhances the robustness and accuracy of MRI pulse sequences in the presence of hardware limitations.