Time optimal control‐based RF pulse design under gradient imperfections

• Published in: Magnetic resonance in medicine 2020-02, Vol.83 (2), p.561-574
• Main Authors: Aigner, Christoph S., Rund, Armin, Abo Seada, Samy, Price, Anthony N., Hajnal, Joseph V., Malik, Shaihan J., Kunisch, Karl, Stollberger, Rudolf
• Format: Article
• Language: English
• Subjects: Algorithms; Brain - diagnostic imaging; Computer Simulation; Control systems; Defects; Design; Economic models; Eddy currents; Equipment Design; Fourier Analysis; Full Papers—Imaging Methodology; gradient imperfections; gradient impulse response function; Humans; Image Processing, Computer-Assisted - methods; Impulse response; Magnetic Resonance Imaging - instrumentation; Magnetic Resonance Imaging - methods; Male; Models, Statistical; Optimal control; Phantoms, Imaging; pulse design; Radio frequency; Radio Waves; Response functions; Scanners; Shape optimization; simultaneous multi‐slice excitation; Slew rate; Time optimal control; Waveforms
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.27955

Purpose This study incorporates a gradient system imperfection model into an optimal control framework for radio frequency (RF) pulse design. Theory and Methods The joint design of minimum‐time RF and slice selective gradient shapes is posed as an optimal control problem. Hardware limitations such as maximal amplitudes for RF and slice selective gradient or its slew rate are included as hard constraints to assure practical applicability of the optimized waveforms. In order to guarantee the performance of the optimized waveform with possible gradient system disturbances such as limited system bandwidth and eddy currents, a measured gradient impulse response function (GIRF) for a specific system is integrated into the optimization. Results The method generates optimized RF and pre‐distorted slice selective gradient shapes for refocusing that are able to fully compensate the modeled imperfections of the gradient system under investigation. The results nearly regenerate the optimal results of an idealized gradient system. The numerical Bloch simulations are validated by phantom and in‐vivo experiments on 2 3T scanners. Conclusions The presented design approach demonstrates the successful correction of gradient system imperfections within an optimal control framework for RF pulse design.