Simultaneous multislice refocusing via time optimal control.
PURPOSE
Joint design of minimum duration RF pulses and slice-selective gradient shapes for MRI via time optimal control with strict physical constraints, and its application to simultaneous multislice imaging.


THEORY AND METHODS
The minimization of the pulse duration is cast as a time optimal control problem with inequality constraints describing the refocusing quality and physical constraints. It is solved with a bilevel method, where the pulse length is minimized in the upper level, and the constraints are satisfied in the lower level. To address the inherent nonconvexity of the optimization problem, the upper level is enhanced with new heuristics for finding a near global optimizer based on a second optimization problem.


RESULTS
A large set of optimized examples shows an average temporal reduction of 87.1% for double diffusion and 74% for turbo spin echo pulses compared to power independent number of slices pulses. The optimized results are validated on a 3T scanner with phantom measurements.


CONCLUSION
The presented design method computes minimum duration RF pulse and slice-selective gradient shapes subject to physical constraints. The shorter pulse duration can be used to decrease the effective echo time in existing echo-planar imaging or echo spacing in turbo spin echo sequences.
['Rund|Armin|A|', 'Aigner|Christoph Stefan|CS|', 'Kunisch|Karl|K|', 'Stollberger|Rudolf|R|']
D000465:Algorithms D003198:Computer Simulation D007091:Image Processing, Computer-Assisted D008279:Magnetic Resonance Imaging D019047:Phantoms, Imaging D013997:Time Factors
refocusing time optimal control pulse design simultaneous multislice excitation