Anuj Sharma1,2, Michael Lustig3, William A Grissom1,2,4,5. 1. Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee, USA. 2. Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, USA. 3. Department of Electrical Engineering and Computer Science, University of California Berkeley, Berkeley, California, USA. 4. Department of Radiology, Vanderbilt University, Nashville, Tennessee, USA. 5. Department of Electrical Engineering, Vanderbilt University, Nashville, Tennessee, USA.
Abstract
PURPOSE: To design low peak power multiband refocusing radiofrequency pulses, with application to simultaneous multislice spin echo MRI. THEORY AND METHODS: Multiband Shinnar-Le Roux β polynomials were designed using convex optimization. A Monte Carlo algorithm was used to determine patterns of β polynomial root flips that minimized the peak power of the resulting refocusing pulses. Phase-matched multiband excitation pulses were also designed to obtain linear-phase spin echoes. Simulations compared the performance of the root-flipped pulses with time-shifted and phase-optimized pulses. Phantom and in vivo experiments at 7T validated the function of the root-flipped pulses and compared them to time-shifted spin echo signal profiles. RESULTS: Averaged across number of slices, time-bandwidth product, and slice separation, the root-flipped pulses have 46% shorter durations than time-shifted pulses with the same peak radiofrequency amplitude. Unlike time-shifted and phase-optimized pulses, the root-flipped pulses' excitation errors do not increase with decreasing band separation. Experiments showed that the root-flipped pulses excited the desired slices at the target locations, and that for equivalent slice characteristics, the shorter root-flipped pulses allowed shorter echo times, resulting in higher signal than time-shifted pulses. CONCLUSION: The proposed root-flipped multiband radiofrequency pulse design method produces low peak power pulses for simultaneous multislice spin echo MRI.
PURPOSE: To design low peak power multiband refocusing radiofrequency pulses, with application to simultaneous multislice spin echo MRI. THEORY AND METHODS: Multiband Shinnar-Le Roux β polynomials were designed using convex optimization. A Monte Carlo algorithm was used to determine patterns of β polynomial root flips that minimized the peak power of the resulting refocusing pulses. Phase-matched multiband excitation pulses were also designed to obtain linear-phase spin echoes. Simulations compared the performance of the root-flipped pulses with time-shifted and phase-optimized pulses. Phantom and in vivo experiments at 7T validated the function of the root-flipped pulses and compared them to time-shifted spin echo signal profiles. RESULTS: Averaged across number of slices, time-bandwidth product, and slice separation, the root-flipped pulses have 46% shorter durations than time-shifted pulses with the same peak radiofrequency amplitude. Unlike time-shifted and phase-optimized pulses, the root-flipped pulses' excitation errors do not increase with decreasing band separation. Experiments showed that the root-flipped pulses excited the desired slices at the target locations, and that for equivalent slice characteristics, the shorter root-flipped pulses allowed shorter echo times, resulting in higher signal than time-shifted pulses. CONCLUSION: The proposed root-flipped multiband radiofrequency pulse design method produces low peak power pulses for simultaneous multislice spin echo MRI.
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