Root-flipped multiband refocusing pulses.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency Pulse Design
• Biomedical Engineering

Background:

• Simultaneous multislice (SMS) imaging accelerates MRI acquisition by exciting multiple slices concurrently.
• Conventional SMS techniques often require high peak radiofrequency (RF) power, limiting performance and potentially increasing SAR.
• Efficient refocusing pulses are crucial for maintaining image quality in spin echo sequences.

Purpose of the Study:

• To design low peak power multiband refocusing radiofrequency (RF) pulses.
• To apply these pulses to simultaneous multislice (SMS) spin echo Magnetic Resonance Imaging (MRI).

Main Methods:

• Designed multiband Shinnar-Le Roux β polynomials using convex optimization.
• Employed a Monte Carlo algorithm for β polynomial root flipping to minimize peak power.
• Designed phase-matched multiband excitation pulses for linear-phase spin echoes.
• Validated through simulations, phantom, and in vivo 7T experiments.

Main Results:

• Root-flipped pulses demonstrated 46% shorter durations compared to time-shifted pulses at equivalent peak RF amplitude.
• Excitation errors of root-flipped pulses remained stable with decreasing band separation, unlike other methods.
• Shorter echo times were achieved, leading to higher signal in experimental validation.

Conclusions:

• The novel root-flipped multiband RF pulse design method effectively produces low peak power pulses.
• This method is suitable for simultaneous multislice spin echo MRI applications.
• The developed pulses enhance scan efficiency and signal quality in SMS MRI.