Simultaneous multislice multiband parallel radiofrequency excitation with independent slice-specific transmit B1...

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Engineering
• Biomedical Engineering

Background:

• High and ultrahigh field MRI offer enhanced signal-to-noise ratio but face challenges with B1+ inhomogeneity and high RF power deposition.
• Simultaneous Multiband (MB) excitation accelerates imaging by exciting multiple slices concurrently, but exacerbates B1+ inhomogeneity and power concerns.
• Conventional MB pulses often use circularly polarized (CP) mode, which may not optimally address these challenges.

Purpose of the Study:

• To develop a novel parallel transmit (pTx) pulse design for simultaneous MB excitation.
• To address B1+ inhomogeneity and total RF power limitations in MB MRI.
• To enable optimal RF excitation for slice acceleration in high and ultrahigh field MRI.

Main Methods:

• A new pTx pulse design formalism was developed, incorporating band-specific B1 complex shim weights for each simultaneously excited band.
• The method was validated in human brain imaging at 7 Tesla using a 16-channel pTx system.
• Numerical simulations using measured B1 maps were performed for comparison with conventional MB pulses in CP mode.

Main Results:

• The novel pTx pulse design improved B1+ homogeneity by 60% compared to the conventional CP mode, while maintaining constant total RF power.
• When excitation fidelity was kept constant, the new method reduced total RF power by 72% compared to the CP mode.
• Significant improvements in slice-wise B1+ uniformity and/or substantial reductions in total RF power were observed.

Conclusions:

• A new pTx pulse design formalism has been introduced for MB excitation.
• This formalism effectively targets slice-specific B1+ homogenization while constraining total RF power.
• The developed pulses offer superior B1+ uniformity and/or reduced RF power compared to conventional MB pulses in CP mode.