Local excitation universal parallel transmit pulses at 9.4T

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Pulse Design
• Neuroimaging

Background:

• Precise radiofrequency (RF) pulse design is crucial for targeted excitation in Magnetic Resonance Imaging (MRI).
• Current methods often require time-consuming, subject-specific calibration, limiting efficiency.
• The development of 'universal' pulses could overcome these limitations for local excitation applications.

Purpose of the Study:

• To demonstrate the applicability of universal RF pulses for local brain excitation.
• To achieve targeted excitation of the visual cortex while minimizing excitation in surrounding brain regions.
• To validate the performance of these universal pulses in simulations and in vivo.

Main Methods:

• Acquisition of B0 maps from eight subjects at 9.4T to create a database.
• Calculation of universal RF pulses using an extended spatial domain method for local excitation (visual cortex) with specific flip angles (7° and 90°).
• Validation through Bloch simulations and in vivo experiments at 9.4T using GRE and TurboFLASH sequences on non-database subjects.

Main Results:

• Universal RF pulses demonstrated excellent performance in simulations and in vivo, accurately exciting the visual cortex.
• Minimal signal was observed in non-targeted brain regions, confirming effective local excitation.
• While 3D patterns showed minor inhomogeneity in simulations, it was acceptable in vivo; reduced field-of-view application was successful.

Conclusions:

• A novel design approach successfully created universal local excitation RF pulses for 7° and 90° flip angles.
• These universal parallel transmission (pTx) pulses eliminate the need for subject-specific B0 mapping and pulse calculation during scans.
• This significantly streamlines the MRI process for local excitation applications, improving efficiency and potentially accessibility.