Minimum TR radiofrequency-pulse design for rapid gradient echo sequences.

Area of Science:

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

Background:

• Minimizing repetition time (TR) is crucial for accelerating MRI sequences, particularly in applications like cardiac imaging.
• Existing methods like variable-rate selective excitation (VERSE) can shorten RF pulse duration but may not achieve minimum TR due to specific absorption rate (SAR) constraints.
• Gradient echo sequences are widely used, but their TR is often limited by RF pulse design and hardware/physiological constraints.

Purpose of the Study:

• To present a framework for designing radiofrequency (RF) pulses that specifically minimize the TR of gradient echo sequences.
• To optimize RF pulse design considering hardware limitations and physiological constraints such as SAR and peripheral nerve stimulation (PNS).
• To demonstrate the application of this framework in reducing scan times for advanced MRI techniques.

Main Methods:

• Development of a TR-optimal framework for designing RF pulses.
• Incorporation of constraints including spatial encoding, SAR, and PNS.
• Application of the framework to single-band and multiband (MB) RF pulses, including variable-rate selective excitation (VERSE).
• Consideration of gradient imperfections and their impact on imaging.

Main Results:

• Significant TR reduction achieved: up to 14% for low time-bandwidth product (TBP) pulses (e.g., bSSFP) and up to 72% for high TBP pulses (e.g., slab-selective imaging).
• A breath-hold cardiac exam duration was reduced by 46% using both MB and the TR-optimal framework.
• RF-based correction of gradient imperfections proved important, and PNS was not a limiting factor.

Conclusions:

• The TR-optimal framework effectively designs RF pulses to minimize sequence TR across various pulse parameters.
• This approach enables substantial reductions in MRI scan times, improving patient comfort and throughput.
• The framework offers a valuable tool for optimizing gradient echo sequences in diverse clinical applications.