New method to characterize and correct with sub-µs precision gradient delays in bipolar multispoke RF pulses

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Pulse Design
• Parallel Transmission (pTx)

Background:

• Gradient delays significantly degrade performance of bipolar spoke RF pulses in parallel transmission (pTx).
• Sub-microsecond precision is required to mitigate these detrimental effects.

Purpose of the Study:

• To develop and validate a novel method for characterizing and correcting gradient delays in RF pulses.
• To achieve sub-microsecond precision in gradient delay measurement for enhanced pTx performance.

Main Methods:

• Experimentally determined phase shifts (Δφ) to achieve a 0° flip angle in a bipolar two-spoke pulse configuration.
• Applied the method across multiple slice locations using a multislice gradient echo sequence at 7 Tesla.
• Tested prospective compensation by adjusting the phase of the second RF pulse.

Main Results:

• Achieved an accuracy of approximately 50 nanoseconds in gradient delay measurement.
• Demonstrated that phase corrections up to 180° on the second spoke pulse restore performance.
• Enabled comparable performance to unipolar designs with significantly shorter RF excitations.

Conclusions:

• A simple, accurate gradient-delay calibration method was successfully developed.
• The method facilitates the use of bipolar multispoke pulses in multislice MRI protocols.
• This advancement is crucial for high-performance pTx applications in 2D imaging.