General gradient delay correction method in bipolar multispoke RF pulses using trim blips.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Pulse Design
• Gradient Coil Physics

Background:

• Gradient delays in MRI RF pulses cause signal dephasing, particularly affecting slice-selective and slab-selective excitations.
• This dephasing is dependent on gradient amplitude and position relative to the magnet isocenter.
• Existing methods may not address all scenarios, including tilted acquisitions and anisotropic delays.

Purpose of the Study:

• To introduce and validate a method using gradient trim blips to correct for gradient delays in bipolar-spoke RF pulses.
• To ensure compatibility with tilted acquisitions and anisotropic gradient delays.
• To mitigate RF field inhomogeneity issues in brain imaging at 7 Tesla.

Main Methods:

• Gradient delays were measured on a 7T MRI system using an imaging method.
• Parallel transmission universal bipolar spokes were designed offline.
• Trim blips were implemented to compensate for measured delays and validated through flip angle and temporal SNR measurements in volunteers.

Main Results:

• Trim blip correction significantly reduced the effects of gradient delays.
• Acquisitions using corrected pulses demonstrated high fidelity compared to simulations.
• The method proved effective in real-world volunteer scans at 7T.

Conclusions:

• Gradient trim blips offer a robust solution for correcting gradient delays in various RF pulse schemes.
• This approach is versatile, applicable to slice, slab excitations, and anisotropic delays.
• Trim blip methods provide an advantage over time or phase-shifting techniques for comprehensive delay compensation.