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

Abstract

To correct with gradient trim blips for gradient delays in bipolar-spoke RF pulses in slice-selective and slab-selective excitations, compatible with tilted acquisitions and anisotropic delays. The effect of small gradient delays with respect to RF pulses results in a dephasing of the second RF spoke, proportional to the slab-selection gradient amplitude and the distance of the slice center from the magnet isocenter. Accordingly, adding a trim blip along the corresponding logical gradient axis between the two spokes compensates for the same dephasing, and therefore cancels the gradient delay effects, regardless of position and orientation. Gradient delays on different axes were first measured on a 7T system based on an imaging method. Parallel transmission universal bipolar spokes were designed offline to mitigate the RF field inhomogeneity problem in the human brain. Trim blips were used to compensate for the known delays, which was validated with flip angle and temporal SNR measurements on two different volunteers at 7 T. Pulses corrected with trim blips greatly reduced gradient delay effects. Acquisitions made with corrected and noncorrected pulses showed good fidelity with simulations. Unlike time or phase-shifting approaches, trim blip-based methods apply to all possible bipolar spoke scenarios such as slice excitations, slab excitations, and anisotropy in the gradient delays.