Université Paris-Saclay, CEA, CNRS, BAOBAB, NeuroSpin, Gif-sur-Yvette, France.
Magn Reson Med. 2021 Feb;85(2):1004-1012. doi: 10.1002/mrm.28478. Epub 2020 Aug 27.
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.
在用于切片选择和块选择激发的双极辐射状 RF 脉冲中，通过梯度修剪脉冲来校正梯度延迟，从而兼容倾斜采集和各向异性延迟。
相对于 RF 脉冲的小梯度延迟会导致第二 RF 辐射瓣的去相位，其与块选择梯度幅度和切片中心与磁体等中心的距离成正比。因此，在两个辐射瓣之间沿着相应的逻辑梯度轴添加修剪脉冲可以补偿相同的去相位，从而消除梯度延迟效应，无论位置和方向如何。
首先基于成像方法在 7T 系统上测量不同轴上的梯度延迟。设计了离线的并行传输通用双极辐射瓣，以减轻人脑内 RF 场非均匀性问题。使用修剪脉冲来补偿已知的延迟，这在 7T 上的两名志愿者中进行了翻转角和时间 SNR 测量来验证。
使用修剪脉冲校正的脉冲大大降低了梯度延迟效应。使用校正和未校正脉冲进行的采集与模拟具有很好的一致性。
与时间或相移方法不同，基于修剪脉冲的方法适用于所有可能的双极辐射瓣情况，如切片激发、块激发和梯度延迟的各向异性。
