GE Research, Niskayuna, New York, USA.
Mayo Clinic, Rochester, Minnesota, USA.
Magn Reson Med. 2023 Jan;89(1):262-275. doi: 10.1002/mrm.29452. Epub 2022 Sep 21.
Asymmetric gradient coils introduce zeroth- and first-order concomitant field terms, in addition to higher-order terms common to both asymmetric and symmetric gradients. Salient to compensation strategies is the accurate calibration of the concomitant field spatial offset parameters for asymmetric coils. A method that allows for one-time calibration of the offset parameters is described.
A modified phase contrast pulse sequence with single-sided bipolar flow encoding is proposed to calibrate the offsets for asymmetric, transverse gradient coils. By fitting the measured phase offsets to different gradient amplitudes, the spatial offsets were calculated by fitting the phase variation. This was used for calibrating real-time pre-emphasis compensation of the zeroth- and first-order concomitant fields.
Image quality improvement with the proposed corrections was demonstrated in phantom and healthy volunteers with non-Cartesian and Cartesian trajectory acquisitions. Concomitant field compensation using the calibrated offsets resulted in a residual phase error <3% at the highest gradient amplitude and demonstrated substantial reduction of image blur and slice position/selection artifacts.
The proposed implementation provides an accurate method for calibrating spatial offsets that can be used for real-time concomitant field compensation of zeroth and first-order terms, substantially reducing artifacts without retrospective correction or sequence specific waveform modifications.
非对称梯度线圈会引入零阶和一阶伴随磁场项，除了非对称和对称梯度共有的高阶项。补偿策略的关键是准确校准非对称线圈的伴随场空间偏移参数。本文介绍了一种可一次性校准偏移参数的方法。
提出了一种改进的具有单边双极流动编码的相位对比脉冲序列，用于校准非对称横向梯度线圈的偏移。通过将测量的相位偏移拟合到不同的梯度幅度，通过拟合相位变化来计算空间偏移。这用于校准零阶和一阶伴随场的实时预加重补偿。
在具有非笛卡尔和笛卡尔轨迹采集的体模和健康志愿者中，通过所提出的校正，图像质量得到了改善。使用校准后的偏移量进行伴随场补偿，在最高梯度幅度下残余相位误差 <3%，并显著减少了图像模糊和切片位置/选择伪影。
所提出的实现方法提供了一种准确的校准空间偏移的方法，可用于实时补偿零阶和一阶项的伴随场，在不进行回溯校正或序列特定波形修改的情况下，大大减少伪影。
