Mayo Clinic Department of Radiology, Mayo Clinic, Rochester, Minnesota, USA.
Mayo Graduate School, Mayo Clinic, Rochester, Minnesota, USA.
Magn Reson Med. 2018 Mar;79(3):1538-1544. doi: 10.1002/mrm.26790. Epub 2017 Jun 21.
Imaging gradients result in the generation of concomitant fields, or Maxwell fields, which are of increasing importance at higher gradient amplitudes. These time-varying fields cause additional phase accumulation, which must be compensated for to avoid image artifacts. In the case of gradient systems employing symmetric design, the concomitant fields are well described with second-order spatial variation. Gradient systems employing asymmetric design additionally generate concomitant fields with global (zeroth-order or B ) and linear (first-order) spatial dependence.
This work demonstrates a general solution to eliminate the zeroth-order concomitant field by applying the correct B frequency shift in real time to counteract the concomitant fields. Results are demonstrated for phase contrast, spiral, echo-planar imaging (EPI), and fast spin-echo imaging.
A global phase offset is reduced in the phase-contrast exam, and blurring is virtually eliminated in spiral images. The bulk image shift in the phase-encode direction is compensated for in EPI, whereas signal loss, ghosting, and blurring are corrected in the fast-spin echo images.
A user-transparent method to compensate the zeroth-order concomitant field term by center frequency shifting is proposed and implemented. This solution allows all the existing pulse sequences-both product and research-to be retained without any modifications. Magn Reson Med 79:1538-1544, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
成像梯度会产生伴随场（即麦克斯韦场），在更高的梯度幅度下，这些场变得越来越重要。这些时变场会导致额外的相位积累，必须加以补偿以避免图像伪影。在采用对称设计的梯度系统中，伴随场可以很好地用二阶空间变化来描述。采用非对称设计的梯度系统还会产生具有全局（零阶或 B ）和线性（一阶）空间依赖性的伴随场。
本工作展示了一种通过实时施加正确的 B 频率偏移来消除零阶伴随场的通用解决方案，以抵消伴随场。结果演示了用于相位对比、螺旋、回波平面成像（EPI）和快速自旋回波成像的梯度系统。
在相位对比检查中减少了全局相位偏移，在螺旋图像中几乎消除了模糊。在 EPI 中补偿了相位编码方向的整体图像移位，而在快速自旋回波图像中校正了信号损失、重影和模糊。
提出并实现了一种通过中心频率偏移来补偿零阶伴随场项的用户透明方法。该解决方案允许保留所有现有的脉冲序列——无论是产品还是研究——而无需进行任何修改。磁共振医学 79:1538-1544, 2018。© 2017 国际磁共振学会。
