Bieri O, Markl M, Scheffler K
MR Physics, Department of Medical Radiology, University of Basel, Basel, Switzerland.
Magn Reson Med. 2005 Jul;54(1):129-37. doi: 10.1002/mrm.20527.
Balanced steady-state free precession (SSFP) completely compensates for all gradients within each repetition time (TR), and is thus very sensitive to any magnetic field imperfection that disturbs the perfectly balanced acquisition scheme. It is demonstrated that balanced SSFP is especially sensitive to changing eddy currents that are induced by stepwise changing phase-encoding (PE) gradients. In contrast to the linear k-space trajectory, which has small variations between consecutive encoding steps, other encoding schemes (e.g., centric, random, or segmented orderings) exhibit significant jumps in k-space between adjacent PE steps, and consequently induce rapidly changing eddy currents. The resulting disturbances induce significant image artifacts, such that compensation strategies are essential when nonlinear PE schemes are applied. Although direct annihilation of the induced eddy currents by additional, opposing magnetic fields has been investigated, it is limited by uncertainty regarding the time evolution of induced eddy currents. A generic (and thus system-unrelated) compensation strategy is proposed that consists of "pairing" of consecutive PE steps. Another approach is based on partial dephasing along the slice direction that annihilates eddy-current-induced signal oscillations. Both pairing of the PE steps and "through-slice equilibration" are easy to implement and allow the use of arbitrary k-space trajectories for balanced SSFP.
平衡稳态自由进动（SSFP）在每个重复时间（TR）内完全补偿所有梯度，因此对任何干扰完美平衡采集方案的磁场缺陷都非常敏感。结果表明，平衡SSFP对由逐步变化的相位编码（PE）梯度引起的变化涡流特别敏感。与线性k空间轨迹不同，线性k空间轨迹在连续编码步骤之间变化很小，其他编码方案（例如，中心、随机或分段排序）在相邻PE步骤之间的k空间中表现出显著的跳跃，因此会感应出快速变化的涡流。由此产生的干扰会导致显著的图像伪影，因此在应用非线性PE方案时，补偿策略至关重要。尽管已经研究了通过额外的反向磁场直接消除感应涡流，但它受到感应涡流时间演化不确定性的限制。提出了一种通用的（因此与系统无关的）补偿策略，该策略包括连续PE步骤的“配对”。另一种方法是基于沿切片方向的部分去相位，以消除涡流引起的信号振荡。PE步骤的配对和“切片内平衡”都易于实现，并允许在平衡SSFP中使用任意k空间轨迹。
