Monitoring and correcting spatio-temporal variations of the MR scanner's static magnetic field
Monitoring and correcting spatio-temporal variations of the MR scanner's static magnetic field
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DOI：
10.1007/s10334-006-0050-2
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发表时间：
2006-11-01
影响因子：
2.3
通讯作者：
Atalar, Ergin
中科院分区：
文献类型：
--
作者：
El-Sharkawy, Abdel Monem;Schar, Michael;Atalar, Ergin
静磁场的均匀性和稳定性对于对相位误差和磁场不均匀性敏感的 MR 程序的准确性至关重要。结果表明，三个不同供应商的临床水平孔超导 MR 扫描仪中的强烈梯度利用导致主磁场在长时间尺度上在空间和时间上变化量级为 0.8-2.5 ppm。观察到的空间变化具有线性和二次变化，沿 z 方向最强。结果表明，这种变化的影响足以完全混淆质子共振频移MR测温法测量的热相移，并且肯定会影响精度。此外，场变化会导致 MR 光谱中的信号丢失和谱线展宽，例如在 45 分钟的人脑研究过程中代谢物的谱线展宽四倍。磁场变化与磁体结构的电阻跳动一致。结论是需要校正策略来补偿相敏 MR 协议的这些空间和时间场漂移。事实证明，串行场测绘和相位差成像校正协议可以充分补偿在磁共振测温和光谱实验中观察到的漂移效应。
The homogeneity and stability of the static magnetic field are of paramount importance to the accuracy of MR procedures that are sensitive to phase errors and magnetic field inhomogeneity. It is shown that intense gradient utilization in clinical horizontal-bore superconducting MR scanners of three different vendors results in main magnetic fields that vary on a long time scale both spatially and temporally by amounts of order 0.8-2.5 ppm. The observed spatial changes have linear and quadratic variations that are strongest along the z direction. It is shown that the effect of such variations is of sufficient magnitude to completely obfuscate thermal phase shifts measured by proton-resonance frequency-shift MR thermometry and certainly affect accuracy. In addition, field variations cause signal loss and line-broadening in MR spectroscopy, as exemplified by a fourfold line-broadening of metabolites over the course of a 45 min human brain study. The field variations are consistent with resistive beating of the magnet structures. It is concluded that correction strategies are required to compensate for these spatial and temporal field drifts for phase-sensitive MR protocols. It is demonstrated that serial field mapping and phased difference imaging correction protocols can substantially compensate for the drift effects observed in the MR thermometry and spectroscopy experiments.