Sir Peter Mansfield Magnetic Resonance Centre, School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, UK.
Neuroimage. 2011 Feb 1;54(3):1942-50. doi: 10.1016/j.neuroimage.2010.09.079. Epub 2010 Oct 13.
Large artefacts that compromise EEG data quality are generated when electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) are carried out concurrently. The gradient artefact produced by the time-varying magnetic field gradients is the largest of these artefacts. Although average artefact correction (AAS) and related techniques can remove the majority of this artefact, the need to avoid amplifier saturation necessitates the use of a large dynamic range and strong low-pass filtering in EEG recording. Any intrinsic reduction in the gradient artefact amplitude would allow data with a higher bandwidth to be acquired without amplifier saturation, thus increasing the frequency range of neuronal activity that can be investigated using combined EEG-fMRI. Furthermore, gradient artefact correction methods assume a constant artefact morphology over time, so their performance is compromised by subject movement. Since the resulting, residual gradient artefacts can easily swamp signals from brain activity, any reduction in their amplitude would be highly advantageous for simultaneous EEG-fMRI studies. The aim of this work was to investigate whether adjustment of the subject's axial position in the MRI scanner can reduce the amplitude of the induced gradient artefact, before and after artefact correction using AAS. The variation in gradient artefact amplitude as a function of the subject's axial position was first investigated in six subjects by applying gradient pulses along the three Cartesian axes. The results of this study showed that a significant reduction in the gradient artefact magnitude can be achieved by shifting the subject axially by 4 cm towards the feet relative to the standard subject position (nasion at iso-centre). In a further study, the 4-cm shift was shown to produce a 40% reduction in the RMS amplitude (and a 31% reduction in the range) of the gradient artefact generated during the execution of a standard multi-slice, EPI sequence. By picking out signals occurring at harmonics of the slice acquisition frequency, it was also shown that the 4-cm shift led to a 36% reduction in the residual gradient artefact after AAS. Functional and anatomical MR data quality is not affected by the 4-cm shift, as the head remains in the homogeneous region of the static magnet field and gradients.
当同时进行脑电图 (EEG) 和功能磁共振成像 (fMRI) 时，会产生影响 EEG 数据质量的大伪影。时变磁场梯度产生的梯度伪影是这些伪影中最大的。虽然平均伪影校正 (AAS) 和相关技术可以去除大部分伪影，但为了避免放大器饱和，需要在 EEG 记录中使用大动态范围和强低通滤波。梯度伪影幅度的任何内在降低都将允许采集具有更高带宽的数据而不会发生放大器饱和，从而增加可以使用组合 EEG-fMRI 研究的神经元活动的频率范围。此外，梯度伪影校正方法假设伪影形态随时间保持恒定，因此其性能会受到受试者运动的影响。由于产生的残余梯度伪影很容易淹没来自脑活动的信号，因此降低其幅度对于同时进行 EEG-fMRI 研究将非常有利。本工作的目的是研究在使用 AAS 进行伪影校正之前和之后，通过调整 MRI 扫描仪中受试者的轴向位置是否可以降低感应梯度伪影的幅度。通过沿三个笛卡尔轴施加梯度脉冲，首先在 6 名受试者中研究了梯度伪影幅度随受试者轴向位置的变化。该研究的结果表明，相对于标准受试者位置（鼻根在等中心），将受试者轴向移动 4 厘米朝向脚部可以显著降低梯度伪影的幅度。在进一步的研究中，显示 4 厘米的偏移导致在执行标准多切片 EPI 序列期间产生的梯度伪影的 RMS 幅度降低 40%（范围降低 31%）。通过挑选出在切片采集频率的谐波处发生的信号，还表明 AAS 后 4 厘米的偏移导致残余梯度伪影减少 36%。4 厘米的偏移不会影响功能和解剖学磁共振数据的质量，因为头部仍然处于静磁场和梯度的均匀区域内。
