Centre for functional MRI of the Brain (FMRIB), University of Oxford, Oxford, UK.
Neuroimage. 2011 Aug 15;57(4):1364-75. doi: 10.1016/j.neuroimage.2011.05.070. Epub 2011 Jun 1.
Although 2D echo-planar imaging (EPI) remains the dominant method for functional MRI (FMRI), 3D readouts are receiving more interest as these sequences have favorable signal-to-noise ratio (SNR) and enable imaging at a high isotropic resolution. Spoiled gradient-echo (SPGR) and balanced steady-state free-precession (bSSFP) are rapid sequences that are typically acquired with highly segmented 3D readouts, and thus less sensitive to image distortion and signal dropout. They therefore provide a powerful alternative for FMRI in areas with strong susceptibility offsets, such as deep gray matter structures and the brainstem. Unfortunately, the multi-shot nature of the readout makes these sequences highly sensitive to physiological fluctuations, and large signal instabilities are observed in the inferior regions of the brain. In this work a characterization of the source of these instabilities is given and a new method is presented to reduce the instabilities observed in 3D SPGR and bSSFP. Rapidly acquired single-slice data, which critically sampled the respiratory and cardiac waveforms, showed that cardiac pulsation is the dominant source of the instabilities. Simulations further showed that synchronizing the readout to the cardiac cycle minimizes the instabilities considerably. A real-time synchronization method was therefore developed, which utilizes parallel-imaging techniques to allow cardiac synchronization without alteration of the volume acquisition rate. The implemented method significantly improves the temporal stability in areas that are affected by cardiac-related signal fluctuations. In bSSFP data the tSNR in the brainstem increased by 45%, at the cost of a small reduction in tSNR in the cortical areas. In SPGR the temporal stability is improved by approximately 20% in the subcortical structures and as well as cortical gray matter when synchronization was performed.
尽管二维 echo-planar 成像（EPI）仍然是功能磁共振成像（fMRI）的主要方法，但 3D 读取技术越来越受到关注，因为这些序列具有良好的信噪比（SNR），并能够以高各向同性分辨率进行成像。扰相梯度回波（SPGR）和平衡稳态自由进动（bSSFP）是快速序列，通常使用高度分段的 3D 读取技术获取，因此对图像失真和信号丢失不太敏感。因此，它们为具有强磁化率偏移的区域（如深部灰质结构和脑干）的 fMRI 提供了强大的替代方法。不幸的是，读取技术的多拍性质使这些序列对生理波动非常敏感，并且在大脑的下部区域观察到大的信号不稳定性。在这项工作中，给出了这些不稳定性的来源的特征描述，并提出了一种新的方法来减少 3D SPGR 和 bSSFP 中观察到的不稳定性。快速获取的单切片数据对呼吸和心脏波形进行了临界采样，结果表明心脏搏动是不稳定性的主要来源。模拟进一步表明，将读取技术与心脏周期同步可以大大减少不稳定性。因此，开发了一种实时同步方法，该方法利用并行成像技术在不改变容积采集率的情况下实现心脏同步。所实现的方法显著改善了受心脏相关信号波动影响的区域的时间稳定性。在 bSSFP 数据中，脑干的 tSNR 增加了 45%，而皮质区域的 tSNR 略有降低。在 SPGR 中，当进行同步时，亚皮质结构和皮质灰质的时间稳定性分别提高了约 20%。
