Wang Yicun, van Gelderen Peter, de Zwart Jacco A, Campbell-Washburn Adrienne E, Duyn Jeff H
Advanced MRI Section, Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA.
Cardiovascular Branch, Division of Intramural Research, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland, USA.
Magn Reson Med. 2021 Jun;85(6):3196-3210. doi: 10.1002/mrm.28657. Epub 2021 Jan 21.
Low-field (<1 tesla) MRI scanners allow more widespread diagnostic use for a range of cardiac, musculoskeletal, and neurological applications. However, the feasibility of performing robust fMRI at low field has yet to be fully demonstrated. To address this gap, we investigated task-based fMRI using a highly sensitive transition-band balanced steady-state free precession approach and standard EPI on a 0.55 tesla scanner equipped with modern high-performance gradient coils and a receive array.
TR and flip-angle of transition-band steady-state free precession were optimized for 0.55 tesla by simulations. Static shimming was employed to compensate for concomitant field effects. Visual task-based fMRI data were acquired from 8 healthy volunteers. For comparison, standard EPI data were also acquired with TE = . Retrospective image-based correction for physiological effects (RETROICOR) was used to quantify physiological noise effects.
Activation was robustly detected using both methods in a 4-min scan time. Transition-band steady-state free precession was found to be sensitive to interference from subtle spatial and temporal (field drift, respiration) variations in the magnetic field, counteracting potential advantages of the reduced magnetic susceptibility effects compared to its utilization at high field. These adverse effects could be partially remedied with static shimming and postprocessing approaches. Standard EPI proved more robust against the sources of interference.
BOLD contrast is sufficiently large at 0.55 tesla for robust detection of brain activation and may be employed to broaden the spectrum of applications of low-field MRI. Standard EPI outperforms transition-band steady-state free precession in terms of signal stability.
低场（<1特斯拉）MRI扫描仪可在一系列心脏、肌肉骨骼和神经应用中实现更广泛的诊断用途。然而，在低场进行可靠的功能磁共振成像（fMRI）的可行性尚未得到充分证明。为了填补这一空白，我们在一台配备现代高性能梯度线圈和接收阵列的0.55特斯拉扫描仪上，使用高灵敏度过渡带平衡稳态自由进动方法和标准回波平面成像（EPI）研究了基于任务的fMRI。
通过模拟为0.55特斯拉优化过渡带稳态自由进动的重复时间（TR）和翻转角。采用静态匀场来补偿伴随场效应。从8名健康志愿者获取基于视觉任务的fMRI数据。为作比较，还采集了TE = 的标准EPI数据。使用基于图像的生理效应回顾性校正（RETROICOR）来量化生理噪声效应。
在4分钟的扫描时间内，两种方法均能可靠地检测到激活。发现过渡带稳态自由进动对磁场中细微的空间和时间（场漂移、呼吸）变化的干扰敏感，抵消了与在高场使用相比降低的磁化率效应的潜在优势。这些不利影响可通过静态匀场和后处理方法部分得到补救。标准EPI对干扰源的耐受性更强。
在0.55特斯拉时，血氧水平依赖（BOLD）对比度足够大，可可靠地检测脑激活，可用于拓宽低场MRI的应用范围。在信号稳定性方面标准EPI优于过渡带稳态自由进动。
