Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, Massachusetts, USA.
Department of Radiology, Harvard Medical School, Boston, Massachusetts, USA.
Magn Reson Med. 2021 Jan;85(1):120-139. doi: 10.1002/mrm.28415. Epub 2020 Jul 23.
To alleviate the spatial encoding limitations of single-shot echo-planar imaging (EPI) by developing multi-shot segmented EPI for ultra-high-resolution functional MRI (fMRI) with reduced ghosting artifacts from subject motion and respiration.
Segmented EPI can reduce readout duration and reduce acceleration factors, however, the time elapsed between segment acquisitions (on the order of seconds) can result in intermittent ghosting, limiting its use for fMRI. Here, "FLEET" segment ordering, where segments are looped over before slices, was combined with a variable flip angle progression (VFA-FLEET) to improve inter-segment fidelity and maximize signal for fMRI. Scaling a sinc pulse's flip angle for each segment (VFA-FLEET-Sinc) produced inconsistent slice profiles and ghosting, therefore, a recursive Shinnar-Le Roux (SLR) radiofrequency (RF) pulse design was developed (VFA-FLEET-SLR) to generate unique pulses for every segment that together produce consistent slice profiles and signals.
The temporal stability of VFA-FLEET-SLR was compared against conventional-segmented EPI and VFA-FLEET-Sinc at 3T and 7T. VFA-FLEET-SLR showed reductions in both intermittent and stable ghosting compared to conventional-segmented and VFA-FLEET-Sinc, resulting in improved image quality with a minor trade-off in temporal SNR. Combining VFA-FLEET-SLR with acceleration, we achieved a 0.6-mm isotropic acquisition at 7T, without zoomed imaging or partial Fourier, demonstrating reliable detection of blood oxygenation level-dependent (BOLD) responses to a visual stimulus. To counteract the increased repetition time from segmentation, simultaneous multi-slice VFA-FLEET-SLR was demonstrated using RF-encoded controlled aliasing.
VFA-FLEET with a recursive RF pulse design supports acquisitions with low levels of artifact and spatial blur, enabling fMRI at previously inaccessible spatial resolutions with a "full-brain" field of view.
通过开发用于超高分辨率功能磁共振成像（fMRI）的多段分段 EPI，减轻单次回波平面成像（EPI）的空间编码限制，减少由于受试者运动和呼吸引起的鬼影伪影。
分段 EPI 可以减少读出时间并降低加速因子，但是，段之间的时间间隔（几秒钟）可能会导致间歇性鬼影，限制其在 fMRI 中的应用。在这里，“FLEET”段排序，其中段在切片之前循环，与可变翻转角进展（VFA-FLEET）相结合，以提高段间保真度并最大化 fMRI 的信号。为每个段缩放 sinc 脉冲的翻转角（VFA-FLEET-Sinc）会产生不一致的切片轮廓和鬼影，因此，开发了递归 Shinnar-Le Roux（SLR）射频（RF）脉冲设计（VFA-FLEET-SLR），为每个段生成独特的脉冲，这些脉冲一起产生一致的切片轮廓和信号。
在 3T 和 7T 下，将 VFA-FLEET-SLR 的时间稳定性与传统分段 EPI 和 VFA-FLEET-Sinc 进行了比较。与传统分段和 VFA-FLEET-Sinc 相比，VFA-FLEET-SLR 减少了间歇性和稳定鬼影，从而在 temporal SNR 略有下降的情况下提高了图像质量。将 VFA-FLEET-SLR 与加速相结合，我们在 7T 实现了 0.6 毫米各向同性采集，无需缩放成像或部分傅里叶变换，显示了可靠地检测到视觉刺激的血氧水平依赖（BOLD）反应。为了抵消分段引起的重复时间增加，使用 RF 编码的受控混叠演示了同时多切片 VFA-FLEET-SLR。
具有递归 RF 脉冲设计的 VFA-FLEET 支持具有低水平伪影和空间模糊的采集，从而能够以以前无法达到的空间分辨率实现“全脑”视野的 fMRI。
