使用分段加速VFA-FLEET和递归RF脉冲设计的超高空间分辨率BOLD fMRI
Ultra-high spatial resolution BOLD fMRI in humans using combined segmented-accelerated VFA-FLEET with a recursive RF pulse design
摘要：
目的通过开发用于超高分辨率fMRI的多激发分段EPI，以减少受试者运动和呼吸引起的重影伪影，从而减轻单激发EPI的空间编码限制。方法分段式EPI可以缩短读出持续时间并降低加速因子，然而，分段采集之间经过的时间（以秒为单位）可能会导致间歇性重影，从而限制其在fMRI中的使用。在这里，“FLEET”片段排序-片段在切片之前循环-与可变翻转角进展（VFA-FLEET）相结合，以提高片段间保真度并最大化fMRI信号。缩放每个片段的sinc脉冲翻转角（VFA-FLEET-Sinc）会产生不一致的切片轮廓和重影，因此，开发了递归Shinnar-Le Roux（SLR）RF脉冲设计（VFA-FLEET-SLR），为每个片段生成独特的脉冲，共同产生一致的切片轮廓和信号。结果比较了VFA FLEET SLR与常规分段EPI和VFA FLEET Sinc在3 T和7 T下的时间稳定性。VFA-FLEET-SLR与常规分割和VFA-FLEET-Sinc相比，间歇性和稳定性重影均有所减少，从而提高了图像质量，同时在时间SNR方面略有折衷。结合VFA-FLEET-SLR与加速度，我们在7 T下实现了0.6 mm各向同性采集-没有缩放成像或部分傅立叶-证明了对视觉刺激的BOLD响应的可靠检测。为了抵消分割导致的重复时间增加，使用RF编码控制混叠演示了同步多切片VFA-FLEET-SLR。结论：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，使fMRI能够以以前无法达到的空间分辨率和“全脑”视野进行。
英文摘要：
Purpose To alleviate the spatial encoding limitations of single-shot EPI by developing multi-shot segmented EPI for ultra-high-resolution fMRI with reduced ghosting artifacts from subject motion and respiration. Methods 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) RF pulse design was developed (VFA-FLEET-SLR) to generate unique pulses for every segment that together produce consistent slice profiles and signals. Results The temporal stability of VFA-FLEET-SLR was compared against conventional-segmented EPI and VFA-FLEET-Sinc at 3 T and 7 T. 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 7 T--without zoomed imaging or partial Fourier--demonstrating reliable detection of 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. Conclusions 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.
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