High-Field MR Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany.
Department for Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany.
Magn Reson Med. 2021 Oct;86(4):2076-2083. doi: 10.1002/mrm.28871. Epub 2021 May 31.
To assess the vessel size specificity and sensitivity of rapid CPMG and GRASE for functional BOLD imaging for different echo train lengths, echo spacings, field strength, and refocusing flip angle schemes. In addition, the behavior of signals acquired before and after the refocusing time points is analyzed.
Evolution of magnetization within a network of artificial cylinders is simulated with Monte Carlo methods for all relevant coherence pathways. In addition, measurements on microspheres were performed to confirm some of the theoretical results.
For reduced refocusing flip angles, the peak of the vessel size sensitivity curve is shifting toward larger radii with increasing echo time. Furthermore, the BOLD-related signal change along the echo train depends on the chosen refocusing flip angle scheme and in general does not follow corresponding echo amplitudes.
CPMG or GRASE can be used with low refocusing flip angles without significant loss of sensitivity to BOLD. The evolution of BOLD signal changes along the echo train can be used to design optimal k-space reordering schemes. Signals acquired before or after the spin echo time point show contributions from larger vessels similar to gradient echo sequences. Short echo spacing (time between refocusing pulses) suppresses gradient echo-related contributions from larger vessels, whereas the spin echo-related contribution from small vessels remains constant and is independent of the echo spacing.
评估快速 CPMG 和 GRASE 对不同回波链长度、回波间隔、场强和重聚焦翻转角方案的功能 BOLD 成像的血管大小特异性和敏感性。此外，还分析了在重聚焦时间点前后采集的信号的行为。
采用蒙特卡罗方法模拟了人工圆柱体网络内的磁化演变，用于所有相关的相干途径。此外，还对微球进行了测量，以验证一些理论结果。
对于减小的重聚焦翻转角，血管大小敏感性曲线的峰值随着回波时间的增加而向更大的半径移动。此外，沿着回波链的 BOLD 相关信号变化取决于所选的重聚焦翻转角方案，通常不遵循相应的回波幅度。
CPMG 或 GRASE 可以与低重聚焦翻转角一起使用，而不会对 BOLD 的灵敏度有显著损失。可以使用沿 echo 链的 BOLD 信号变化来设计最佳的 k 空间重排序方案。在自旋回波时间点之前或之后采集的信号显示出与梯度回波序列相似的来自较大血管的贡献。短回波间隔（重聚焦脉冲之间的时间）抑制了来自较大血管的梯度回波相关贡献，而来自小血管的自旋回波相关贡献保持不变，且与回波间隔无关。
