Magnetic Resonance Research Center, Yale University, New Haven, Connecticut, USA.
Neuroimage. 2011 Oct 15;58(4):1060-9. doi: 10.1016/j.neuroimage.2011.06.082. Epub 2011 Jul 12.
Analysis of task-evoked fMRI data ignores low frequency fluctuations (LFF) of the resting-state the BOLD signal, yet LFF of the spontaneous BOLD signal is crucial for analysis of resting-state connectivity maps. We characterized the LFF of resting-state BOLD signal at 11.7T in α-chloralose and domitor anesthetized rat brain and modeled the spontaneous signal as a scale-free (i.e., fractal) distribution of amplitude power (|A|²) across a frequency range (f) compatible with an |A(f)|² ∝ 1/f(β) model where β is the scaling exponent (or spectral index). We compared β values from somatosensory forelimb area (S1FL), cingulate cortex (CG), and caudate putamen (CPu). With α-chloralose, S1FL and CG β values dropped from ~0.7 at in vivo to ~0.1 at post mortem (p<0.0002), whereas CPu β values dropped from ~0.3 at in vivo to ~0.1 at post mortem (p<0.002). With domitor, cortical (S1FL, CG) β values were slightly higher than with α-chloralose, while subcortical (CPu) β values were similar with α-chloralose. Although cortical and subcortical β values with both anesthetics were significantly different in vivo (p<0.002), at post mortem β values in these regions were not significantly different and approached zero (i.e., range of -0.1 to 0.2). Since a water phantom devoid of susceptibility gradients had a β value of zero (i.e., random), we conclude that deoxyhemoglobin present in voxels post-sacrifice still impacts tissue water diffusion. These results suggest that in the anesthetized rat brain the LFF of BOLD signal at 11.7T follow a general 1/f(β) model of fractality where β is a variable responding to physiology. We describe typical experimental pitfalls which may elude detection of fractality in the resting-state BOLD signal.
在分析任务诱发的 fMRI 数据时，研究人员往往忽略了静息状态血氧水平依赖信号的低频波动（LFF），而自发的 BOLD 信号的 LFF 对于静息状态连接图的分析至关重要。我们在α-氯醛和安定麻醉的大鼠脑中研究了静息状态 BOLD 信号的 LFF，并将自发信号建模为幅度功率（|A|²）在频率范围内的无标度（即分形）分布（f），与|A（f）|²∝1/f（β）模型相匹配，其中β是标度指数（或谱指数）。我们比较了感觉前肢区（S1FL）、扣带皮层（CG）和尾壳核（CPu）的β 值。在α-氯醛麻醉的情况下，S1FL 和 CG 的β 值从体内的0.7 下降到死后的0.1（p<0.0002），而 CPu 的β 值从体内的0.3 下降到死后的0.1（p<0.002）。在安定麻醉的情况下，皮质（S1FL、CG）的β 值略高于α-氯醛，而皮质下（CPu）的β 值与α-氯醛相似。尽管两种麻醉剂的皮质和皮质下β 值在体内差异显著（p<0.002），但这些区域的死后β 值没有显著差异，接近零（即范围在-0.1 到 0.2 之间）。由于没有磁敏感度梯度的水模的β 值为零（即随机），我们得出结论，在牺牲后体素中仍然存在去氧血红蛋白会影响组织水扩散。这些结果表明，在麻醉大鼠脑中，11.7T 下的 BOLD 信号的 LFF 遵循分形的一般 1/f（β）模型，其中β 是一个响应生理变化的变量。我们描述了一些可能会导致静息状态 BOLD 信号的分形性无法被检测到的典型实验陷阱。
