Shmuel Amir, Yacoub Essa, Chaimow Denis, Logothetis Nikos K, Ugurbil Kamil
Center for MR Research, University of Minnesota Medical School, 2021 6th St. SE, Minneapolis, MN, USA.
Neuroimage. 2007 Apr 1;35(2):539-52. doi: 10.1016/j.neuroimage.2006.12.030. Epub 2007 Jan 4.
This study investigated the spatio-temporal properties of blood-oxygenation level-dependent (BOLD) functional MRI (fMRI) signals in gray matter, excluding the confounding, inaccurate contributions of large blood vessels. We quantified the spatial specificity of the BOLD response, and we investigated whether this specificity varies as a function of time from stimulus onset. fMRI was performed at 7 Tesla (T), where mapping signals of parenchymal origin are easily detected. Two abutting visual stimuli were adjusted to elicit responses centered on a flat gray matter region in V1. fMRI signals were sampled at high-resolution orthogonal to the retinotopic boundary between the representations of the stimuli. Signals from macro-vessels were masked out. Principal component analysis revealed that the first component in space accounted for 96.2+/-1.6% of the variance over time. The spatial profile of this time-invariant response was fitted with a model consisting of the convolution of a step function and a Gaussian point-spread-function (PSF). The mean full-width at half-maximal-height of the fitted PSF was 2.34+/-0.20 mm. Based on simulations of confounding effects, we estimate that BOLD PSF in human gray matter is smaller than 2 mm. A time-point to time-point analysis revealed that the PSF obtained during the 3rd (1.52 mm) and 4th (1.99 mm) seconds of stimulation were narrower than the mean PSF obtained from the 5th second on (2.42+/-0.15 mm). The position of the edge of the responding region was offset (1.72+/-0.07 mm) from the boundary of the stimulated region, indicating a spatial non-linearity. Simulations showed that the effective contrast between active and non-active columns is reduced 25-fold when imaged using a PSF whose width is equal to the cycle of the imaged columnar organization. Thus, the PSF of the hyper-oxygenated BOLD response in human gray matter is narrower than that reported at 1.5 T, where macro-vessels dominate the mapping signals. The initial phase of this response is more spatially specific than later phases. Data acquisition methods that suppress macro-vascular signals should increase the spatial specificity of BOLD fMRI. The choice of optimal stimulus duration represents a trade-off between the spatial specificity and the overhead associated with short stimulus duration.
本研究调查了灰质中血氧水平依赖（BOLD）功能磁共振成像（fMRI）信号的时空特性，排除了大血管产生的混淆性、不准确的信号贡献。我们量化了BOLD反应的空间特异性，并研究了这种特异性是否随刺激开始后的时间而变化。fMRI在7特斯拉（T）下进行，在此场强下，来自实质组织的信号很容易被检测到。调整两个相邻的视觉刺激，以引发以V1区平坦灰质区域为中心的反应。fMRI信号在与刺激表征之间的视网膜拓扑边界正交的高分辨率下进行采样。来自大血管的信号被屏蔽掉。主成分分析表明，空间中的第一个成分在时间上占方差的96.2±1.6%。这个随时间不变的反应的空间分布用一个由阶跃函数和高斯点扩散函数（PSF）卷积组成的模型进行拟合。拟合后的PSF的半高宽均值为2.34±0.20毫米。基于对混淆效应的模拟，我们估计人类灰质中的BOLD PSF小于2毫米。逐点分析表明，在刺激的第3秒（1.52毫米）和第4秒（1.99毫米）期间获得的PSF比从第5秒开始获得的平均PSF（2.42±0.15毫米）更窄。反应区域边缘的位置相对于刺激区域的边界偏移了（1.72±0.07毫米），表明存在空间非线性。模拟表明，当使用宽度等于成像柱状组织周期的PSF进行成像时，活跃柱和非活跃柱之间的有效对比度降低了25倍。因此，人类灰质中高氧BOLD反应的PSF比在1.5 T时报道的更窄，在1.5 T时大血管主导映射信号。该反应的初始阶段在空间上比后期阶段更具特异性。抑制大血管信号的数据采集方法应能提高BOLD fMRI的空间特异性。最佳刺激持续时间的选择代表了空间特异性与短刺激持续时间相关的额外成本之间的权衡。
