Tomasi Dardo G, Caparelli Elisabeth C
Medical Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
J Cereb Blood Flow Metab. 2007 Jan;27(1):33-42. doi: 10.1038/sj.jcbfm.9600314. Epub 2006 Apr 12.
Brain activation maps of blood oxygenation level dependent (BOLD) signals during functional magnetic resonance imaging (fMRI) are sensitive to unwanted contributions from large vessels. Most BOLD-fMRI studies are based on a stimulus-correlated modulation of the MRI signal amplitude that is sensitive to desired microvascular effects and unwanted macrovascular effects. Aiming to suppress macrovascular effects in activation patterns, this BOLD-fMRI study evaluates brain activation during a verbal working memory task (2-back) in healthy volunteers (n=18) using the amplitude and phase components of the MRI signal. The use of the first time point as a phase reference allowed us to eliminate phase wrapping artifacts and increase the statistical power of 'phase' activation, and this information was used to filter out voxels with significant macrovascular (i.e., draining and pial veins) contribution in 'amplitude' activation patterns. Across subjects, the task produced large modulations of the relative phase in the occipital, dorsolateral prefrontal, and parietal cortices, suggesting a common distribution of draining veins in these regions across subjects, and in the rostral frontal cortex, probably associated to stimulus-correlated motion of the head. The phase filtering method partially suppressed BOLD responses in the superior and lateral prefrontal, parietal, and occipital cortices; therefore the commonly reported brain activation in these cortices during working memory tasks may include significant macrovascular contributions. This study suggests that the phase information embedded in the MRI signal can be used to suppress unwanted macrovascular contributions in fMRI studies.
在功能磁共振成像（fMRI）期间，基于血氧水平依赖（BOLD）信号的脑激活图对大血管的不必要贡献很敏感。大多数BOLD-fMRI研究基于对MRI信号幅度的刺激相关调制，这种调制对所需的微血管效应和不必要的大血管效应敏感。为了抑制激活模式中的大血管效应，这项BOLD-fMRI研究使用MRI信号的幅度和相位分量，评估了18名健康志愿者在言语工作记忆任务（2-back）期间的脑激活情况。将第一个时间点用作相位参考，使我们能够消除相位包裹伪影，并提高“相位”激活的统计功效，该信息用于在“幅度”激活模式中滤除具有显著大血管（即引流静脉和软脑膜静脉）贡献的体素。在所有受试者中，该任务在枕叶、背外侧前额叶和顶叶皮质产生了相对相位的大幅调制，表明这些区域的引流静脉在受试者之间具有共同分布，而在额叶前部皮质，可能与头部的刺激相关运动有关。相位滤波方法部分抑制了前额叶上外侧、顶叶和枕叶皮质的BOLD反应；因此，在工作记忆任务期间这些皮质中通常报道的脑激活可能包括显著的大血管贡献。这项研究表明，MRI信号中嵌入的相位信息可用于在fMRI研究中抑制不必要的大血管贡献。
