Goense Jozien B M, Logothetis Nikos K
Department of Physiology of Cognitive Processes, Max-Planck Institute for Biological Cybernetics, 72076 Tübingen, Germany.
Magn Reson Imaging. 2006 May;24(4):381-92. doi: 10.1016/j.mri.2005.12.032. Epub 2006 Mar 13.
The lamination of mammalian neocortex is widely used as reference for describing a wide range of anatomical and physiological data. Its value lies in the observation that in all examined species, cortical afferents, intrinsic cells and projection neurons organize themselves with respect to the laminae. The comprehension of the computations, carried out by the neocortical microcircuits, critically relies on the study of the interlaminar connectivity patterns and the intralaminar physiological processes in vivo. High-resolution functional neuroimaging, enabling the visualization of activity in individual cortical laminae or columns, may greatly contribute in such studies. Yet, the BOLD effect, as measured with the commonly used GE-EPI, contains contributions from both macroscopic venous blood vessels and capillaries. The low density of the cortical veins limits the effective spatial specificity of the fMRI signal and yields maps that are weighted toward the macrovasculature, which thus can be significantly different from the actual site of increased neuronal activity. Spin-echo (SE) sequences yielding apparent T2-weighted BOLD images have been shown to improve spatial specificity by increasing the sensitivity of the signal to spins of the parenchyma, particularly at high magnetic fields. Here we used SE-fMRI at 4.7 T to examine the specificity and resolution of functional maps obtained by stimulating the primary visual cortex of monkeys. Cortical layers could be clearly visualized, and functional activity was predominantly localized in cortical layer IV/Duvernoy layer 3. The choice of sequence parameters influences the fMRI signal, as the SE-EPI is by nature sensitive to T2* in addition to its T2 dependency. Using parameters that limit T2* effects yielded higher specificity and better visualization of the cortical laminae. Because the demands of high-spatial resolution using SE severely decreases temporal resolution, we used a stimulus protocol that allows sampling at higher effective temporal resolution. This way, it was possible to acquire high-spatial and high-temporal resolution SE-fMRI data.
哺乳动物新皮层的分层被广泛用作描述各种解剖学和生理学数据的参考。其价值在于观察到，在所有被研究的物种中，皮质传入神经、固有细胞和投射神经元都相对于各层进行自我组织。对新皮层微电路所执行的计算的理解，严重依赖于对层间连接模式和体内层内生理过程的研究。高分辨率功能神经成像能够可视化单个皮质层或柱中的活动，可能会对这类研究做出巨大贡献。然而，用常用的梯度回波平面成像（GE-EPI）测量的血氧水平依赖（BOLD）效应包含来自宏观静脉血管和毛细血管的贡献。皮质静脉的低密度限制了功能磁共振成像（fMRI）信号的有效空间特异性，并产生偏向大血管系统的图谱，因此可能与神经元活动增加的实际部位有显著差异。已证明产生表观T2加权BOLD图像的自旋回波（SE）序列通过提高信号对实质自旋的敏感性来改善空间特异性，特别是在高磁场下。在这里，我们使用4.7T的SE-fMRI来检查通过刺激猴子的初级视觉皮层获得的功能图谱的特异性和分辨率。皮质层可以清晰地可视化，并且功能活动主要定位在皮质第IV层/迪韦尔诺伊第3层。序列参数的选择会影响fMRI信号，因为SE-EPI本质上除了依赖T2外还对T2敏感。使用限制T2效应的参数可产生更高的特异性和更好的皮质层可视化效果。由于使用SE实现高空间分辨率的要求会严重降低时间分辨率，我们采用了一种允许以更高有效时间分辨率进行采样的刺激方案。通过这种方式，有可能获得高空间和高时间分辨率的SE-fMRI数据。
