Wellcome Centre for Integrative Neuroscience - Centre for Functional Magnetic Resonance Imaging of the Brain (FMRIB), John Radcliffe Hospital, University of Oxford, UK.
Wellcome Centre for Integrative Neuroscience - Centre for Functional Magnetic Resonance Imaging of the Brain (FMRIB), John Radcliffe Hospital, University of Oxford, UK.
Neuroimage. 2018 Aug 1;176:417-430. doi: 10.1016/j.neuroimage.2018.04.040. Epub 2018 Apr 21.
When axonal fibres approach or leave the cortex, their trajectories tend to closely follow the cortical convolutions. To quantify this tendency, we propose a three-dimensional coordinate system based on the gyral geometry. For every voxel in the brain, we define a "radial" axis orthogonal to nearby surfaces, a "sulcal" axis along the sulcal depth gradient that preferentially points from deep white matter to the gyral crown, and a "gyral" axis aligned with the long axis of the gyrus. When compared with high-resolution, in-vivo diffusion MRI data from the Human Connectome Project, we find that in superficial white matter the apparent diffusion coefficient (at b = 1000) along the sulcal axis is on average 16% larger than along the gyral axis and twice as large as along the radial axis. This is reflected in the vast majority of observed fibre orientations lying close to the tangential plane (median angular offset < 7°), with the dominant fibre orientation typically aligning with the sulcal axis. In cortical grey matter, fibre orientations transition to a predominantly radial orientation. We quantify the width and location of this transition and find strong reproducibility in test-retest data, but also a clear dependence on the resolution of the diffusion data. The ratio of radial to tangential diffusion is fairly constant throughout most of the cortex, except for a decrease of the diffusivitiy ratio in the sulcal fundi and the primary somatosensory cortex (Brodmann area 3) and an increase in the primary motor cortex (Brodmann area 4). Although only constrained by cortical folds, the proposed gyral coordinate system provides a simple and intuitive representation of white and grey matter fibre orientations near the cortex, and may be useful for future studies of white matter development and organisation.
当轴突纤维接近或离开皮质时，它们的轨迹往往紧密跟随皮质的脑回。为了量化这种趋势，我们提出了一种基于脑回几何形状的三维坐标系。对于大脑中的每个体素，我们定义了一个“径向”轴，它与附近的表面正交，一个“脑沟”轴沿着脑沟深度梯度，优先从深部白质指向脑回的冠部，还有一个“脑回”轴与脑回的长轴一致。与来自人类连接组计划的高分辨率、体内扩散 MRI 数据相比，我们发现，在浅部白质中，沿着脑沟轴的表观扩散系数（b=1000 时）平均比沿着脑回轴大 16%，比沿着径向轴大两倍。这反映在绝大多数观察到的纤维方向接近切平面（中位数角度偏移<7°），主要纤维方向通常与脑沟轴一致。在皮质灰质中，纤维方向转变为主要的径向方向。我们量化了这种转变的宽度和位置，并在测试-再测试数据中发现了很强的重现性，但也发现了对扩散数据分辨率的明显依赖。除了在脑沟底部和初级躯体感觉皮层（Brodmann 区域 3）中扩散比降低，以及在初级运动皮层（Brodmann 区域 4）中扩散比增加外，整个大部分皮质的径向与切向扩散比相当稳定。尽管仅受皮质褶皱的限制，但所提出的脑回坐标系为皮质附近的白质和灰质纤维方向提供了一种简单直观的表示方法，可能对未来的白质发育和组织研究有用。
