Champalimaud Research, Champalimaud Centre for the Unknown, Lisbon, Portugal.
Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, United States.
Elife. 2020 Feb 12;9:e49855. doi: 10.7554/eLife.49855.
Axon caliber plays a crucial role in determining conduction velocity and, consequently, in the timing and synchronization of neural activation. Noninvasive measurement of axon radii could have significant impact on the understanding of healthy and diseased neural processes. Until now, accurate axon radius mapping has eluded in vivo neuroimaging, mainly due to a lack of sensitivity of the MRI signal to micron-sized axons. Here, we show how - when confounding factors such as extra-axonal water and axonal orientation dispersion are eliminated - heavily diffusion-weighted MRI signals become sensitive to axon radii. However, diffusion MRI is only capable of estimating a single metric, the effective radius, representing the entire axon radius distribution within a voxel that emphasizes the larger axons. Our findings, both in rodents and humans, enable noninvasive mapping of critical information on axon radii, as well as resolve the long-standing debate on whether axon radii can be quantified.
轴突口径在决定传导速度方面起着至关重要的作用，因此，对神经激活的时间和同步性也有影响。非侵入性测量轴突半径可能对理解健康和患病的神经过程有重大影响。到目前为止，活体神经影像学一直未能准确地映射轴突半径，主要是因为 MRI 信号对微米级轴突的敏感性不足。在这里，我们展示了在消除了诸如轴外水和轴突方向分散等混杂因素的情况下，重度扩散加权 MRI 信号如何变得对轴突半径敏感。然而，扩散 MRI 只能估计单个度量，即有效半径，代表一个体素内整个轴突半径分布，强调较大的轴突。我们在啮齿动物和人类中的发现，使我们能够无创地绘制有关轴突半径的关键信息图，并解决了关于是否可以量化轴突半径的长期争论。
