Andersson Mariam, Pizzolato Marco, Kjer Hans Martin, Skodborg Katrine Forum, Lundell Henrik, Dyrby Tim B
Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Amager and Hvidovre, Copenhagen 2650, Denmark; Department of Applied Mathematics and Computer Science, Technical University of Denmark, Kongens Lyngby 2800, Denmark.
Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Amager and Hvidovre, Copenhagen 2650, Denmark; Department of Applied Mathematics and Computer Science, Technical University of Denmark, Kongens Lyngby 2800, Denmark; Signal Processing Laboratory (LTS5), École Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland.
Neuroimage. 2022 Mar;248:118718. doi: 10.1016/j.neuroimage.2021.118718. Epub 2021 Nov 10.
Noninvasive estimation of axon diameter with diffusion MRI holds the potential to investigate the dynamic properties of the brain network and pathology of neurodegenerative diseases. Recent studies use powder averaging to account for complex white matter architectures, but these have not been validated for real axonal geometries from regions that contain fibre crossings. Here, we present 120-304μm long segmented axons from X-ray nano-holotomography volumes of a splenium and crossing fibre region of a vervet monkey brain. We show that the axons in the complex crossing fibre region, which contains callosal, association, and corticospinal connections, exhibit a wider diameter distribution than those of the splenium region. To accurately estimate the axon diameter in these regions, therefore, sensitivity to a wide range of diameters is required. We demonstrate how the q-value, b-value, signal-to-noise ratio and the assumed intra-axonal parallel diffusivity influence the range of measurable diameters with powder average approaches. Furthermore, we show how Gaussian distributed noise results in a wider range of measurable diameter at high b-values than Rician distributed noise, even at high signal-to-noise ratios of 100. The number of gradient directions is also shown to impose a lower bound on measurable diameter. Our results indicate that axon diameter estimation can be performed with only few b-shells, and that additional shells do not improve the accuracy of the estimate. For strong gradients available on human Connectom and preclinical scanners, Monte Carlo simulations of diffusion confirm that powder averaging techniques succeed in providing accurate estimates of axon diameter across a range of diameters, sequence parameters and diffusion times, even in complex white matter architectures. At relatively low b-values, the diameter estimate becomes sensitive to axonal microdispersion and the intra-axonal parallel diffusivity shows time dependency at both in vivo and ex vivo intrinsic diffusivities.
利用扩散磁共振成像无创估计轴突直径，有望研究脑网络的动态特性和神经退行性疾病的病理学。最近的研究使用粉末平均法来考虑复杂的白质结构，但这些方法尚未针对包含纤维交叉区域的真实轴突几何结构进行验证。在这里，我们展示了来自黑长尾猴脑胼胝体和交叉纤维区域的X射线纳米全息断层扫描体积中120 - 304μm长的分段轴突。我们表明，在包含胼胝体、联合和皮质脊髓连接的复杂交叉纤维区域中的轴突，其直径分布比胼胝体区域的轴突更宽。因此，为了准确估计这些区域的轴突直径，需要对广泛的直径范围具有敏感性。我们展示了q值、b值、信噪比以及假定的轴突内平行扩散率如何通过粉末平均法影响可测量直径的范围。此外，我们还展示了即使在信噪比高达100时，高斯分布噪声在高b值下导致的可测量直径范围比莱斯分布噪声更宽。梯度方向的数量也被证明对可测量直径施加了下限。我们的结果表明，仅用少数几个b壳层就能进行轴突直径估计，额外的壳层并不能提高估计的准确性。对于人类连接体和临床前扫描仪上可用的强梯度，扩散的蒙特卡罗模拟证实，即使在复杂的白质结构中，粉末平均技术也能成功地在一系列直径、序列参数和扩散时间内提供准确的轴突直径估计。在相对较低的b值下，直径估计对轴突微分散变得敏感，并且轴突内平行扩散率在体内和体外固有扩散率下均显示出时间依赖性。
