论文标题
使用扩散MRI逼真的轴突形对轴突直径估计的影响
The impact of realistic axonal shape on axon diameter estimation using diffusion MRI
论文作者
论文摘要
为了研究具有扩散MRI的轴突微观结构，通常将轴突建模为直接不可渗透的圆柱体，从而使横向扩散MRI信号可以使圆柱体的内径敏感。但是，真实轴突的形状沿轴突方向变化，轴突方向将整个轴突方向的纵向和横向扩散构成。在这里，我们基于3D扩散基于轴突形状的粗粒形成轴内扩散MRI信号的理论。我们证明了内径的估计值如何被直径变化（串珠）以及沿轴突方向（悬垂）的局部变化混淆。我们通过分析扩散MRI指标，例如时间依赖性的径向扩散率D（T）和峰度K（T），与轴突形状相关，并使用蒙特卡洛模拟在合成的轴突中验证我们的理论，这些轴突具有随机定位珠的合成轴突，以及从真实的轴突中构造了来自Electon Brain Mainsicpy Imales White Phite Morse White White Matery White Phiteshithy White White Phiteshite hyder White White Thictle Matery White Phicter Phite Phicter White White Trainter Phister Phite tosem nouss White White Trainter tosem nouss White Phicte toss的理论。我们表明（i）在狭窄的脉冲极限中，由于轴突机芯变化和起伏的组合，D（t）的内径被高估了大约两倍（每个效果大小）； （ii）狭窄的脉冲kurtosis k $ _ {t \ to \ infty} $由于口径变化而在理想的圆柱体中与之偏离；我们还以数值计算宽脉冲极限的理想圆柱体的四阶累积剂，这与内径高估有关。 （iii）在宽的脉冲极限中，轴突直径高估主要是由于低扩散权重B处的起伏。 （iv）可以通过高-B信号的方向平均来大大降低起伏的影响，而轴突机芯（由最厚的轴突主导），机芯变化和剩余贡献的轴突机芯的组合给出了明显的内径。
To study axonal microstructure with diffusion MRI, axons are typically modeled as straight impermeable cylinders, whereby the transverse diffusion MRI signal can be made sensitive to the cylinder's inner diameter. However, the shape of a real axon varies along the axon direction, which couples the longitudinal and transverse diffusion of the overall axon direction. Here we develop a theory of the intra-axonal diffusion MRI signal based on coarse-graining of the axonal shape by 3d diffusion. We demonstrate how the estimate of the inner diameter is confounded by the diameter variations (beading), and by the local variations in direction (undulations) along the axon. We analytically relate diffusion MRI metrics, such as time-dependent radial diffusivity D(t) and kurtosis K(t), to the axonal shape, and validate our theory using Monte Carlo simulations in synthetic undulating axons with randomly positioned beads, and in realistic axons reconstructed from electron microscopy images of mouse brain white matter. We show that (i) In the narrow pulse limit, the inner diameter from D(t) is overestimated by about twofold due to a combination of axon caliber variations and undulations (each contributing a comparable effect size); (ii) The narrow-pulse kurtosis K$_{t\to\infty}$ deviates from that in an ideal cylinder due to caliber variations; we also numerically calculate the fourth-order cumulant for an ideal cylinder in the wide pulse limit, which is relevant for inner diameter overestimation; (iii) In the wide pulse limit, the axon diameter overestimation is mainly due to undulations at low diffusion weightings b; and (iv) The effect of undulations can be considerably reduced by directional averaging of high-b signals, with the apparent inner diameter given by a combination of the axon caliber (dominated by the thickest axons), caliber variations, and the residual contribution of undulations.
