Monte Carlo Simulations of Water Exchange Through Myelin Wraps: Implications for Diffusion MRI
Monte Carlo Simulations of Water Exchange Through Myelin Wraps: Implications for Diffusion MRI
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DOI：
10.1109/tmi.2019.2894398
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发表时间：
2019-06-01
影响因子：
10.6
通讯作者：
Nilsson, Markus
中科院分区：
文献类型：
--
作者：
Brusini, Lorenza;Menegaz, Gloria;Nilsson, Markus
扩散磁共振成像（dMRI）产生的参数敏感的脑组织的微观结构。这种微结构的一个重要结构方面是髓鞘包裹在轴突周围。本文研究了关于水交换是否通过螺旋结构的髓鞘可以有意义地有助于在dMRI的信号的正向问题。蒙特卡洛模拟进行了系统内轴突，髓鞘，和轴突外室。髓鞘中的扩散被模拟为缠绕轴突的螺旋，具有自定义的缠绕数量。交换（或轴突内驻留）时间进行了分析的各种数量的包裹和轴突直径。脉冲梯度序列被用来模拟dMRI信号，这是使用不同的方法进行分析。扩散峰度成像分析得到径向扩散率（RD）和径向峰度（RK），而二室Karger模型得到轴突内体积分数（v（ic））和交换时间（tau）的估计值。结果表明，在轴突直径小于1.0 μ m、缠绕数小于8圈的几何体中，tau蛋白的表达水平为亚秒级。否则，与典型的实验持续时间相比，交换可以忽略不计，τ为秒或更长。在交换影响信号的情况下，RK和v（ic）的估计值随着缠绕次数的增加而增加，而RD则减少。来自模拟信号的tau估计与预测的一致。总之，通过螺旋髓鞘的交换允许亚秒级tau用于小直径和低缠绕数。这种情况可能出现在发育中的大脑或神经退行性疾病中，因此这些结果可以帮助解释dMRI研究。
Diffusion magnetic resonance imaging (dMRI) yields parameters sensitive to brain tissue microstructure. A structurally important aspect of this microstructure is the myelin wrapping around the axons. This paper investigated the forward problem concerning whether water exchange via the spiraling structure of the myelin can meaningfully contribute to the signal in dMRI. Monte Carlo simulations were performed in a system with intra-axonal, myelin, and extra-axonal compartments. Diffusion in the myelin was simulated as a spiral wrapping the axon, with a custom number of wraps. Exchange (or intra-axonal residence) times were analyzed for various number of wraps and axon diameters. Pulsed gradient sequences were employed to simulate the dMRI signal, which was analyzed using different methods. Diffusional kurtosis imaging analysis yielded the radial diffusivity (RD) and radial kurtosis (RK), while the two-compartment Karger model yielded estimates the intra-axonal volume fraction (v(ic)) and exchange time (tau). Results showed that tau was on the sub-second level for geometrieswith axon diameters below 1.0 mu m and less than eight wraps. Otherwise, exchangewas negligible compared to typical experimental durations, with tau of seconds or longer. In situations where exchange influenced the signal, estimates of RK and v(ic) increasedwith the number of wraps, while RD decreased. tau estimates from simulated signals were in agreement with predicted ones. In conclusion, exchange through spiraling myelin permits sub-second tau for small diameters and low number of wraps. Such conditions may arise in the developing brain or in neurodegenerative disease, and thus the results could aid the interpretation of dMRI studies.