Searching for the neurite density with diffusion MRI: Challenges for biophysical modeling
Searching for the neurite density with diffusion MRI: Challenges for biophysical modeling
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DOI：
10.1002/hbm.24542
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发表时间：
2019-06-01
影响因子：
4.8
通讯作者：
Nilsson, Markus
中科院分区：
文献类型：
--
作者：
Lampinen, Bjorn;Szczepankiewicz, Filip;Nilsson, Markus
用弥散磁共振成像(DMRI)对轴突密度进行活体定位是一个高难度但具有挑战性的目标。首先，目前尚不清楚是否所有的神经突起都表现出完全的各向异性(“棒状”)扩散。第二，组织成分的“密度”可能会受到T2松弛等非扩散特性的影响。第三，对所估计的参数作为神经突起密度指标的有效范围进行了不完全的探索。我们通过使用“b-张量编码”和在方向一致性低的大脑区域和白质病变中获取多次回声时间来研究这些挑战。结果表明，b张量数据的微观各向异性与有髓轴突有关，但与树突无关。此外，b张量数据和多次回波时间采集的数据表明，白质病变的无偏密度估计需要数据驱动的间隔特定T2值的估计。最后，不同生物物理模型的“棒状”部分通常不能作为健康大脑和脑白质病变中的轴突密度指数，比较的结果取决于限制条件的选择。特别是，限制特定于脑室的T2值在健康大脑中是模棱两可的，并对估计值有很大影响。总而言之，估计轴突密度通常需要考虑轴突和树突之间不同的扩散和/或T2特性。受约束的“索引”参数可以在有限的领域内有效，这些领域应该由未来的研究来描述。
In vivo mapping of the neurite density with diffusion MRI (dMRI) is a high but challenging aim. First, it is unknown whether all neurites exhibit completely anisotropic ("stick-like") diffusion. Second, the "density" of tissue components may be confounded by non-diffusion properties such as T2 relaxation. Third, the domain of validity for the estimated parameters to serve as indices of neurite density is incompletely explored. We investigated these challenges by acquiring data with "b-tensor encoding" and multiple echo times in brain regions with low orientation coherence and in white matter lesions. Results showed that microscopic anisotropy from b-tensor data is associated with myelinated axons but not with dendrites. Furthermore, b-tensor data together with data acquired for multiple echo times showed that unbiased density estimates in white matter lesions require data-driven estimates of compartment-specific T2 values. Finally, the "stick" fractions of different biophysical models could generally not serve as neurite density indices across the healthy brain and white matter lesions, where outcomes of comparisons depended on the choice of constraints. In particular, constraining compartment-specific T2 values was ambiguous in the healthy brain and had a large impact on estimated values. In summary, estimating neurite density generally requires accounting for different diffusion and/or T2 properties between axons and dendrites. Constrained "index" parameters could be valid within limited domains that should be delineated by future studies.