Separating blood and water: Perfusion and free water elimination from diffusion MRI in the human brain.
Separating blood and water: Perfusion and free water elimination from diffusion MRI in the human brain.
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DOI：
10.1016/j.neuroimage.2017.04.023
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发表时间：
2017-08-01
期刊：
影响因子：
5.7
通讯作者：
Pasternak O
中科院分区：
文献类型：
--
作者：
Rydhög AS;Szczepankiewicz F;Wirestam R;Ahlgren A;Westin CF;Knutsson L;Pasternak O
脑中游离水分数的评估提供了关于细胞外过程的重要信息，例如各种临床状况以及正常发育和衰老中的萎缩和神经炎症。假设来自扩散MRI的游离水估计值解释了细胞外空间中自由扩散的水分子，但可能会受到快速随机运动中的其他分子池的影响，例如血液的体素内非相干运动（IVIM），其中水分子灌注在随机取向的毛细血管网络中。这项工作的目标是分离的自由水和其他脑扩散的灌注血液的信号贡献。通过模拟弥散MRI数据中的灌注，研究了血管腔室对游离水分数和其他弥散系数估计的影响。模拟中的灌注效应是显著的，特别是对于游离水分数的估计，并且只要分析中包括低b值数据，就保持灌注效应。本研究探索了两种降低灌注效应的方法：（i）增加拟合中使用的最小b值，以及（ii）使用明确考虑毛细血管血液中水分子的三室模型。排除低b值的模型参数估计降低了灌注效应，但对噪声高度敏感。三室模型拟合更稳定，此外，还提供了毛细血管血室体积分数的估计值。因此，三室模型解开了自由水扩散和灌注的影响，这具有重要的临床意义，因为脑中这些组分的变化可能指示不同的病理学，即，起源于细胞外间隙的那些，如神经炎症和萎缩，以及与血管间隙相关的那些，如血管舒张、血管收缩和毛细血管密度。使用多个b壳从健康志愿者中采集的弥散MRI数据表明，血液组分的预期非零贡献，并表明未考虑灌注效应可能解释了先前研究中显示的游离水组分的高估。最后，该方法的适用性证明了使用临床可行的协议，具有更短的采集时间和更少的b壳采集的数据集。
The assessment of the free water fraction in the brain provides important information about extracellular processes such as atrophy and neuroinflammation in various clinical conditions as well as in normal development and aging. Free water estimates from diffusion MRI are assumed to account for freely diffusing water molecules in the extracellular space, but may be biased by other pools of molecules in rapid random motion, such as the intravoxel incoherent motion (IVIM) of blood, where water molecules perfuse in the randomly oriented capillary network. The goal of this work was to separate the signal contribution of the perfusing blood from that of free-water and of other brain diffusivities. The influence of the vascular compartment on the estimation of the free water fraction and other diffusivities was investigated by simulating perfusion in diffusion MRI data. The perfusion effect in the simulations was significant, especially for the estimation of the free water fraction, and was maintained as long as low b-value data were included in the analysis. Two approaches to reduce the perfusion effect were explored in this study: (i) increasing the minimal b-value used in the fitting, and (ii) using a three-compartment model that explicitly accounts for water molecules in the capillary blood. Estimation of the model parameters while excluding low b-values reduced the perfusion effect but was highly sensitive to noise. The three-compartment model fit was more stable and additionally, provided an estimation of the volume fraction of the capillary blood compartment. The three-compartment model thus disentangles the effects of free water diffusion and perfusion, which is of major clinical importance since changes in these components in the brain may indicate different pathologies, i.e., those originating from the extracellular space, such as neuroinflammation and atrophy, and those related to the vascular space, such as vasodilation, vasoconstriction and capillary density. Diffusion MRI data acquired from a healthy volunteer, using multiple b-shells, demonstrated an expected non-zero contribution from the blood fraction, and indicated that not accounting for the perfusion effect may explain the overestimation of the free water fraction evinced in previous studies. Finally, the applicability of the method was demonstrated with a dataset acquired using a clinically feasible protocol with shorter acquisition time and fewer b-shells.
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