Xu Junzhong, Li Hua, Li Ke, Harkins Kevin D, Jiang Xiaoyu, Xie Jingping, Kang Hakmook, Dortch Richard D, Anderson Adam W, Does Mark D, Gore John C
Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA; Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, TN, USA; Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, USA.
NMR Biomed. 2016 Apr;29(4):400-10. doi: 10.1002/nbm.3484. Epub 2016 Jan 19.
Mapping axon diameter is of interest for the potential diagnosis and monitoring of various neuronal pathologies. Advanced diffusion-weighted MRI methods have been developed to measure mean axon diameters non-invasively, but suffer major drawbacks that prevent their direct translation into clinical practice, such as complex non-linear data fitting and, more importantly, long scanning times that are usually not tolerable for most human subjects. In the current study, temporal diffusion spectroscopy using oscillating diffusion gradients was used to measure mean axon diameters with high sensitivity to small axons in the central nervous system. Axon diameters have been found to be correlated with a novel metric, DDR⊥ (the rate of dispersion of the perpendicular diffusion coefficient with gradient frequency), which is a model-free quantity that does not require complex data analyses and can be obtained from two diffusion coefficient measurements in clinically relevant times with conventional MRI machines. A comprehensive investigation including computer simulations and animal experiments ex vivo showed that measurements of DDR⊥ agree closely with histological data. In humans in vivo, DDR⊥ was also found to correlate well with reported mean axon diameters in human corpus callosum, and the total scan time was only about 8 min. In conclusion, DDR⊥ may have potential to serve as a fast, simple and model-free approach to map the mean axon diameter of white matter in clinics for assessing axon diameter changes.
绘制轴突直径对于各种神经元病变的潜在诊断和监测具有重要意义。先进的扩散加权磁共振成像（MRI）方法已被开发用于无创测量平均轴突直径，但存在严重缺陷，阻碍了它们直接应用于临床实践，例如复杂的非线性数据拟合，更重要的是，扫描时间长，这对于大多数人类受试者来说通常是无法忍受的。在当前研究中，使用振荡扩散梯度的时间扩散光谱法被用于测量中枢神经系统中小轴突的平均轴突直径，且具有高灵敏度。已发现轴突直径与一种新的指标DDR⊥（垂直扩散系数随梯度频率的离散率）相关，DDR⊥是一个无需复杂数据分析的无模型量，可通过传统MRI机器在临床相关时间内进行两次扩散系数测量获得。包括计算机模拟和离体动物实验在内的全面研究表明，DDR⊥的测量结果与组织学数据密切吻合。在人体活体研究中，还发现DDR⊥与报道的人类胼胝体平均轴突直径密切相关，且总扫描时间仅约8分钟。总之，DDR⊥可能有潜力作为一种快速、简单且无模型的方法，用于在临床上绘制白质的平均轴突直径，以评估轴突直径变化。
