Department of Physics & Astronomy, University of British Columbia, Vancouver, BC, Canada.
UBC MRI Research Centre, University of British Columbia, Vancouver, BC, Canada.
NMR Biomed. 2019 Jul;32(7):e4092. doi: 10.1002/nbm.4092. Epub 2019 Apr 30.
Brain myelin and iron content are important parameters in neurodegenerative diseases such as multiple sclerosis (MS). Both myelin and iron content influence the brain's R relaxation rate. However, their quantification based on R maps requires a realistic tissue model that can be fitted to the measured data. In structures with low myelin content, such as deep gray matter, R shows a linear increase with increasing iron content. In white matter, R is not only affected by iron and myelin but also by the orientation of the myelinated axons with respect to the external magnetic field. Here, we propose a numerical model which incorporates iron and myelin, as well as fibre orientation, to simulate R decay in white matter. Applying our model to fibre orientation-dependent in vivo R data, we are able to determine a unique solution of myelin and iron content in global white matter. We determine an averaged myelin volume fraction of 16.02 ± 2.07% in non-lesional white matter of patients with MS, 17.32 ± 2.20% in matched healthy controls, and 18.19 ± 2.98% in healthy siblings of patients with MS. Averaged iron content was 35.6 ± 8.9 mg/kg tissue in patients, 43.1 ± 8.3 mg/kg in controls, and 47.8 ± 8.2 mg/kg in siblings. All differences in iron content between groups were significant, while the difference in myelin content between MS patients and the siblings of MS patients was significant. In conclusion, we demonstrate that a model that combines myelin-induced orientation-dependent and iron-induced orientation-independent components is able to fit in vivo R data.
脑髓鞘和铁含量是多发性硬化症（MS）等神经退行性疾病的重要参数。髓鞘和铁含量都影响大脑的 R 弛豫率。然而，基于 R 图谱对它们进行定量需要一个现实的组织模型，该模型可以拟合测量数据。在髓鞘含量低的结构中，如深部灰质，R 随铁含量的增加呈线性增加。在白质中，R 不仅受铁和髓鞘的影响，还受有髓轴突相对于外磁场的方向的影响。在这里，我们提出了一个数值模型，该模型将铁和髓鞘以及纤维方向纳入其中，以模拟白质中的 R 衰减。将我们的模型应用于纤维方向依赖性的体内 R 数据，我们能够确定全局白质中髓鞘和铁含量的唯一解。我们确定 MS 患者非病变性白质中的平均髓鞘体积分数为 16.02±2.07%，匹配的健康对照组为 17.32±2.20%，MS 患者的健康兄弟姐妹为 18.19±2.98%。患者的平均铁含量为 35.6±8.9mg/kg 组织，对照组为 43.1±8.3mg/kg，健康兄弟姐妹为 47.8±8.2mg/kg。组间铁含量的所有差异均有统计学意义，而 MS 患者与 MS 患者的健康兄弟姐妹之间的髓鞘含量差异有统计学意义。总之，我们证明了一种将髓鞘诱导的方向依赖性和铁诱导的方向独立性成分结合在一起的模型能够拟合体内 R 数据。
