Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, United States.
Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, United States.
Neuroimage. 2018 Nov 15;182:469-478. doi: 10.1016/j.neuroimage.2018.01.004. Epub 2018 Jan 12.
Diffusion microstructural imaging techniques have attracted great interest in the last decade due to their ability to quantify axon diameter and volume fraction in healthy and diseased human white matter. The estimates of compartment size and volume fraction continue to be debated, in part due to the lack of a gold standard for validation and quality control. In this work, we validate diffusion MRI estimates of compartment size and volume fraction using a novel textile axon ("taxon") phantom constructed from hollow polypropylene yarns with distinct intra- and extra-taxonal compartments to mimic white matter in the brain. We acquired a comprehensive set of diffusion MRI measurements in the phantom using multiple gradient directions, diffusion times and gradient strengths on a human MRI scanner equipped with maximum gradient strength (G) of 300 mT/m. We obtained estimates of compartment size and restricted volume fraction through a straightforward extension of the AxCaliber/ActiveAx frameworks that enables estimation of mean compartment size in fiber bundles of arbitrary orientation. The voxel-wise taxon diameter estimates of 12.2 ± 0.9 μm were close to the manufactured inner diameter of 11.8 ± 1.2 μm with G = 300 mT/m. The estimated restricted volume fraction demonstrated an expected decrease along the length of the fiber bundles in accordance with the known construction of the phantom. When G was restricted to 80 mT/m, the taxon diameter was overestimated, and the estimates for taxon diameter and packing density showed greater uncertainty compared to data with G = 300 mT/m. In conclusion, the compartment size and volume fraction estimates resulting from diffusion measurements on a human scanner were validated against ground truth in a phantom mimicking human white matter, providing confidence that this method can yield accurate estimates of parameters in simplified but realistic microstructural environments. Our work also demonstrates the importance of a biologically analogous phantom that can be applied to validate a variety of diffusion microstructural imaging methods in human scanners and be used for standardization of diffusion MRI protocols for neuroimaging research.
扩散微观结构成像技术在过去十年中引起了极大的兴趣，因为它们能够量化健康和患病人类白质中的轴突直径和体积分数。由于缺乏验证和质量控制的金标准，对隔室大小和体积分数的估计仍存在争议。在这项工作中，我们使用一种新型纺织轴突（“taxon”）幻影来验证扩散 MRI 对隔室大小和体积分数的估计，该幻影由具有独特的内外隔室的空心聚丙烯纱线制成，以模拟大脑中的白质。我们使用配备最大梯度强度 (G) 为 300 mT/m 的人类 MRI 扫描仪，在幻影中使用多个梯度方向、扩散时间和梯度强度采集了一套全面的扩散 MRI 测量值。我们通过扩展 AxCaliber/ActiveAx 框架直接获得了隔室大小和受限体积分数的估计值，该框架能够估计任意方向纤维束中平均隔室大小。在 G=300 mT/m 时，12.2±0.9 μm 的 taxon 直径估计值与制造的内直径 11.8±1.2 μm 接近。根据已知的幻影结构，受限体积分数沿纤维束长度呈现出预期的下降。当 G 限制为 80 mT/m 时，taxon 直径被高估，与 G=300 mT/m 相比，taxon 直径和包装密度的估计值显示出更大的不确定性。总之，在模仿人类白质的幻影中，通过人类扫描仪上的扩散测量得出的隔室大小和体积分数估计值与真实值进行了验证，这为该方法能够在简化但现实的微观结构环境中产生准确的参数估计提供了信心。我们的工作还表明，类似生物的幻影的重要性，该幻影可用于在人类扫描仪上验证各种扩散微观结构成像方法，并可用于扩散 MRI 协议的标准化，以进行神经影像学研究。
