Centre for Functional and Metabolic Mapping, Robarts Research Institute, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada.
Centre for Functional and Metabolic Mapping, Robarts Research Institute, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada; Department of Clinical Neurological Sciences, Division of Neurosurgery, Western University, 1151 Richmond St. N., London, ON N6A 5B7, Canada.
Neuroimage. 2021 Jan 1;224:117373. doi: 10.1016/j.neuroimage.2020.117373. Epub 2020 Sep 17.
Most neuroanatomical studies are based on T-weighted MR images, whose intensity profiles are not solely determined by the tissue's longitudinal relaxation times (T), but also affected by varying non-T contributions, hampering data reproducibility. In contrast, quantitative imaging using the MP2RAGE sequence, for example, allows direct characterization of the brain based on the tissue property of interest. Combined with 7 Tesla (7T) MRI, this offers unique opportunities to obtain robust high-resolution brain data characterized by a high reproducibility, sensitivity and specificity. However, specific MP2RAGE parameter choices - e.g., to emphasize intracortical myelin-dependent contrast variations - can substantially impact image quality and cortical analyses through remnants of B-related intensity variations, as illustrated in our previous work. To follow up on this: we (1) validate this protocol effect using a dataset acquired with a particularly B insensitive set of MP2RAGE parameters combined with parallel transmission excitation; and (2) extend our analyses to evaluate the effects on hippocampal morphometry. The latter remained unexplored initially, but can provide important insights related to generalizability and reproducibility of neurodegenerative research using 7T MRI. We confirm that B inhomogeneities have a considerably variable effect on cortical T estimates, as well as on hippocampal morphometry depending on the MP2RAGE setup. While T differed substantially across datasets initially, we show the inter-site T comparability improves after correcting for the spatially varying B field using a separately acquired Sa2RAGE B map. Finally, removal of B residuals affects hippocampal volumetry and boundary definitions, particularly near structures characterized by strong intensity changes (e.g. cerebral spinal fluid). Taken together, we show that the choice of MP2RAGE parameters can impact T comparability across sites and present evidence that hippocampal segmentation results are modulated by B inhomogeneities. This calls for careful (1) consideration of sequence parameters when setting acquisition protocols, as well as (2) acquisition of a B map to correct MP2RAGE data for potential B variations to allow comparison across datasets.
大多数神经解剖学研究都是基于 T 加权磁共振图像，其强度分布不仅取决于组织的纵向弛豫时间 (T)，还受到不同的非 T 贡献的影响，从而影响数据的可重复性。相比之下，使用 MP2RAGE 序列进行定量成像可以根据感兴趣的组织特性直接对大脑进行特征描述。结合 7 特斯拉 (7T) MRI，这为获得具有高可重复性、灵敏度和特异性的稳健高分辨率大脑数据提供了独特的机会。然而，特定的 MP2RAGE 参数选择（例如，强调皮质内髓鞘依赖性对比变化）可能会通过与 B 相关的强度变化的残余物，显著影响图像质量和皮质分析，正如我们之前的工作所说明的那样。为了跟进这一点：我们 (1) 使用一组特别不敏感的 MP2RAGE 参数和并行传输激励来验证该协议的效果；(2) 将我们的分析扩展到评估对海马形态测量学的影响。后者最初尚未得到探索，但可以为使用 7T MRI 的神经退行性研究的可推广性和可重复性提供重要的见解。我们确认 B 不均匀性对皮质 T 估计以及海马形态测量学有相当大的可变影响，具体取决于 MP2RAGE 配置。虽然数据集之间的 T 差异很大，但我们表明，使用单独获取的 Sa2RAGE B 图校正空间变化的 B 场后，站点间 T 的可比性会提高。最后，去除 B 残余物会影响海马体的体积和边界定义，特别是在具有强烈强度变化的结构附近（例如，脑脊液）。总之，我们表明，MP2RAGE 参数的选择会影响站点之间的 T 可比性，并提供证据表明，海马体分割结果受到 B 不均匀性的调制。这需要在设置采集协议时（1）仔细考虑序列参数，以及（2）采集 B 图以校正 MP2RAGE 数据以避免潜在的 B 变化，从而允许在数据集之间进行比较。
