Most neuroanatomical studies are based on T1-weighted MR images, whose intensity profiles are not solely determined by the tissue's longitudinal relaxation times (T1), but also affected by varying non-T1 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 B1+-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 B1+ 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 B1+ inhomogeneities have a considerably variable effect on cortical T1 estimates, as well as on hippocampal morphometry depending on the MP2RAGE setup. While T1 differed substantially across datasets initially, we show the inter-site T1 comparability improves after correcting for the spatially varying B1+ field using a separately acquired Sa2RAGE B1+ map. Finally, removal of B1+ 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 T1 comparability across sites and present evidence that hippocampal segmentation results are modulated by B1+ inhomogeneities. This calls for careful (1) consideration of sequence parameters when setting acquisition protocols, as well as (2) acquisition of a B1+ map to correct MP2RAGE data for potential B1+ variations to allow comparison across datasets.

中文翻译：

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MP2RAGE B1+ 敏感性对 7T 时位点间 T1 再现性和海马形态测量的影响

大多数神经解剖学研究基于 T1 加权 MR 图像，其强度分布不仅由组织的纵向弛豫时间 (T1) 决定，而且还受不同的非 T1 贡献的影响，阻碍了数据的可重复性。相比之下，例如，使用 MP2RAGE 序列的定量成像允许根据感兴趣的组织特性直接表征大脑。结合 7 特斯拉 (7T) MRI，这为获得具有高重现性、灵敏度和特异性的强大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的对比度变化——可以通过 B1+ 相关强度变化的残余显着影响图像质量和皮质分析，如我们之前的工作所示。为了跟进这一点：我们（1）使用一个数据集来验证这个协议效果，该数据集使用一组特别 B1+ 不敏感的 MP2RAGE 参数与并行传输激励相结合；(2) 扩展我们的分析以评估对海马形态测量的影响。后者最初仍未被探索，但可以提供与使用 7T MRI 的神经退行性研究的普遍性和可重复性相关的重要见解。我们确认 B1+ 不均匀性对皮质 T1 估计值以及海马形态测量学具有相当大的可变影响，具体取决于 MP2RAGE 设置。虽然最初的数据集 T1 差异很大，但我们显示，在使用单独获取的 Sa2RAGE B1+ 地图校正空间变化的 B1+ 场后，站点间 T1 的可比性有所提高。最后，去除 B1+ 残留物会影响海马体积和边界定义，特别是在强度变化强烈的结构附近（例如脑脊液）。总之，我们表明 MP2RAGE 参数的选择可以影响跨位点的 T1 可比性，并提供证据表明海马分割结果受 B1+ 不均匀性调节。这要求在设置采集协议时仔细 (1) 考虑序列参数，以及 (2) 采集 B1+ 图以校正 MP2RAGE 数据的潜在 B1+ 变化，从而允许跨数据集进行比较。我们表明 MP2RAGE 参数的选择可以影响跨位点的 T1 可比性，并提供证据表明海马分割结果受 B1+ 不均匀性调节。这要求在设置采集协议时仔细 (1) 考虑序列参数，以及 (2) 采集 B1+ 图以校正 MP2RAGE 数据的潜在 B1+ 变化，从而允许跨数据集进行比较。我们表明 MP2RAGE 参数的选择可以影响跨位点的 T1 可比性，并提供证据表明海马分割结果受 B1+ 不均匀性调节。这要求在设置采集协议时仔细 (1) 考虑序列参数，以及 (2) 采集 B1+ 图以校正 MP2RAGE 数据的潜在 B1+ 变化，从而允许跨数据集进行比较。