Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands.
Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands.
Neuroimage. 2018 Mar;168:162-171. doi: 10.1016/j.neuroimage.2017.03.039. Epub 2017 Mar 20.
Several magnetic resonance imaging (MRI) contrasts are sensitive to myelin content in gray matter in vivo which has ignited ambitions of MRI-based in vivo cortical histology. Ultra-high field (UHF) MRI, at fields of 7T and beyond, is crucial to provide the resolution and contrast needed to sample contrasts over the depth of the cortex and get closer to layer resolved imaging. Ex vivo MRI of human post mortem samples is an important stepping stone to investigate MRI contrast in the cortex, validate it against histology techniques applied in situ to the same tissue, and investigate the resolutions needed to translate ex vivo findings to in vivo UHF MRI. Here, we investigate key technology to extend such UHF studies to large human brain samples while maintaining high resolution, which allows investigation of the layered architecture of several cortical areas over their entire 3D extent and their complete borders where architecture changes. A 16 channel cylindrical phased array radiofrequency (RF) receive coil was constructed to image a large post mortem occipital lobe sample (~80×80×80mm) in a wide-bore 9.4T human scanner with the aim of achieving high-resolution anatomical and quantitative MR images. Compared with a human head coil at 9.4T, the maximum Signal-to-Noise ratio (SNR) was increased by a factor of about five in the peripheral cortex. Although the transmit profile with a circularly polarized transmit mode at 9.4T is relatively inhomogeneous over the large sample, this challenge was successfully resolved with parallel transmit using the kT-points method. Using this setup, we achieved 60μm anatomical images for the entire occipital lobe showing increased spatial definition of cortical details compared to lower resolutions. In addition, we were able to achieve sufficient control over SNR, B and B homogeneity and multi-contrast sampling to perform quantitative T* mapping over the same volume at 200μm. Markov Chain Monte Carlo sampling provided maximum posterior estimates of quantitative T* and their uncertainty, allowing delineation of the stria of Gennari over the entire length and width of the calcarine sulcus. We discuss how custom RF receive coil arrays built to specific large post mortem sample sizes can provide a platform for UHF cortical layer-specific quantitative MRI over large fields of view.
几种磁共振成像（MRI）对比剂对体内灰质中的髓鞘含量敏感，这激发了基于 MRI 的皮质组织学的活体研究。超高磁场（UHF）MRI 在 7T 及以上的磁场中至关重要，可为皮质深度采样对比度提供所需的分辨率和对比度，并更接近层分辨成像。对人类死后样本的离体 MRI 是研究皮质 MRI 对比的重要垫脚石，它可以验证其与应用于同一组织的原位组织学技术的一致性，并研究将离体发现转化为活体 UHF MRI 所需的分辨率。在这里，我们研究了将这些 UHF 研究扩展到大型人类大脑样本的关键技术，同时保持高分辨率，从而可以研究几个皮质区域的分层结构及其完整边界，这些边界的结构会发生变化。构建了一个 16 通道圆柱形相控阵射频（RF）接收线圈，以在宽孔径 9.4T 人体扫描仪中对大型死后枕叶样本（~80×80×80mm）进行成像，旨在获得高分辨率解剖和定量 MRI 图像。与 9.4T 时的人头线圈相比，外周皮质的最大信噪比（SNR）提高了约五倍。尽管在大样本中，采用圆极化发射模式的发射轮廓在 9.4T 时相对不均匀，但这一挑战通过使用 kT 点方法的并行传输成功得到解决。使用该设置，我们实现了整个枕叶的 60μm 解剖图像，与较低分辨率相比，皮质细节的空间分辨率更高。此外，我们能够对 SNR、B 和 B 均匀性以及多对比度采样进行充分控制，以便在 200μm 时对同一体积进行定量 T映射。马尔可夫链蒙特卡罗采样提供了定量 T及其不确定性的最大后验估计，从而可以在整个距状沟的长度和宽度上描绘出 Gennari 纹。我们讨论了如何根据特定的大型死后样本大小构建定制的 RF 接收线圈阵列，为大视野范围内的 UHF 皮质层特异性定量 MRI 提供平台。
