Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA.
Department of Electric and Electronic Engineering, University of Bristol, Bristol, UK.
NMR Biomed. 2024 Dec;37(12):e5234. doi: 10.1002/nbm.5234. Epub 2024 Aug 4.
Understanding the effects of white matter (WM) axon fibre microstructure on T1 relaxation is important for neuroimaging. Here, we have studied the interrelationship between T1 and axon fibre configurations at 3T and 7T. T1 and S0 (=signal intensity at zero TI) were computed from MP2RAGE images acquired with six inversion recovery times. Multishell diffusion MRI images were analysed for fractional anisotropy (FA); MD; V1; the volume fractions for the first (f), second (f) and third (f) fibre configuration; and fibre density cross-section images for the first (fdc), second (fdc) and third (fdc) fibres. T1 values were plotted as a function of FA, f, f, f, fdc, fdc and fdc to examine interrelationships between the longitudinal relaxation and the diffusion MRI microstructural measures. T1 values decreased with increasing FA, f and f in a nonlinear fashion. At low FA values (from 0.2 to 0.4), a steep shortening of T1 was followed by a shallow shortening by 6%-10% at both fields. The steep shortening was associated with decreasing S0 and MD. T1 also decreased with increasing fdc values in a nonlinear fashion. Instead, only a small T1 change as a function of either f or fdc was observed. In WM areas selected by fdc only masks, T1 was shorter than in those with fdc/fdc. In WM areas with high single fibre populations, as delineated by f/fdc masks, T1 was shorter than in tissue with high complex fibre configurations, as segmented by f/fdc or f/fdc masks. T1 differences between these WM areas are attributable to combined effects by T1 anisotropy and lowered FA. The current data show strong interrelationships between T1, axon fibre configuration and orientation in healthy WM. It is concluded that diffusion MRI microstructural measures are essential in the effort to interpret quantitative T1 images in terms of tissue state in health and disease.
了解白质 (WM) 轴突纤维微观结构对 T1 弛豫的影响对于神经影像学很重要。在这里，我们研究了在 3T 和 7T 下 T1 与轴突纤维结构之间的相互关系。使用六个反转恢复时间从 MP2RAGE 图像中计算 T1 和 S0（TI 为零时的信号强度）。对多壳扩散 MRI 图像进行分析以获得分数各向异性（FA）；MD；V1；第一（f）、第二（f）和第三（f）纤维结构的体积分数；以及第一（fdc）、第二（fdc）和第三（fdc）纤维的纤维密度截面图像。将 T1 值绘制为 FA、f、f、f、fdc、fdc 和 fdc 的函数，以检查纵向弛豫与扩散 MRI 微观结构测量之间的相互关系。T1 值随 FA、f 和 f 呈非线性方式降低。在低 FA 值（0.2 到 0.4）下，T1 急剧缩短，然后在两个场中以 6%到 10%的斜率变浅。急剧缩短与 S0 和 MD 的减少有关。T1 值也随 fdc 值的增加呈非线性方式降低。相反，仅观察到 T1 随 f 或 fdc 的变化很小。在仅由 fdc 值掩模选择的 WM 区域中，T1 比在具有 fdc/fdc 的区域中短。在由 f/fdc 掩模描绘的具有高单纤维群体的 WM 区域中，T1 比在由 f/fdc 或 f/fdc 掩模分割的具有高复杂纤维结构的组织中短。这些 WM 区域之间的 T1 差异归因于 T1 各向异性和 FA 降低的综合效应。目前的数据显示 T1、轴突纤维结构和方向之间存在很强的相互关系，这在健康 WM 中。可以得出结论，扩散 MRI 微观结构测量对于解释健康和疾病状态下定量 T1 图像的组织状态是必不可少的。
