Department of Radiology, Wayne State University, 3990 John R, 4201 St Antoine, Detroit Receiving Hospital 3L-8, Detroit, MI, 48201, USA.
Department of Neurology, Detroit Medical Center, Wayne State University, University Health Center-8th floor, 4201 St Antoine, Detroit, MI, 48201, USA.
Neuroimage. 2024 Oct 15;300:120857. doi: 10.1016/j.neuroimage.2024.120857. Epub 2024 Sep 19.
White matter (WM) fiber tracts in the brainstem communicate with various regions in the cerebrum, cerebellum, and spinal cord. Clinically, small lesions, malformations, or histopathological changes in the brainstem can cause severe neurological disorders. A direct and non-invasive assessment approach could bring valuable information about the intricate anatomical variations of the white matter fiber tracts and nuclei. Although tractography from diffusion tensor imaging has been commonly used to map the WM fiber tracts connectivity, it is difficult to differentiate the complex WM tracts anatomically. Both high field MRI methods and ultrahigh-field MRI methods at 7T and 11.7 T have been used to enhance the contrast of WM fiber tracts. Despite their promising results, it is still challenging to achieve wide clinical adoption at 3T. In this study, we explored a clinically feasible method using a proton density weighted (PDW) 3D gradient echo (GRE) sequence to directly image the WM fiber tracts in the brainstem at 3T in vivo.
We optimized a 3D high resolution, double echo, short TR, PDW GRE sequence on 5 healthy volunteers using a clinical 3T scanner to visualize the complicated anatomy of WM fiber tracts in the brain stem. Tissue properties including T1, proton density and T2* from in vivo quantitative MRI data were used for simulations to determine the optimal flip angle for the sequence. The visualization of multiple WM fiber tracts in the brainstem was assessed qualitatively and quantitatively using relative contrast and contrast-to-noise ratio (CNR). To improve the CNR, the final images were created by averaging over all echoes from two consecutive scans at the optimal flip angle. The results were compared to anatomical atlases and histology sections to identify the major fiber tracts. All the identified major fiber tracts were labeled on axial, sagittal and coronal slices.
The WM fiber tracts were found to have distinct hypointense signal throughout the brainstem and most of the major WM fiber tracts, such as the corticospinal tract, medial lemniscus, medial longitudinal fasciculus, and central tegmental tract, in the brainstem up to and including the thalamus were identified in all subjects. Both qualitative and quantitative evaluations showed that the 3° scan offered the best contrast for WM fiber tracts for a TR of 20 ms. The average over the first two echo times and two consecutive 3° scans gave a CNR of 47.8 ± 6.2 for the pyramidal tracts in particular and CNRs values greater than 6.5 ± 2.4 for the rest of the fiber tracts.
All the major fiber tracts in the brainstem could be visualized. Given the reasonably short scan time of 10 min at 3T, double echo PDW GRE sequence is a very practical approach for clinical adoption.
脑干部位的白质（WM）纤维束与大脑、小脑和脊髓的各个区域相连。临床上，脑干部位的小病变、畸形或组织病理学变化都可能导致严重的神经障碍。一种直接的、非侵入性的评估方法可以提供关于 WM 纤维束和核的复杂解剖变异的有价值信息。虽然基于弥散张量成像的示踪技术已经广泛用于描绘 WM 纤维束的连通性，但在解剖学上区分复杂的 WM 纤维束仍然具有挑战性。高磁场 MRI 方法和超高磁场 MRI 方法（7T 和 11.7T）已被用于增强 WM 纤维束的对比度。尽管这些方法具有广阔的应用前景，但在 3T 下实现广泛的临床应用仍然具有挑战性。在这项研究中，我们探索了一种在临床上可行的方法，即在 3T 下使用质子密度加权（PDW）3D 梯度回波（GRE）序列在体内直接成像脑干部位的 WM 纤维束。
我们使用临床 3T 扫描仪对 5 名健康志愿者优化了一种高分辨率、双回波、短重复时间、PDW GRE 序列，以可视化脑干部位 WM 纤维束的复杂解剖结构。使用体内定量 MRI 数据的 T1、质子密度和 T2*等组织特性进行模拟，以确定序列的最佳翻转角。使用相对对比度和对比噪声比（CNR）对脑干部位的多个 WM 纤维束进行定性和定量评估。为了提高 CNR，在最佳翻转角下，通过对两次连续扫描的所有回波进行平均，创建最终图像。将结果与解剖图谱和组织学切片进行比较，以识别主要的纤维束。在所有被试者中，所有识别出的主要纤维束均在轴位、矢状位和冠状位切片上进行了标记。
在脑干部位发现 WM 纤维束呈现出明显的低信号，在所有被试者中，大多数主要的 WM 纤维束，如皮质脊髓束、内侧丘系、内侧纵束和中央被盖束，以及包括丘脑在内的脑干部位的 WM 纤维束均被识别出来。定性和定量评估均表明，对于 TR 为 20ms 的 WM 纤维束，3°扫描提供了最佳的对比度。对前两个回波时间和两次连续 3°扫描的平均，使锥体束的 CNR 达到 47.8±6.2，其余纤维束的 CNR 值大于 6.5±2.4。
脑干部位的所有主要纤维束均能被可视化。考虑到在 3T 下 10 分钟的扫描时间相对较短，双回波 PDW GRE 序列是一种非常实用的临床应用方法。
