Stanford University, Stanford, CA 94305-5105, USA.
Neuroimage. 2012 Sep;62(3):2065-82. doi: 10.1016/j.neuroimage.2012.05.065. Epub 2012 Jun 5.
Diseases involving the medial temporal lobes (MTL) such as Alzheimer's disease and mesial temporal sclerosis pose an ongoing diagnostic challenge because of the difficulty in identifying conclusive imaging features, particularly in pre-clinical states. Abnormal neuronal connectivity may be present in the circuitry of the MTL, but current techniques cannot reliably detect those abnormalities. Diffusion tensor imaging (DTI) has shown promise in defining putative abnormalities in connectivity, but DTI studies of the MTL performed to date have shown neither dramatic nor consistent differences across patient populations. Conventional DTI methodology provides an inadequate depiction of the complex microanatomy present in the medial temporal lobe because of a typically employed low isotropic resolution of 2.0-2.5 mm, a low signal-to-noise ratio (SNR), and echo-planar imaging (EPI) geometric distortions that are exacerbated by the inhomogeneous magnetic environment at the skull base. In this study, we pushed the resolving power of DTI to near-mm isotropic voxel size to achieve a detailed depiction of mesial temporal microstructure at 3 T. High image fidelity and SNR at this resolution are achieved through several mechanisms: (1) acquiring multiple repetitions of the minimum field of view required for hippocampal coverage to boost SNR; (2) utilizing a single-refocused diffusion preparation to enhance SNR further; (3) performing a phase correction to reduce Rician noise; (4) minimizing distortion and maintaining left-right distortion symmetry with axial-plane parallel imaging; and (5) retaining anatomical and quantitative accuracy through the use of motion correction coupled with a higher-order eddy-current correction scheme. We combined this high-resolution methodology with a detailed segmentation of the MTL to identify tracks in all subjects that may represent the major pathways of the MTL, including the perforant pathway. Tractography performed on a subset of the data identified similar tracks, although they were lesser in number. This detailed analysis of MTL substructure may have applications to clinical populations.
涉及内侧颞叶（MTL）的疾病，如阿尔茨海默病和内侧颞叶硬化症，由于难以识别明确的成像特征，尤其是在临床前状态下，这给诊断带来了持续的挑战。MTL 回路中的异常神经元连接可能存在，但目前的技术无法可靠地检测到这些异常。弥散张量成像（DTI）在定义连接的可疑异常方面显示出了前景，但迄今为止对 MTL 进行的 DTI 研究在患者群体中既没有显示出显著差异，也没有显示出一致的差异。传统的 DTI 方法学提供了对内侧颞叶复杂微观解剖结构的不充分描述，因为典型地采用 2.0-2.5 毫米的低各向同性分辨率、低信噪比（SNR）和 EPI 几何变形，这些变形因颅底不均匀的磁场环境而加剧。在这项研究中，我们将 DTI 的分辨率提高到接近毫米各向同性体素大小，以在 3T 下实现内侧颞叶微观结构的详细描述。在这个分辨率下，通过几种机制实现了高图像保真度和 SNR：（1）采集多次覆盖海马所需的最小视野重复，以提高 SNR；（2）利用单重聚焦扩散准备进一步提高 SNR；（3）进行相位校正以减少瑞利噪声；（4）通过使用轴向平面平行成像最小化失真并保持左右失真对称；（5）通过使用运动校正和更高阶的涡流校正方案来保留解剖学和定量准确性。我们将这种高分辨率方法与 MTL 的详细分割相结合，以识别所有受试者中可能代表 MTL 主要通路的轨迹，包括穿通通路。在数据的一个子集上进行的轨迹分析确定了类似的轨迹，尽管它们的数量较少。对 MTL 子结构的详细分析可能对临床人群有应用。
