Tissue longitudinal relaxation characterized by recovery time T1 or rate R1 is a fundamental MRI contrast mechanism that is increasingly being used to study the brain's myelination patterns in both health and disease. Nevertheless, the quantitative relationship between T1 and myelination, and its dependence on B0 field strength, is still not well known. It has been theorized that in much of brain tissue, T1 field-dependence is driven by that of macromolecular protons (MP) through a mechanism called magnetization transfer (MT). Despite the explanatory power of this theory and substantial support from in-vitro experiments at low fields (<3 T), in-vivo evidence across clinically relevant field strengths is lacking. In this study, T1-weighted MRI was acquired in a group of eight healthy volunteers at four clinically relevant field strengths (0.55, 1.5, 3 and 7 T) using the same pulse sequence at a single site, and jointly analyzed based on the two-pool model of MT. MP fraction and free-water pool T1 were obtained in several brain structures at 3 and 7 T, which allowed distinguishing between contributions from macromolecular content and iron to tissue T1. Based on this, the T1 of MP in white matter, indirectly determined by assuming a field independent T1 of free water, was shown to increase approximately linearly with B0. This study advances our understanding of the T1 contrast mechanism and its relation to brain myelin content across the wide range of currently available MRI strengths, and it has the potential to inform design of T1 mapping methods for improved reproducibility in the human brain.

中文翻译：

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人脑 MRI T1 弛豫的 B0 场依赖性


以恢复时间 T1 或速率 R1 为特征的组织纵向松弛是一种基本的 MRI 对比机制，越来越多地用于研究健康和疾病状态下大脑的髓鞘形成模式。然而，T1 和髓鞘形成之间的定量关系及其对 B0 场强的依赖性仍不清楚。据推测，在大部分脑组织中，T1 场依赖性是由大分子质子 (MP) 通过磁化转移 (MT) 机制驱动的。尽管该理论具有解释力，并且得到低场（<3 T）体外实验的大力支持，但缺乏临床相关场强的体内证据。在本研究中，在单个部位使用相同的脉冲序列，在四个临床相关场强（0.55、1.5、3 和 7 T）下对 8 名健康志愿者进行了 T1 加权 MRI 采集，并根据两个-MT池模型。在 3 T 和 7 T 下，在多个脑结构中获得了 MP 分数和自由水池 T1，这使得可以区分大分子含量和铁对组织 T1 的贡献。基于此，白质中 MP 的 T1（通过假设自由水的场独立 T1 间接确定）显示出与 B0 近似线性增加。这项研究增进了我们对 T1 对比机制及其与当前可用 MRI 强度范围内的脑髓磷脂含量的关系的理解，并且有可能为 T1 映射方法的设计提供信息，以提高人脑的可重复性。