Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom.
Centre for the Developing Brain, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom.
Magn Reson Med. 2022 Jan;87(1):220-235. doi: 10.1002/mrm.28984. Epub 2021 Aug 21.
Magnetization transfer (MT) and inhomogeneous MT (ihMT) contrasts are used in MRI to provide information about macromolecular tissue content. In particular, MT is sensitive to macromolecules, and ihMT appears to be specific to myelinated tissue. This study proposes a technique to characterize MT and ihMT properties from a single acquisition, producing both semiquantitative contrast ratios and quantitative parameter maps.
Building on previous work that uses multiband RF pulses to efficiently generate ihMT contrast, we propose a cyclic steady-state approach that cycles between multiband and single-band pulses to boost the achieved contrast. Resultant time-variable signals are reminiscent of an MR fingerprinting acquisition, except that the signal fluctuations are entirely mediated by MT effects. A dictionary-based low-rank inversion method is used to reconstruct the resulting images and to produce both semiquantitative MT ratio and ihMT ratio maps, as well as quantitative parameter estimates corresponding to an ihMT tissue model.
Phantom and in vivo brain data acquired at 1.5 Tesla demonstrate the expected contrast trends, with ihMT ratio maps showing contrast more specific to white matter, as has been reported by others. Quantitative estimation of semisolid fraction and dipolar T was also possible and yielded measurements consistent with literature values in the brain.
By cycling between multiband and single-band pulses, an entirely MT-mediated fingerprinting method was demonstrated. This proof-of-concept approach can be used to generate semiquantitative maps and quantitatively estimate some macromolecular-specific tissue parameters.
磁化传递（MT）和不均匀磁化传递（ihMT）对比在 MRI 中用于提供关于大分子组织含量的信息。特别是，MT 对大分子敏感，而 ihMT 似乎对髓鞘组织特异。本研究提出了一种从单次采集即可对 MT 和 ihMT 特性进行特征描述的技术，既能产生半定量对比比，又能产生定量参数图。
基于先前使用多频带 RF 脉冲高效生成 ihMT 对比的工作，我们提出了一种循环稳态方法，在多频带和单频带脉冲之间循环，以提高所获得的对比。由此产生的时变信号类似于磁共振指纹采集，只是信号波动完全由 MT 效应介导。字典基低秩反演方法用于重建所得图像，并产生半定量 MT 比和 ihMT 比图，以及对应于 ihMT 组织模型的定量参数估计。
在 1.5T 下采集的体模和活体脑数据显示了预期的对比趋势，ihMT 比图显示了更特异于白质的对比，这是其他人已经报道过的。半固态分数和偶极 T 的定量估计也是可能的，并且得到的测量值与大脑中的文献值一致。
通过在多频带和单频带脉冲之间循环，证明了一种完全由 MT 介导的指纹方法。这种概念验证方法可用于生成半定量图，并定量估计一些大分子特异性组织参数。
