School of Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom.
Centre for the Developing Brain, King's College London, London, United Kingdom.
Magn Reson Med. 2019 Feb;81(2):907-920. doi: 10.1002/mrm.27442. Epub 2018 Sep 14.
This study demonstrates magnetization transfer (MT) effects directly affect relaxometry measurements and develops a framework that allows single-pool models to be valid in 2-pool MT systems.
A theoretical framework is developed in which a 2-pool MT system effectively behaves as a single-pool if the RMS RF magnetic field ( {\text{B}}_{1}^{{{\text{rms}}}}) is kept fixed across all measurements. A practical method for achieving controlled saturation magnetization transfer (CSMT) using multiband RF pulses is proposed. Numerical, Phantom, and in vivo validations were performed directly comparing steady state (SS) estimation approaches that under correct single-pool assumptions would be expected to vary in precision but not accuracy.
Numerical simulations predict single-pool estimates obtained from MT model generated data are not consistent for different SS estimation methods, and a systematic underestimation of T is expected. Neither effect occurs under the proposed CSMT approach. Both phantom and in vivo experiments corroborate the numerical predictions. Experimental data highlights that even when using the same relaxometry method, different estimates are obtained depending on which combination of flip angles (FAs) and TRs are used if the CSMT approach is not used. Using CSMT, stable measurements of both T and T are obtained. The measured T ) depends on {\text{B}}_{1}^{{{\text{rms}}}}, which is therefore an important parameter to specify.
This work demonstrates that conventional single pool relaxometry, which is highly efficient for human studies, results in unreliable parameter estimates in biological tissues because of MT effects. The proposed CSMT framework is shown to allow single-pool assumptions to be valid, enabling reliable and efficient quantitative imaging to be performed.
本研究直接证明磁化转移（MT）效应对弛豫测量有影响，并提出一个框架，使单池模型在双池 MT 系统中有效。
开发了一个理论框架，在该框架中，如果 RMS RF 磁场（B1rms）在所有测量中保持固定，则双池 MT 系统有效地表现为单池。提出了一种使用多频带 RF 脉冲实现受控饱和磁化转移（CSMT）的实用方法。进行了数值、幻影和体内验证，直接比较了在正确的单池假设下，预计会在精度而不是准确性上有所变化的稳态（SS）估计方法。
数值模拟预测，从 MT 模型生成的数据中获得的单池估计对于不同的 SS 估计方法是不一致的，并且预计 T 会被系统低估。在提出的 CSMT 方法下，不会出现这两种影响。幻影和体内实验都证实了数值预测。实验数据突出表明，即使使用相同的弛豫测量方法，如果不使用 CSMT 方法，也会根据使用的翻转角（FA）和 TR 的组合获得不同的估计值。使用 CSMT 可以稳定地测量 T 和 T。测量的 T1 取决于 B1rms，因此这是一个需要指定的重要参数。
这项工作表明，在生物组织中，由于 MT 效应，对于人体研究非常高效的传统单池弛豫测量会导致不可靠的参数估计。所提出的 CSMT 框架被证明允许单池假设有效，从而能够进行可靠和高效的定量成像。
