Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, SE1 7EH, United Kingdom.
Centre for the Developing Brain, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, SE1 7EH, United Kingdom.
Magn Reson Med. 2018 Aug;80(2):767-779. doi: 10.1002/mrm.27040. Epub 2017 Dec 15.
An extended phase graph framework (EPG-X) for modeling systems with exchange or magnetization transfer (MT) is proposed.
EPG-X models coupled two-compartment systems by describing each compartment with separate phase graphs that exchange during evolution periods. There are two variants: EPG-X(BM) for systems governed by the Bloch-McConnell equations, and EPG-X(MT) for the pulsed MT formalism. For the MT case, the "bound" protons have no transverse components, so their phase graph consists of only longitudinal states.
The EPG-X model was validated against steady-state solutions and isochromat-based simulation of gradient-echo sequences. Three additional test cases were investigated: (i) MT effects in multislice turbo spin-echo; (ii) variable flip angle gradient-echo imaging of the type used for MR fingerprinting; and (iii) water exchange in multi-echo spin-echo T relaxometry.
EPG-X was validated successfully against isochromat based transient simulations and known steady-state solutions. EPG-X(MT) simulations matched in-vivo measurements of signal attenuation in white matter in multislice turbo spin-echo images. Magnetic resonance fingerprinting-style experiments with a bovine serum albumin (MT) phantom showed that the data were not consistent with a single-pool model, but EPG-X(MT) could be used to fit the data well. The EPG-X(BM) simulations of multi-echo spin-echo T relaxometry suggest that exchange could lead to an underestimation of the myelin-water fraction.
The EPG-X framework can be used for modeling both steady-state and transient signal response of systems exhibiting exchange or MT. This may be particularly beneficial for relaxometry approaches that rely on characterizing transient rather than steady-state sequences. Magn Reson Med 80:767-779, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
提出了一种用于建模具有交换或磁化转移（MT）的系统的扩展相位图框架（EPG-X）。
EPG-X 通过描述每个隔室具有单独的相位图来对耦合两隔室系统进行建模，这些相位图在演化期间进行交换。有两种变体：用于受 Bloch-McConnell 方程控制的系统的 EPG-X（BM），以及用于脉冲 MT 形式的 EPG-X（MT）。对于 MT 情况，“束缚”质子没有横向分量，因此它们的相位图仅由纵向状态组成。
EPG-X 模型通过稳态解和梯度回波序列的等离振子模拟进行验证。还研究了三个附加测试案例：（i）多切片涡轮自旋回波中的 MT 效应；（ii）用于磁共振指纹识别的可变翻转角梯度回波成像；（iii）多回波自旋回波 T 弛豫测量中的水交换。
EPG-X 成功地通过等离振子瞬态模拟和已知的稳态解进行了验证。EPG-X（MT）模拟与多切片涡轮自旋回波图像中白质的信号衰减的体内测量相匹配。牛血清白蛋白（MT）幻影的磁共振指纹样式实验表明，数据与单池模型不一致，但 EPG-X（MT）可以很好地拟合数据。多回波自旋回波 T 弛豫测量的 EPG-X（BM）模拟表明，交换可能导致对髓鞘-水分数的低估。
EPG-X 框架可用于对表现出交换或 MT 的系统的稳态和瞬态信号响应进行建模。这对于依赖于对瞬态而不是稳态序列进行特征描述的弛豫测量方法可能特别有益。磁共振医学 80：767-779，2018。©2017 作者 磁共振医学由 Wiley 期刊出版公司代表国际磁共振学会出版。这是在知识共享署名许可条款下的许可，允许在任何媒介中使用、分发和复制，只要原始作品正确引用。
