On the performance of multi-compartment relaxometry for myelin water imaging (MCR-MWI)

In this study, we optimized the variable flip angle (VFA) acquisition scheme using numerical simulations to shorten the acquisition time of multicompartment relaxometry for myelin water imaging (MCR-MWI) to a clinically practical range in the absence of advanced image reconstruction methods. As the primary objective of this study, the test-retest repeatability of myelin water fraction (MWF) measurements of MCR-MWI is evaluated on three gradient echo (GRE) sequence settings using the optimized VFA schemes with different echo times and repetition times, emulating various scanner setups. The cross-protocol reproducibility of MCR-MWI and MCR with diffusion-informed myelin water imaging (MCR-DIMWI) is also examined. As a secondary objective, we explore the bundle-specific profiles of various microstructural parameters from MCR-(DI)MWI and their cross-correlations to determine if these parameters possess supplementary microstructure information beyond myelin concentration.

Numerical simulations indicate that MCR-MWI can be performed with a minimum of three flip angles covering a wide range of T1 weightings without adding significant bias. This is supported by the results of an in vivo experiment, allowing whole-brain 1.5 mm isotropic MWF maps to be acquired in 9 min, reducing the total scan time to 40% of the original implementation without significant quality degradation. Good test-retest repeatability is observed for MCR-MWI for all three GRE protocols. While good correlations can also be found in MWF across protocols, systematic differences are observed. Bundle-specific MWF analysis reveals that certain white matter bundles are similar in all participants. We also found that microstructure relaxation parameters have low linear correlations with MWF. MCR-MWI is a reproducible measure of myelin. However, attention should be paid to the protocol related MWF differences when comparing different studies, as the MWF bias up to 0.5% can be observed across the protocols examined in this work.

本研究通过数值模拟优化了可变翻转角（variable flip angle, VFA）采集方案，旨在在无需先进图像重建方法的前提下，将用于髓鞘水成像的多室弛豫测量法（multicompartment relaxometry for myelin water imaging, MCR-MWI）的采集时长缩短至临床实用范围内。本研究的首要目标为：针对三种梯度回波（gradient echo, GRE）序列设置，采用不同回波时间与重复时间的优化VFA方案，模拟不同扫描仪配置，评估MCR-MWI的髓鞘水分数（myelin water fraction, MWF）测量的重测重复性。同时本研究还考察了MCR-MWI与扩散信息辅助髓鞘水成像多室弛豫测量法（diffusion-informed myelin water imaging, MCR-DIMWI）的跨协议可重复性。作为次要目标，我们探索了来自MCR-(DI)MWI的各类微观结构参数的束特异性特征及其互相关性，以判断这些参数是否具备超出髓鞘浓度之外的额外微观结构信息。

数值模拟结果表明，仅需至少三个覆盖宽范围T1加权的翻转角即可开展MCR-MWI扫描，且不会引入显著偏倚。体内实验结果验证了这一结论，使得全脑1.5mm各向同性MWF图的采集时长仅需9分钟，将总扫描时间缩减至原方案的40%，且未出现明显的图像质量下降。三种GRE协议下的MCR-MWI均表现出良好的重测重复性。尽管不同协议间的MWF也呈现出良好的相关性，但仍可观察到系统差异。束特异性MWF分析显示，部分白质束在所有受试者中表现出相似特征。我们还发现，微观结构弛豫参数与MWF的线性相关性较低。MCR-MWI是一种可重复的髓鞘测量指标，但在比较不同研究时需注意协议相关的MWF差异，本研究中考察的不同协议间可观察到最高达0.5%的MWF偏倚。