Golub Marc, Neto Henriques Rafael, Gouveia Nunes Rita
ISR-Lisboa/LARSyS and Department of Bioengineering, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal.
Champalimaud Research, Champalimaud Centre for the Unknown, Lisbon, Portugal.
Magn Reson Med. 2021 May;85(5):2537-2551. doi: 10.1002/mrm.28599. Epub 2020 Dec 3.
Free-water elimination DTI (FWE-DTI) has been used widely to distinguish increases of free-water partial-volume effects from tissue's diffusion in healthy aging and degenerative diseases. Because the FWE-DTI fitting is only well-posed for multishell acquisitions, a regularized gradient descent (RGD) method was proposed to enable application to single-shell data, more common in the clinic. However, the validity of the RGD method has been poorly assessed. This study aims to quantify the specificity of FWE-DTI procedures on single-shell and multishell data.
Different FWE-DTI fitting procedures were tested on an open-source in vivo diffusion data set and single-shell and multishell synthetic signals, including the RGD and standard nonlinear least-squares methods. Single-voxel simulations were carried out to compare initialization approaches. A multivoxel phantom simulation was performed to evaluate the effect of spatial regularization when comparing between methods. To test the algorithms' specificity, phantoms with two different types of lesions were simulated: with altered mean diffusivity or with modified free water.
Plausible parameter maps were obtained with RGD from single-shell in vivo data. The plausibility of these maps was shown to be determined by the initialization. Tests with simulated lesions inserted into the in vivo data revealed that the RGD approach cannot distinguish free water from tissue mean-diffusivity alterations, contrarily to the nonlinear least-squares algorithm.
The RGD FWE-DTI method has limited specificity; thus, its results from single-shell data should be carefully interpreted. When possible, multishell acquisitions and the nonlinear least-squares approach should be preferred instead.
自由水消除扩散张量成像（FWE-DTI）已被广泛用于区分健康衰老和退行性疾病中自由水部分容积效应的增加与组织扩散情况。由于FWE-DTI拟合仅在多壳采集时是适定的，因此提出了一种正则化梯度下降（RGD）方法，以使该技术能够应用于临床更常见的单壳数据。然而，RGD方法的有效性尚未得到充分评估。本研究旨在量化FWE-DTI程序对单壳和多壳数据的特异性。
在一个开源的体内扩散数据集以及单壳和多壳合成信号上测试了不同的FWE-DTI拟合程序，包括RGD和标准非线性最小二乘法。进行了单个体素模拟以比较初始化方法。在比较不同方法时，进行了多体素体模模拟以评估空间正则化的效果。为了测试算法的特异性，模拟了具有两种不同类型病变的体模：平均扩散率改变或自由水改变。
使用RGD从单壳体内数据获得了合理的参数图。这些图的合理性被证明是由初始化决定的。对插入体内数据的模拟病变进行测试发现，与非线性最小二乘算法相反，RGD方法无法区分自由水和组织平均扩散率的改变。
RGD FWE-DTI方法的特异性有限；因此，应谨慎解释其从单壳数据获得的结果。可能的话，应优先选择多壳采集和非线性最小二乘方法。
