Department of Radiology & Nuclear Medicine, Erasmus MC University Medical Center, Doctor Molewaterplein 40, 3015 GD Rotterdam, The Netherlands.
Department of Radiology & Nuclear Medicine, Erasmus MC University Medical Center, Doctor Molewaterplein 40, 3015 GD Rotterdam, The Netherlands.
Magn Reson Imaging. 2020 Jul;70:91-97. doi: 10.1016/j.mri.2020.04.005. Epub 2020 Apr 14.
Quantification of the T relaxation time constant is relevant in various magnetic resonance imaging applications. Mono- or bi-exponential models are typically used to determine these parameters. However, in case of complex, heterogeneous tissues these models could lead to inaccurate results. We compared a model, provided by the fractional-order extension of the Bloch equation with the conventional models.
Axial 3D ultra-short echo time (UTE) scans were acquired using a 3.0 T MRI and a 16-channel surface coil. After image registration, voxel-wise T was quantified with mono-exponential, bi-exponential and fractional-order fitting. We evaluated all three models repeatability and the bias of their derived parameters by fitting at various noise levels. To investigate the effect of the SNR for the different models, a Monte-Carlo experiment with 1000 repeats was performed for different noise levels for one subject. For a cross-sectional investigation, we used the mean fitted values of the ROIs in five volunteers.
Comparing the mono-exponential and the fractional order T maps, the fractional order fitting method yielded enhanced contrast and an improved delineation of the different tissues. In the case of the bi-exponential method, the long T component map demonstrated the anatomy clearly with high contrast. Simulations showed a nonzero bias of the parameters for all three mathematical models. ROI based fitting showed that the T values were different depending on the applied method, and they differed most for the patellar tendon in all subjects.
In high SNR cases, the fractional order and bi-exponential models are both performing well with low bias. However, in all observed cases, one of the bi-exponential components has high standard deviation in T. The bi-exponential model is suitable for T mapping, but we recommend using the fractional order model for cases of low SNR.
T 弛豫时间常数的量化在各种磁共振成像应用中都很重要。通常使用单指数或双指数模型来确定这些参数。然而，在复杂、不均匀的组织中，这些模型可能会导致不准确的结果。我们比较了分数阶布洛赫方程模型与传统模型。
使用 3.0 T MRI 和 16 通道表面线圈采集轴向 3D 超短回波时间（UTE）扫描。图像配准后，使用单指数、双指数和分数阶拟合对体素 T 进行量化。我们在不同噪声水平下评估了所有三种模型的重复性及其导出参数的偏差。为了研究不同模型的 SNR 对其的影响，我们针对一个体素，对不同噪声水平进行了 1000 次重复的蒙特卡罗实验。为了进行横截面研究，我们在五个志愿者中使用 ROI 的平均拟合值。
与单指数 T 图相比，分数阶拟合方法增强了对比度，并改善了不同组织的轮廓。在双指数方法中，长 T 分量图以高对比度清晰地显示了解剖结构。模拟表明，所有三种数学模型的参数都存在非零偏差。基于 ROI 的拟合表明，参数值取决于所应用的方法，在所有受试者中，髌腱的差异最大。
在高 SNR 情况下，分数阶和双指数模型的性能都很好，偏差较小。然而，在所有观察到的情况下，双指数模型的一个分量的 T 值具有较高的标准偏差。双指数模型适用于 T 映射，但我们建议在 SNR 较低的情况下使用分数阶模型。
