Kokeny Paul, Cheng Yu-Chung N, Xie He
Department of Biomedical Engineering, Wayne State University, United States.
Department of Radiology, Wayne State University, United States.
Magn Reson Imaging. 2018 May;48:129-137. doi: 10.1016/j.mri.2017.12.019. Epub 2017 Dec 26.
Modeling MRI signal behaviors in the presence of discrete magnetic particles is important, as magnetic particles appear in nanoparticle labeled cells, contrast agents, and other biological forms of iron. Currently, many models that take into account the discrete particle nature in a system have been used to predict magnitude signal decays in the form of R2* or R2' from one single voxel. Little work has been done for predicting phase signals. In addition, most calculations of phase signals rely on the assumption that a system containing discrete particles behaves as a continuous medium. In this work, numerical simulations are used to investigate MRI magnitude and phase signals from discrete particles, without diffusion effects. Factors such as particle size, number density, susceptibility, volume fraction, particle arrangements for their randomness, and field of view have been considered in simulations. The results are compared to either a ground truth model, theoretical work based on continuous mediums, or previous literature. Suitable parameters used to model particles in several voxels that lead to acceptable magnetic field distributions around particle surfaces and accurate MR signals are identified. The phase values as a function of echo time from a central voxel filled by particles can be significantly different from those of a continuous cubic medium. However, a completely random distribution of particles can lead to an R2' value which agrees with the prediction from the static dephasing theory. A sphere with a radius of at least 4 grid points used in simulations is found to be acceptable to generate MR signals equivalent from a larger sphere. Increasing number of particles with a fixed volume fraction in simulations reduces the resulting variance in the phase behavior, and converges to almost the same phase value for different particle numbers at each echo time. The variance of phase values is also reduced when increasing the number of particles in a fixed voxel. These results indicate that MRI signals from voxels containing discrete particles, even with a sufficient number of particles per voxel, cannot be properly modeled by a continuous medium with an equivalent susceptibility value in the voxel.
在存在离散磁性颗粒的情况下对磁共振成像（MRI）信号行为进行建模很重要，因为磁性颗粒存在于纳米颗粒标记的细胞、造影剂以及其他铁的生物形态中。目前，许多考虑系统中离散颗粒性质的模型已被用于预测来自单个体素的以R2*或R2'形式的幅度信号衰减。在预测相位信号方面所做的工作很少。此外，大多数相位信号的计算依赖于这样的假设，即包含离散颗粒的系统表现为连续介质。在这项工作中，使用数值模拟来研究离散颗粒的MRI幅度和相位信号，不考虑扩散效应。模拟中考虑了诸如颗粒大小、数密度、磁化率、体积分数、颗粒排列的随机性以及视野等因素。将结果与地面真值模型、基于连续介质的理论工作或先前的文献进行比较。确定了用于在几个体素中对颗粒进行建模的合适参数，这些参数能在颗粒表面周围产生可接受的磁场分布并给出准确的磁共振信号。来自由颗粒填充的中心体素的相位值作为回波时间的函数可能与连续立方介质的相位值有显著差异。然而，颗粒的完全随机分布可导致一个与静态去相位理论预测相符的R2'值。发现在模拟中使用半径至少为4个网格点的球体来生成与更大球体等效的磁共振信号是可以接受的。在模拟中增加具有固定体积分数的颗粒数量会降低相位行为的最终方差，并且在每个回波时间对于不同的颗粒数量收敛到几乎相同的相位值。在固定体素中增加颗粒数量时，相位值的方差也会降低。这些结果表明，即使每个体素中有足够数量的颗粒，包含离散颗粒的体素的MRI信号也不能通过在体素中具有等效磁化率值的连续介质来正确建模。
