Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Cracow, Poland.
Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Cracow, Poland.
NMR Biomed. 2019 Nov;32(11):e4130. doi: 10.1002/nbm.4130. Epub 2019 Jul 25.
Diffusion tensor imaging (DTI) is a powerful MRI modality that allows the investigation of the microstructure of tissues both in vivo and noninvasively. Its reliability is strictly dependent on the performance of diffusion-sensitizing gradients, of which spatial nonuniformity is a known issue in the case of virtually all clinical MRI scanners. The influence of diffusion gradient inhomogeneity on the accuracy of the diffusion tensor imaging was investigated by means of computer simulations supported by an MRI experiment performed at the isocenter and 15 cm away. The DTI measurements of two diffusion phantoms were simulated assuming a nonuniform diffusion-sensitizing gradient and various levels of noise. Thereafter, the tensors were calculated by two methods: (i) assuming a spatially constant b-matrix (standard DTI) and (ii) applying the b-matrix spatial distribution in the DTI (BSD-DTI) technique, a method of indicating the b-matrix for each voxel separately using an anisotropic phantom as a standard of diffusion. The average eigenvalues and fractional anisotropy across the homogeneous region of interest were calculated and compared with the expected values. Diffusion gradient inhomogeneity leads to overestimation of the largest eigenvalue, underestimation of the smallest one and thus overestimation of fractional anisotropy. The effect is similar to that caused by noise; however, it could not be corrected by increasing SNR. The MRI measurements, performed using a 3 T clinical scanner, revealed that the split of the eigenvalues measured 15 cm away from the isocenter is significant (up to 25%). The BSD-DTI calibration allowed the reduction of the measured fractional anisotropy of the isotropic medium from 0.174 to 0.031, suggesting that gradient inhomogeneity was the main cause of this error. For the phantom measured at the isocenter, however, the split was almost not observed; the average eigenvalues were shifted from the expected value by ~ 5%.
弥散张量成像（DTI）是一种强大的 MRI 模态，可以在体内和非侵入性地研究组织的微观结构。其可靠性严格依赖于扩散敏感梯度的性能，而实际上所有临床 MRI 扫描仪都存在空间非均匀性问题。通过计算机模拟和在等中心和 15cm 处进行的 MRI 实验，研究了扩散梯度非均匀性对扩散张量成像准确性的影响。在假设扩散敏感梯度不均匀且存在不同噪声水平的情况下，模拟了两个扩散体模的 DTI 测量。此后，通过两种方法计算张量：（i）假设空间常数 b 矩阵（标准 DTI）和（ii）应用 b 矩阵空间分布在 DTI（BSD-DTI）技术中，该方法使用各向异性体模作为扩散标准，为每个体素分别指示 b 矩阵。在均匀感兴趣区域内计算并比较平均特征值和各向异性分数。扩散梯度非均匀性会导致最大特征值高估、最小特征值低估，从而导致各向异性分数高估。这种效果类似于噪声引起的效果；但是，增加 SNR 并不能纠正这种效果。使用 3T 临床扫描仪进行的 MRI 测量显示，在离等中心 15cm 处测量的特征值分裂是显著的（高达 25%）。BSD-DTI 校准允许将各向同性介质的测量各向异性分数从 0.174 降低到 0.031，表明梯度非均匀性是这种误差的主要原因。然而，对于在等中心测量的体模，几乎没有观察到分裂；平均特征值与预期值相差约 5%。
