Mapping of distributions of a local b-matrix cross-term strength using diffusion tensor MRI in patients with Alzheimers disease

摘要

Purpose: Mapping of the distribution of local susceptibility strength could be very important in understanding the mechanisms of progression in neurodegenerative diseases, due to depositions of iron and iron-containing plaques. The goal of this study is to map the voxelwise distribution of local cross-term b-value (b c) effect caused by interactions between the external and internal gradients, in subjects with Alzheimers disease (AD), mild cognitive impairment (MCI), and cognitively normal (CN) elderly, using a diffusion tensor (DT) MRI. Methods: Two DT-MRI experiments were conducted with opposite polarities of the external diffusion-sensitizing gradients, and the authors modeled the 3 × 3 tensor matrix of b c maps and drove a rotationally independent mean b c (i.e., MB c) maps. To test whether AD has higher cross-term gradients than MCI andor CN, 15 AD patients, 18 MCI patients, and 16 CN controls were acquired from DT-MRI data, with six diffusion encoding directions, five b-values (0, 160, 360, 640, and 1000 smm 2), and positively and negatively alternating polarities of the external diffusion-sensitizing gradients. The b c and MB c maps were calculated and were spatially normalized into a study specific template for all subjects. The differences of MB c maps across the three subject groups were investigated with voxelwise one-way ANOVA tests for each b-value. The differences of MB c maps, among the four b-values, were also investigated with a voxelwise one-way within-subject ANOVA test for each group. Results: The authors successfully mapped the local cross-term strength, using a DT-MRI data in the three groups. The MB c differences between the groups were increased with increasing b-values. Compared with the CN group and the MCI group, MB c values in the AD group were significantly increased. However, compared with the CN group, MB c values in the MCI group were not significantly different for all the b-values. Conclusions: In order to map the b-matrix cross-term effect, the authors developed a rotationally invariant index of MB c, and the index was applied in AD, MCI, and CN subjects. In the AD group, compared with the MCI and CN groups, MB c values were increased. AD patients may have much more local intrinsic gradients in the brain than those MCI or CN subjects. MB c maps may be used to detect the intrinsically susceptible materials in the human brain, such as iron-containing plaques in the brain with AD.