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

Abstract

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 Alzheimer's disease (AD), mild cognitive impairment (MCI), and cognitively normal (CN) elderly, using a diffusion tensor (DT) MRI. 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 and∕or 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 s∕mm(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. 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. 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.