DTI studies in patients with Alzheimer's disease, mild cognitive impairment, or normal cognition with evaluation of the intrinsic background gradients

INTRODUCTION: The objective of the study was to explore the impact of the background gradients on diffusion tensor (DT) magnetic resonance imaging (DT-MRI) in patients with Alzheimer's disease (AD), mild cognitive impairment (MCI), or cognitively normal (CN) aging. METHODS: Two DT-MRI sets with positive and negative polarities of the diffusion-sensitizing gradients were obtained in 15 AD patients, 18 MCI patients, and 16 CN control subjects. The maps of mean diffusivity (MD) and fractional anisotropy (FA) were computed separately for positive (p: pMD and pFA) and negative (n: nMD and nFA) polarities, and we computed the geometric mean (gm) of the DT-MRI to obtain the gmFA and gmMD with reducing the background gradient effects. Regional variations were assessed across the groups using one-way analysis of variance. RESULTS: Increased regional gmMD values in the AD subjects, as compared to the regional gmMD values in the MCI and CN subjects, were found primarily in the frontal, limbic, and temporal lobe regions. We also found increased nMD and pMD values in the AD subjects compared to those values in the MCI and CN subjects, including in the temporal lobe and the left limbic parahippocampal gyrus white matter. Results of comparisons among the three methods showed that the left limbic parahippocampal gyrus and right temporal gyrus were the increased MD in the AD patients for all three methods. CONCLUSION: Background gradients affect the DT-MRI measurements in AD patients. Geometric average diffusion measures can be useful to minimize the intrinsic local magnetic susceptibility variations in brain tissue.