The gamma-parameter of anomalous diffusion quantified in human brain by MRI depends on local magnetic susceptibility differences

Motivated by previous results obtained in vitro, we investigated the dependence of the anomalous diffusion (AD) MRI technique on local magnetic susceptibility differences (Delta chi) driven by magnetic field inhomogeneity in human brains. The AD-imaging contrast investigated here is quantified by the stretched-exponential parameter gamma, extracted from diffusion weighted (DW) data collected by varying diffusion gradient strengths. We performed T-2* and DW experiments in eight healthy subjects at 3.0 T.T-2*-weighted images at different TEs = (10,20,35,55) ms and DW-EPI images with fourteen b-values from 0 to 5000 s/mm(2) were acquired. AD-metrics and Diffusion Tensor Imaging (DTI) parameters were compared and correlated to R-2* and to Delta chi values taken from literature for the gray (GM) and the white (WM) matter. Pearson's correlation test and Analysis of Variance with Bonferroni post-hoc test were used. Significant strong linear correlations were found between AD gamma-metrics and R-2* in both GM and WM of the human brain, but not between DTI-metrics and R-2*. Depending on Delta chi driven magnetic field inhomogeneity, the new contrast provided by AD-gamma imaging reflects Delta chi due to differences in myelin orientation and iron content within selected regions in the WM and GM, respectively. This feature of the AD-gamma imaging due to the fact that gamma is quantified by using MM, may be an alternative strategy to investigate, at high magnetic fields, microstructural changes in myelin, and alterations due to iron accumulation. Possible clinical applications might be in the field of neurodegenerative diseases.