High-resolution diffusion MRI brain atlases at 3.0 tesla and application to voxel-based analysis.

摘要

Quantitative measurements of brain diffusion parameters (apparent diffusion coefficient [ADC] and fractional anisotropy [FA]) provide new insight compared to conventional MRI. In this work we utilize affine and free form transformations to successfully create a minimally distorted average morphometric atlas of diffusion parameters (FA, ADC) acquired at 3.0 T. The quantitative atlas can be utilized to characterize morphological and functional information in order to distinguish subtle anatomical and functional variations between population cohorts.;MR diffusion weighted images based on echo planar image acquisition suffer both from low SNR as well as geometric distortions. In an effort to increase SNR, image acquisition was performed at 3T. The increased geometric distortions at the higher field were addressed by two methods: integration of the recently introduced parallel image reconstruction methods with DTI and post-processing to correct for residual geometric distortions. As part of this work, we developed quantitative metrics to determine the optimum acquisition parameters for diffusion-weighted images integrated with parallel acquisition. We also developed and implemented a post-processing algorithm based on optic flow that successfully reduces the geometric distortions to the sub-voxel level.;The next step was creation of shape-normalized atlases of the clinically relevant diffusion indices: ADC and FA. Image studies from two populations of ten normal subjects each with mean ages of 31 and 56 years respectively were used to study the effect of aging on the diffusion indices. Our voxel level analysis revealed age related decline in fractional anisotropy in frontal white matter, genu and body of corpus callosum, posterior limb of internal capsule and anterior and posterior limbs of external capsule. ROI based measurements on selective regions confirmed the voxel-based analysis. Further, we also evaluated the inter-hemispheric diffusion asymmetry with the voxel-based methodology. The preliminary findings, the first such report, indicate significant decline in hemispheric asymmetry with aging. Younger age group demonstrated anisotropy asymmetry in genu, splenium and body of corpus callosum along with thalamus, internal and external capsule and midbrain. However, for the middle-age group, the asymmetry was limited to the internal and external capsule.