Chapter 3.2 - DTI and Tractography in the Autistic Brain

Abstract

Diffusion tensor imaging (DTI) and associated fiber tractography are an emerging MRI technique for studying white matter of the brain. This chapter presents an introduction to the physical and biological bases of diffusion in white matter and the development and analysis of diffusion tensor imaging. It also includes visualization of white matter fiber tracts and quantification of physical diffusion parameters, such as the mean diffusivity (MD) and fractional anisotropy (FA) that might be used to index white matter maturation. A review of the recent findings made using DTI in ASD (autism spectrum disorder) is presented, focusing on studies with large (>20) sample sizes. Common themes of elevated mean diffusivity and diminished fractional anisotropy emerge, especially in structures of the frontal and temporal lobes, but also in the corpus callosum. Voxel-based as well as regional connectivity approaches to extracting quantitative information from DTI are discussed along with approaches involving machine learning of pattern classifiers to distinguish ASD from TD and also identify key features (structures, regions or connections) that contribute most to that discrimination ability. Limitations of tractography based on DTI are discussed along with the emerging advance of high angular resolution diffusion imaging (HARDI) as a means to overcome DTI limitations in regions of complex white matter organization.