Diffusion Tensor Imaging to Study Brain Connectivity

Introduction MRI has been a powerful tool to map anatomy and function of human brains. To assign functions of the cortex, fMRI has been playing important roles. Classically, anatomy – function correlation studies had relied on postmortem examination of patients with localized lesions. Now we have a tool to non-invasively investigate functional centers of the brain. Compared to the cortical mapping, white matter mapping has been a challenging target. The white matter plays an important role to connect different regions of the brain. To understand brain functions, it is essential to understand brain connectivity. Interestingly, our understanding about brain connectivity is quite limited. One of the reasons could be its sheer complexity. In addition, we haven’t had good tools to investigate brain connectivity either. All existing methods are based on invasive techniques, which can’t be applied to human. In addition to our scarce knowledge about the brain connectivity, conventional MRI has also been powerless in this front. In T1 and T2-weighted images, most white matter regions look homogeneous. This situation has changed since the introduction of diffusion imaging and diffusion tensor imaging (DTI) in ‘90s. (1-3) This technology can provide us with approximate orientations of axonal tracts within each pixel. (3-8) It has been also shown that trajectories of prominent white tracts can be three-dimensionally reconstructed based on DTI results. (9-16) There is a possibility that we can obtain much needed information about human brain connectivity. By combining with fMRI data, we can ask questions like, “how two fMRI-activated regions are connected?” Although this is a quite exciting technique, it is also becoming increasingly clear that great care is needed to interpret DTI and DTI-based tractography results. (17, 18) In this course, I would like to go over limitations of DTI technique and various techniques to study brain connectivity.