Читать книгу Dental Neuroimaging - Chia-shu Lin - Страница 37

While the T1‐weighted image provides a spatial feature of different brain regions, it provides less information regarding how the brain forms a connectional network. The key to understanding the connection between brain regions is to estimate the orientation of neural fibres. Diffusion magnetic resonance imaging (dMRI) is an MRI method to estimate the distribution of the ‘fibrous’ space in the brain. The method is based on the phenomenon that water molecules spread less freely in the compartment abundant of axons because the freedom to spread is limited by the axons aligned in the same direction. In contrast, the molecules spread more freely in the fluid space, such as the ventricles, where less hindrance exists to restrict the direction of spreading. Diffusion tensor imaging (DTI) is developed to quantify the directionality of diffusion. There are two major applications of dMRI. Firstly, it helps to examine the microstructural properties of the white matter (Jenkinson and Chappell 2018). For example, fractional anisotropy (FA) is a widely used index related to axonal density, the myelination of nerve fibres, and the membrane permeability (Jones et al. 2013). Secondly, dMRI is useful for exploring the structural connectivity of the brain, i.e. how the brain is wired by neural fibres. At present, it is the only tool that can probe the structural connectivity of the human brain in vivo (Jenkinson and Chappell 2018). Tractography has been used to visualize the streamlines that pass between different brain regions. The results provide further information about how brain regions are wired to form a network (see Section 2.3).