The human brain is made of neural fibers and they are the main component of the brain white matter. Diffusion Magnetic Resonance Imaging (dMRI) is a none invasive procedure that measures the diffusion of the water molecules in biological tissues in vivo. In an isotropic medium (e.g. inside a glass of water), water molecules naturally move randomly according to turbulence and Brownian motion. In biological tissues however, the diffusion may be anisotropic. For instance a water molecule inside an axon of a neuron has a low probability of crossing the myelin membrane. Therefore, the molecule moves mainly along the axis of the neural fiber. The measurement of the diffusion within the brain is devised in voxels (e.g. 2mm x 2mm x 2mm). If we know that water molecules in a particular voxel diffuse in some directions, we can make the assumption that the majority of the white matter fibers are proportionally going parallel to these directions. This enables the examination of the brain connectivity and moreover, this enables the reconstruction of a 3D model of the brain fiber networks. The procedure is called tractography.
Tractography offers new ways of studying the brain. Neurodegenerative diseases, like Parkinson's and Alzheimer's could be better understood with these tools. This could also be a very useful application for neurosurgical planning, as it offers a powerful tool for studying the brain connectivity reconstruction before and after surgery. It can also serve to study brain development. Tractography is a young research field. There still much to do in order to improve the quality of the 3D reconstructed model of the brain fiber networks.
The images bellow show some white matter fibers reconstruction. The image on top shows a whole brain, the coloring sheme is based on a T2 weighted image: blue regions are gray matter (mostly fiber ending), light-blue refer to white matter regions. The middle image uses a coloring scheme based on a tumeral region. Every fibers that pass trough the tumeral region is red, fibers that pass near are yellow and healthy fibers are green. The bottom image shows the same fibers as the middle one, but it also shows the functional MRI activation map (green, cyan, blue) over the cortex and activation volumes in yellow near the tumoral region. These images have been obtained with the FiberNavigator software.