The microarchitecture specific to nerve tissues causes diffusion anisotropy in the white matter of the brain: water molecule diffusion preferably follows the direction of the fibers and is restricted perpendicularly to the fibers.
By performing diffusion-weighted acquisitions in at least 6 directions (and far more in angular high resolution imaging), it is possible to extract the diffusion tensor which synthesizes all the data.
The diffusion tensor characterizes diffusion: anisotropic coefficient, preferred directions and restrictions in space. Different images will be obtained depending on the complexity of the post-processing of this data (figures 13.9 and 13.10):
The limitations of the tensor model in precisely determining the directions of water molecule displacements in the case of fiber crossing, for example, have led to the development of new models, which require a greater number of measurements: