Diffusion imaging focuses on the micromovements (random, brownian) of the water molecules inside voxels. These motions encounter different obstacles in the body (cell membranes, proteins, macromolecules, fibers…), which vary according to the tissues and certain pathological modifications (intracellular edema, abscess, tumors…).
Essentially, extracellular water is the main object of exploration in diffusion imaging. Diffusion data provides indirect information about the structure surrounding these water molecules.
Stated simply, the displacement of water molecules can be summarized into three different types of freedom of movement.
the water molecules displace freely in all spatial directions. A typical example of this corresponds to fluids such as cerebrospinal fluid.
Restricted isotropic diffusion
water molecule displacement is restricted, in whatever spatial direction, by numerous obstacles (proteins, cells)
Example: abscess, tumor at high cell density
Restricted anisotropic diffusion
Certain structured tissues create obstacles that orientate the motion of the water molecules (tendency to displace themselves in one or several particular directions). Diffusion is only restricted in certain spatial directions.
Example: nerve fibers (organization in bundles of axons running in parallel, with concentric layers of myelin restricting transversal diffusion)
DW-MRI aims at highlighting the differences in water molecule mobility, irrespective of their direction of displacement.
Diffusion tensor MRI, on the other hand, studies the directions of water molecule motion to determine, for example, whether or not they diffuse in all directions (fractional anisotropy), or attempts to render the direction of a particular diffusion (which can be applied to indirectly reconstituting the nerve fiber trajectory).