I am trying to understand the physical property which is measured in diffusion weighted imaging (DWI) and diffusion tensor imaging (DTI). I read that these methods estimate the apparent diffusion coefficient (in DWI) or the diffusion tensor (in DTI).
My understanding is that diffusion usually refers to the net movement of a solute (e.g. salt) dissolved in a solvent (e.g. water) from a region of higher concentration to lower concentration.
My understanding is that osmosis usually refers to the net movement of water across a semi-permeable membrane from a region of low solute concentration to region of higher solute concentration.
I read that DWI and DTI measure the "diffusion of water". For example, in the white matter tracts of the brain, "water diffuses at a higher rate along the tract direction than perpendicular to the tract direction". What physical process is this referring to? Is this referring to a net flow of water along the white matter tracts (osmosis?), or the rate at which some solute dissolved in this water could diffuse?
I am struggling to phrase this question clearly, but the main thing I am trying to understand is the physical significance of the quantities measured by DWI and DTI -- the phrase "diffusion of water" does not mean much to me.