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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.

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Diffusion of water.

Osmosis is movement of water across a membrane due to solute changes. In the case of diffusion tensor MRI, the whole point is that there is more diffusion along the same axis as white matter axons are traveling (either within or outside, but not across, those membranes), compared to a perpendicular axis. There isn't any net flow, it's just that water is more mobile in one axis because of the barriers that the membranes provide. We call this "anisotropy" because the diffusion is not "isotropic" which would mean the same in all directions. DWI is similar but we don't care about the direction, just the general ability of water to move freely in different parts of the brain.

Le Bihan, D., Mangin, J. F., Poupon, C., Clark, C. A., Pappata, S., Molko, N., & Chabriat, H. (2001). Diffusion tensor imaging: concepts and applications. Journal of Magnetic Resonance Imaging: An Official Journal of the International Society for Magnetic Resonance in Medicine, 13(4), 534-546.

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  • Good refresher, I forgot how it worked! ^__^ – DoctorWhom May 19 '19 at 22:37

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