Quantification of exchange in the mouse brain using double diffusion encodings with fixed total diffusion-weighting

Motivation: Exchange is an important effect in diffusion MR of the brain but remains difficult to quantify using conventional methods and signal models due to parameter degeneracy. Goal(s): To develop and demonstrate robust measurement of exchange in the mouse brain. Approach: A method based on double diffusion encoding was previously developed to probe exchange isolated from other effects, yielding robust exchange time measurements. We apply this method in vivo for the first time. Results: We report a fast in vivo exchange time of approximately 38 ms as compared to 146 ms in a fixed sample, obtained by averaging through a slice. Impact: Cellular water exchange reflects not only structural characteristics but has also been linked to metabolism. Quantifying exchange may yield rich information, yet methods to do so are not mature. Here, we demonstrate a unique, isolated measurement of exchange in vivo.