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Brain metabolites, such as N-acetylaspartate or myo-inositol, are constantlyprobing their local cellular environment under the effect of diffusion.Diffusion-weighted NMR spectroscopy therefore presents unparalleled potentialto yield cell-type specific microstructural information. Double diffusionencoding (DDE) relies on two diffusion blocks which relative directionsdescribe a varying angle during the course of the experiment. Unlike singlediffusion encoding, DDE measurements at long mixing time display some angularmodulation of the signal amplitude which reflects compartment shape anisotropy,while requiring relatively low gradient strength. This angular dependence hasbeen formerly used to quantify cell fiber diameter using a model ofisotropically oriented infinite cylinders. However, it has been little exploredhow additional features of the cell microstructure, such as cell body diameter,fiber length and branching may also influence the DDE signal. Here, we used acryoprobe as well as state-of-the-art post-processing to perform DDEacquisitions with high accuracy and precision in the mouse brain at 11.7 T. Wethen compared our results to simulated DDE datasets obtained in various 3D cellmodels in order to pinpoint which features of cell morphology may influence themost the angular dependence of the DDE signal. While the infinite cylindermodel poorly fits our experimental data, we show that incorporating branchedfiber structure in our model is paramount to sensibly interpret the DDE signal.Lastly, experiments and simulations in the short mixing time regime suggestthat some sensitivity to cell body diameter might be retrieved, althoughadditional experiments would be required to further support this statement.

Revisiting double diffusion encoding MRS in the mouse brain at 11.7T: which microstructural features are we sensitive to?