  Double diffusion encoding (DDE) magnetic resonance measurements of the water
signal offers a unique ability to separate the effect of microscopic
anisotropic diffusion in structural units of tissue from the overall
macroscopic orientational distribution of cells. However, the specificity in
detected microscopic anisotropy is limited as the signal is averaged over
different cell types and across tissue compartments. Performing side-by-side
metabolite DDE spectroscopy (DDES) and water DDES in which a wide range of
b-values is used to gradually eliminate the extracellular contribution provides
complementary measures from which intracellular and extracellular microscopic
fractional anisotropies ($\mu$FA) and diffusivities can be estimated.
Metabolites are largely confined to the intracellular space and therefore
provide a benchmark for intracellular diffusivity of specific cell types. Here,
we aimed to estimate tissue- and compartment-specific human brain
microstructure by combining water and metabolites DDES experiments. We
performed DDES in human subjects in two brain regions that contain widely
different amounts of white matter (WM) and gray matter (GM): parietal white
matter (PWM) and occipital gray matter (OGM) on a 7 T MRI scanner. Results of
the metabolite DDES experiments in both PWM and OGM suggest a highly
anisotropic intracellular space within neurons and glia, with the possible
exception of gray matter glia. Tortuosity values in the cytoplasm for water and
tNAA, obtained with correlation analysis of microscopic parallel diffusivity
with respect to GM/WM tissue fraction in the volume of interest, are remarkably
similar for both molecules, while exhibiting a clear difference between gray
and white matter, suggesting a more crowded cytoplasm and more complex
cytomorphology of neuronal cell bodies and dendrites in GM than those found in
long-range axons in WM.
