Purpose: To demonstrate how triple diffusion encoding (TDE) MRI can be applied to separately estimate the intra-axonal and extra-axonal diffusion tensors in white matter (WM). Methods: Using a TDE pulse sequence with an axially symmetric b-matrix, diffusion MRI data were acquired at 3T for 3 healthy adults with an axial b-value of 4000 s/mm2, a radial b-value of 307 s/mm2, and 64 diffusion encoding directions. This acquisition was then repeated with the radial b-value set to 0. A previously proposed theory was applied to these data in order to estimate the intra-axonal diffusivity and axonal water fraction for each WM voxel. Conventional single diffusion encoding data were also obtained with b-values of 1000 and 2000 s/mm2, which provided additional information sufficient for determining both the intra-axonal and extra-axonal diffusion tensors. Results: From the TDE data, the average intra-axonal diffusivity in WM was found to be 2.24 ± 0.18 µm2/ms, and the average axonal water fraction was found to be 0.60 ± 0.11. From the 2 diffusion tensors, average WM values were estimated for several compartment-specific diffusion parameters. In particular, the extra-axonal mean diffusivity was 1.09 ± 0.19 µm2/ms, the intra-axonal fractional anisotropy was 0.50 ± 0.14, and the extra-axonal fractional anisotropy was 0.23 ± 0.13. Conclusion: By using a simple TDE pulse sequence with an axially symmetric b-matrix, the diffusion tensors for the intra-axonal and extra-axonal spaces can be separately estimated in adult WM. This allows one to determine compartment-specific diffusion properties for these 2 water pools.
Bibliographical noteFunding Information:
This work was supported, in part, by National Institutes of Health grants F31NS108623 (to H. Moss), T32GM008716 (to N. DeMore), T32DC014435 (to J. Dubno), and P41EB027061 (to S. Ramanna) as well as by a grant from the Litwin Foundation (to J. Helpern).
- axially symmetric
- diffusion tensor
- triple diffusion encoding
- white matter